NTID300.13
TRANSPORTATION NOISE
AND NOISE FROM EQUIPMENT POWERED BY
INTERNAL COMBUSTION ENGINES
DECEMBER 31, 1971
J.S. Environmental Protection Agenc'
Washington, D.C. 20460
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NTID300.13
TRANSPORTATION NOISE
AND NOISE FROM EQUIPMENT POWERED BY
INTERNAL COMBUSTION ENGINES
DECEMBER 31, 1971
Prepared by
WYLE LABORATORIES
under
CONTRACT 68-04-0046
for the
U.S. Environmental Protection Agency
Office of Noise Abatement and Control
Washington, D.C. 20460
This report has been approved for general availability. The contents of this
report reflect the views of the contractor, who is responsible for the facts
and the accuracy of the data presented herein, and do not necessarily
reflect the official views or policy of EPA. This report does not constitute
a standard, specification, or regulation.
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.
Price $4.30 domestic postpaid or $3.75 OPO Bookstore
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TABLE OF CONTENTS
PAGE
1 .0 INTRODUCTION 1
2.0 TRANSPORTATION SYSTEMS 5
2.1 Commercial Aircraft 8
2.1.1 Introduction 8
2.1.2 Source Noise Characteristics 9
2.1.3 Environmental Noise Characteristics 19
2.1 .4 Industry Efforts in Noise Reduction 27
2.1.5 Noise Reduction Potential 45
2.2 V/STOL Aviation 53
2.2.1 Introduction 53
2.2.2 Source Noise Characteristics 54
2.2.3 Environmental Noise Characteristics 60
2.2.4 Industry Efforts in Noise Reduction 64
2.2.5 Noise Reduction Potential 72
2.3 General Aviation Aircraft 74
2.3.1 Introduction 74
2.3.2 Source Noise Characteristics 75
2.3.3 Environmental Noise Characteristics 79
2.3.4 Industry Efforts in Noise Reduction 83
2.3.5 Noise Reduction Potential 87
2.4 Highway Vehicles 92
2.4.1 Introduction 92
2.4.2 Source Noise Characteristics 94
in
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PAGE
2.4.3 Environmental Noise Characteristics 112
2.4.4 Industry Efforts Toward Noise Reduction 117
2.4.5 Noise Reduction Potential 119
2.5 Rail Systems 133
2.5.1 Introduction 133
2.5.2 Source Noise Characteristics 133
2.5.3 Environmental Noise Characteristics 145
2.5.4 Industry Efforts in Noise Reduction 147
2.5.5 Noise Reduction Potential 157
2.6 Ships 161
2.6.1 Introduction 161
2.6.2 Source Noise Characteristics 161
2.6.3 Environmental Noise Characteristics 165
2.7 Recreation Vehicles 166
2.7.1 Introduction 166
2.7.2 Source Noise Characteristics 167
2.7.3 Environmental Noise Characteristics 176
2.7.4 Noise Reduction-Industry Efforts and Potential 179
3.0 DEVICES POWERED BY SMALL INTERNAL COMBUSTION
ENGINES 190
3.1 Introduction 190
3.2 Source Noise Characteristics 190
3.3 Environmental Noise Characteristics 198
3.4 Industry Efforts Towards Noise Reduction 201
3.5 Noise Reduction Potential 205
4.0 ENVIRONMENTAL IMPACT FOR TRANSPORTATION VEHICLES
AND SMALL INTERNAL COMBUSTION ENGINES 206
4.1 Total Noise Energy Output per Day for
Transportation Systems 208
IV
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4.2 Contribution of Transportation System Components
to the Residual Background Noise Level in an
Average Community
4.3 Relative Annoyance Potential of Intruding
Single Event Noise
4.4 Overall Assessment of Noise Impact by the
Transportation System on Non-Participants
4.5 Impact on Participant or Passengers in
Transportation Systems
4.6 Environmental Impact for Internal Combustion
Engine Devices
5.0 CONCLUSIONS AND RECOMMENDATIONS
5.1 Noise Impact on People
5.2 Interaction Between Public and Industry
5.3 Federal Action to Reduce Source Noise
5.4 Recommendations for Noise Reduction
REFERENCES
APPENDIX A
APPENDIX B
MEASUREMENT STANDARDS
FAR36 - Summary
ISO Recommendation R362 - Summary
SAE Standards - Summary
California Administration Codes - Summary
Part 36 - Noise Standards Aircraft Type Certification
METHODOLOGY FOR IMPACT ANALYSIS
B.I Total Noise Energy
B.2 Residual Noise Levels
B.3 Single Event Noise Levels for Major Transportation
Noise Sources as a Function of Distance
PAGE
213
215
221
226
230
233
234
240
242
243
252
A-l
A-4
A-5
A-6
A-10
A-13
B-l
B-l
B-6
B-25
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PAGE
B.4 Noise Impacted Land Areas Around Freeways
and Airports B-28
B.5 Noise Impact on Operators or Passengers of
Transportation Vehicles and Internal Combustion
Engines B-40
B.6 Glossary of Terminology B-41
References B-45
APPENDIX C NOISE GENERATOR CHARACTERISTICS C-l
C.I Jet Engine Noise C-l
C.2 Propeller and Rotor Noise C-22
C.3 Internal Combustion Engine Noise C-31
C.4 Tire Noise C-47
References C-59
VI
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LIST OF TABLES
Table
Number Page
2-1 Growth in the Transportation System, 1950 - 1970 7
2.1-1 Passenger Environment 22
2.1-2 Maximum Perceived Noise Levels of the DC-10-10
Relative to Those of Current 4-Engine Jet Transport 39
2.1-3 NASA Acoustically Treated Nacelle Program 40
2.1-4 NASA Quiet Engine Program 42
2.1-5 Federal Noise Abatement Programs 46
2.1-6 Estimated Aircraft Noise Reduction Potential for Takeoff 46
2.2-1 Estimated Noise Reduction Potential for Helicopters 73
2.3-1 Interior Noise Level Objectives 87
2.3-2 Potential General Aviation Aircraft Noise Reduction 90
2.4-1 Diesel Truck Noise Component Contributions to
Maximum Noise Levels at 50 Feet from Vehicle 102
2.5-1 Summary of the Noise Reduction Potential by Applying
Current Technology to Existing Transit Vehicles 158
2.7-1 Example of Further Noise Reduction Using
Existing Technology 188
3-1 Estimated Noise Reduction Potential for Devices
Powered by Internal Combustion Engines 205
4-1 Example of Potential Noise Reduction for Externally
Radiated Noise for Transportation System Categories 209
4-2 Estimated Noise Reduction Potential for Devices
Powered by Internal Combustion Engines 210
4-3 Estimated Noise Energy for Transportation System
Categories in 1970 211
4-4 Example of Estimated Future Change in Noise Energy
for Major Surface Transportation System Categories
with Three Options for Noise Reduction 212
4-5 Predicted Contributions to Daytime Residual Noise
Levels by Highway Vehicles for a Typical Urban
Community in 1970 214
VII
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Table
Number Page
4-6 Comparison of Major Surface Transportation System
Categories According to Typical Maximum Noise
Levels, Single Event Noise Exposure Levels (SENEL),
and the Distance Within Which the SENEL is
Greater than 72 dB 219
4-7 Summary of Estimated Noise Impacted Land (Within
CNEL 65 Contour) Near Airports and Freeways from
1955 to the Year 2000 with Future Estimates Based on
Option 2 and Option 3 Examples 225
4-8 Typical Passenger Separation Distances and Speech
Interference Criteria Compared to Average Internal
Noise Levels for Major Transportation Categories 229
4-9 Summary of Noise Impact Characteristics of Internal
Combustion Engines 231
5-1 Approximate Number of People (Operators and
Passengers) in Non-Occupational Situations Exposed
to Potentially Hazardous Noise with Respect to
Hearing Damage from Various Significant Sources 239
VIII
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LIST OF FIGURES
Figure
Number Page
2-1 General Characteristics of the Transportation
Industry in 1970 6
2.1-1 Characteristics of Commercial Aircraft 10
2.1-2 Noise Limits and Spectra of 4-Engine Low Bypass
Ratio Turbofan Aircraft 11
2.1-3 Noise Levels and Spectra of 2-3 Engine Low Bypass
Ratio Turbofan Aircraft 13
2.1-4 Noise Levels and Spectra of 4-Engine High Bypass
Ratio Turbofan Aircraft 14
2.1-5 Mean Noise Level Spectra for Various Types of Aircraft
at Approximately 1000 Ft Altitude, During
Takeoff 15
2.1-6 Mean Noise Level Spectra for Various Types of Aircraft
at Approximately 1000 Ft Altitude, During Landing 16
2.1-7 Nature of the Sonic Boom Phenomenon 18
2.1-8 Time Histories of Typical Cabin Noise Levels 20
2.1-9 Interior Cabin Noise Levels and Spectra for
Commercial Jet Aircraft 21
2.1-10 Noise Exposure Forecast vs Slant Range (Takeoff)
for an Equivalent Large Airport 24
2.1-11 Noise Exposure Forecast vs Slant Range (Approach)
for an Equivalent Large Airport 25
2.1-12 NEF 30 Contours for Representative (Single Runway) Airport 26
2.1-13 Projected Change in Commercial Aircraft Fleet Composition
(Based on Projected Passenger Capacity Demand Increasing
at 5 Percent/Year) 29
2.1-14 Projected Growth of Commercial Aircraft Traffic 30
2.1-15 FAR-36 Noise Certification Measurement Positions 33
2.1-16 Effective Perceived Noise Level for Landing
for Today's Aircraft Compared to FAR-36 Limits 35
IX
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Figure
Number ftjge
2.1-17 Effective Perceived Noise Level for Takeoff for Today's
Aircraft Compared to FAR-36 Limits 36
2. 1-18 Effective Perceived Noise Level at Sideline for Today's
Aircraft Compared to FAR-36 Limits 37
2.1-19 Trends in Jet Engine Noise Generation 38
2. 1-20 Flyover Noise Levels - DC-10-10 Compared to Current
Jet Transports Powered by (4) JT3D-3B Engines 41
2.1-21 Effects of Flight Procedure Changes on Noise Reduction
for the Case of a 4-Engine Turbofan Aircraft 44
2. 1-22 Estimated Noise-Impacted Areas (30 NEF or Higher) as
Function of Jet Engine Noise Reduction Goals 49
2. 1-23 Noise- Impacted Areas (30 NEF or Higher) vs Traffic
Growth (Projection for Year 3000) 51
2.2-1 Characteristics of V/STOL Aircraft 55
2.2-2 Potential 1985 U.S. V/STOL Fleet 56
2.2-3 Typical Noise Spectra of Light Piston-Engined Helicopters 57
2.2-4 Typical Noise Spectra of Heavy Helicopters 58
2.2-5 Typical Frequency Characteristics of Propulsion Systems
for 40 to 80 Passenger STOL Aircraft (excluding power
plant) at 500 Feet 61
2.2-6 Comparison of Typical V/STOL Aircraft Noise Levels with
Community Ambient Noise Levels (Lp/J 63
2.2-7 Current Design Approaches to Helicopter Noise Reduction 66
2.2-8 Effect of Design Changes on a Light Helicopter Noise
Signature (Demonstrated) 66
2.2-9 Demonstrated Noise Reduction of a Heavy- Helicopter
Twin-Rotor System 67
2.2-10 Exhaust Noise Suppression of Light Piston-Engined Helicopters 67
2.2-11 Trend of Helicopter Noise Levels with Gross Weight
and Seating Capacity 71
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Figure
Number Page
2.3-1 Characteristics of General Aviation Aircraft 76
2.3-2 Noise Levels and Spectra of General Aviation
Propeller Aircraft 77
2.3-3 Noise Levels and Spectra of Executive Jet Aircraft 78
2.3-4 Interior Cabin Noise Levels and Spectra for
General Aviation Propeller Aircraft 80
2.3-5 Noise Exposure Forecast Values for an Example of a
Representative General Aviation Airport with
Daytime Use Only (Takeoff) 81
2.3-6 Noise Exposure Forecast Values for an Example of a
Representative General Aviation Airport with
Daytime Use Only (Approach) 82
2.3-7 Number of General Aviation Aircraft with
Projections for 1975, 1980, and 1985 85
2.3-8 Comparison of Noise Levels at Various Angles from
Engine at Approximately 3000 Ibs Thrust 88
2.3-9 Noise Levels Generated by General Aviation Aircraft 91
2.4-1 Characteristics of Highway Vehicles 93
2.4-2 Single Vehicle Noise Output as a Function of Vehicle Speed 95
2.4-3 Diesel Truck and Automobile Noise at Highway Speeds,
Cruise and Coasting (at 50 Feet) 96
2.4-4 Typical Octave Band Spectra for Diesel Truck and Automobile
(Full Throttle Acceleration at 35 mph at 50 Feet) 98
2.4-5 Octave Band Spectra of Diesel Engine Inlet and Exhaust
Noise Illustrating the Effects of Silencing for the
Exhaust and the Intake 99
2.4-6 Typical Example of Diesel Truck Noise 101
2.4-7 Effect of Various Tires Mounted on the Drive Axle,
Loaded Single Chassis Vehicle Operating on Concrete
Road Surface (Levels at 50 Feet) 106
2.4-8 Typical Examples of Automobile Noise 109
2.4-9 Typical Octave Band Spectra for Highway Bus 111
XI
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Figure
Number Page
2.4-10 Typical Octave Band Spectra for Garbage Compactor
During Compacting Operations (Levels at 50 Feet) 113
2.4-11 Typical Average Noise Levels Near Freeway
(Diesel Trucks <3 Percent of Total) 114
2.4-12 Range of Typical Interior Noise Levels for Domestic
and Imported Passenger Cars at Highway Speeds 116
2.4-13 Typical Industry Production/Timing Schedule 120
2.4-14 Effect of Potential Noise Reduction for Highway Vehicles 121
2.4-15 Effect of Potential Noise Reduction for Diesel Trucks 124
2.5-1 Characteristics of Rail Systems 134
2.5-2 Wayside Noise Levels and Spectra of Railroad Equipment 136
2.5-3 Train Vehicle Interior Noise Levels and Spectra 139
2.5-4 Wayside Noise Levels and Spectra for Rapid Transit Vehicles 143
2.5-5 Subway Noise Levels and Spectra 144
2.5-6 Noise Level as a Function of Distance from Train
for Railroad and Rapid Transit 146
2.5-7 Comparison of Interior Noise Levels in Subway Transit Vehicles 150
2.5-8 Noise Levels at 50 Feet from Steel Wheel Transit Vehicles
on Tie and Ballast Trackbed for Maximum Vehicle Speed 155
2.6-1 Noise Levels and Spectra on Ships 164
2.7-1 Characteristics of Recreation Vehicles 168
2.7-2 Typical Ranges of Noise Levels Produced by Various
Pleasure Boat Types (db(A) at 50 Feet) 170
2.7-3 Motorcycle Noise Levels for Various Operating Modes 171
2.7-4 Motorcycle Noise 172
2.7-5 Snowmobile Noise 175
2.7-6 Typical Octave Band Spectra for V.W. Powered Dune Buggy
Equipped with "Megaphone" Exhausts (Levels at 50 Feet) 177
2.7-7 Potential Noise Reduction for Recreational Vehicles . 180
XII
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Figure
Number Page
2.7-8 Examples of Demonstrated Further Noise Reduction 185
3-1 Characteristics of Devices Powered by Internal
Combustion Engines 191
3-2 Typical Noise Characteristics of Generators 193
3-3 Typical Noise Characteristics of Rotary Lawnmowers 194
3-4 Typical Noise Characteristics of Chain Saws 196
3-5 Measured Noise Characteristics of Muffled and
Unmuffled Model Airplane Engines 197
4-1 Estimated Long-Term Trend in Outdoor Residual Noise
Levels in a Typical Residential Urban Community 216
4-2 Decay of Noise Level with Distance from Single Source
Defines Relative Bounds of Annoyance Zone 217
4-3 Approximate Growth of a Few Types of Noisy Recreational
Vehicles and Outdoor Home Equipment 220
4-4 Potential Hearing Damage Contributions from Transportation
System Categories in Terms of Equivalent 8-Hour Exposure
Levels, for Passengers or Operators 227
XIII
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1.0 INTRODUCTION
The outdoor noise environment for man today is the summation of noise
energy generated by all of the machines used to transport people and goods, machines
used to make and build things or save human labor, machines used by the consumer for
leisure activity, machines to make the other machines run, and people in their various
activities. Development of this machinery has been fostered by growth in technology
itself, as well as by pressures induced by changes in our life style and by population
growth. This report presents a detailed evaluation of noise of transportation vehicles
including those used commercially, as well as many of the private and non-industrial
devices powered by internal combustion engines.
The report has been prepared by Wyle Laboratories for the Environmental
Protection Agency in response to the directives contained in the Clean Air Amend-
ments Act of 1970, specifically, Section 401 "Noise Pollution and Abatement Act
of 1970." It forms part of the major study accomplished by the Office of Noise Abate-
ment and Control, of the Environmental Protection Agency, which is summarized in its
•*
report to Congress.
The noise sources considered in this report are encountered throughout
man's residential, recreational and working community. Sound is important to most
of the animal kingdom, including man. Some sounds provide warnings of danger,
which are essential for survival. These sounds may evoke basic reactions of startle,
fear or anger, which in turn assist in causing an appropriate response. Acoustic
warning devices such as sirens and horns utilize this principle, and the noise of an
approaching automobile is often the first clue of danger to the pedestrian or the child
playing ball in the street.
Other sounds evoke pleasure or are generated by an animal to reinforce or
communicate pleasure. The purring of a kitten and the ecstatic shouts of a child at play
Throughout this report, references are identified by superscripts.
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are familiar examples to all. Some pleasant sounds are relaxing, lulling an animal to
sleep, and others are stimulating. Music, developed by man, covers this broad spec-
trum, appealing to a wide variety of desires and needs at both the basic and intel-
lectual levels.
For man, sound has even more importance. His ability to communicate by
speech is the keystone of civilization and its spiritual, social, political, economic
and technical progress. Without speech communication, man would never have emerged
from a primitive state and developed the body of knowledge which could be passed from
generation to generation. Nor would he be able to interact with his fellow man in any-
thing beyond the most rudimentary levels such as are displayed by the higher primates.
The undesirability or desirability of noise in the environment must be judged
with reference to its effects on man's basic and intellectual perceptions and actions.
Noise is undesirable when it causes impairment of hearing acuity, interferes with speech
communication, causes unnecessary distraction, or warns of danger when none is present.
However, noise is desirable when it provides a relatively steady background which masks
unwanted distractive sounds, or provides speech privacy so that others do not overhear a
private conversation. Consequently, the goals for noise control must be designed such
that the desirable qualities are retained and the undesirable qualities are minimized. This
is a most difficult task, particularly with transportation noise which provides the all-
pervasive almost steady outdoor residual noise level essential for speech privacy, and
also is responsible for many of the most intrusive and undesirable noises.
To provide a clear understanding of the significance of noise from these
sources on our environment, several aspects are considered in this report:
• Nature and economic significance of the industry associated
with the source.
• Basic noise characteristics of each type of source.
• Environmental noise attributes of each type of source.
• Past and present efforts toward reducing noise.
• Estimated potential noise reduction for the future with
today's technology.
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Chapter 2 presents these findings for all types of vehicles in our trans-
portation system, including those used for recreation. Chapter 3 considers these same
aspects for many of the devices powered by internal combustion engines. This over-
view of the existing and potential noise characteristics of these sources provides the
basis for an assessment of the impact of their contribution to our total noise environ-
ment which is presented in Chapter 4. The impact is discussed from several viewpoints
for each basic source type in our transportation system, as well as for internal com-
bustion engine devices, and a projection is made of possible future impact to the
year 2000. Finally, the implications of the overall results of this study are sum-
marized in Chapter 5 and recommendations made for further action to reduce the over-
all noise impact of the noise sources considered.
Appendix A summarizes several of the more significant national standards
for noise measurement or control which are applicable to this report. It includes a copy
of pertinent sections of Federal Aviation Regulation (FAR) Part 36— Noise Standards:
Aircraft Type Certification. This regulation represents the most complete and compre-
hensive noise measurement and noise regulation standard ever developed by the Federal
Government and is playing a major role in fostering development of quieter non-military
jet aircraft.
Appendix B presents in more detail the basis for the various impact evalu-
ation models utilized in Chapter 4. Appendix C gives a detailed discussion of the
principal sources which dominate the noise generation by all of the systems or devices
considered in this report. These are the propulsion systems of aircraft and motor vehicles,
including turbojets,-turbofans, propellers, rbtors, reciprocating engines and tires.
Throughout this report, single-number noise levels are commonly specified
in terms of A-weighted noise levels in decibels, abbreviated dB(A), defined in
Appendix B. The A-weighted sound (or noise) level is the most commonly-used single-
number scale for quantifying approximately the subjective noisiness of sounds, par-
ticularly those from vehicles other than aircraft. It is also readily measured with the
use of a standard sound level meter employing the A-weighting network. Other
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single-number scales for evaluating aircraft noise are introduced as necessary. Wnere
appropriate, frequency content of the noise generated by the various sources are
specified in terms of octave or one-third octave band sound pressure levels in decibels
2
relative to 20 newtons per square meter (equivalent to 0.0002 dynes/cm ).
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2.0 TRANSPORTATION SYSTEMS
One of the most significant forces acting on the life style in the United
States is the ever-in creasing demand for improved modes of transportation. This force
is, in itself, a natural product of the pressure of increasing population and economic
growth. As the size of urban areas has increased, so has the demand for methods of
transporting people to and from their residences and places of employment. As the
interdependency between and within urban areas has increased, so has the demand for
transporting goods and services between and within our urban centers. These demands
have been met by an ever-increasing development of more efficient, larger and faster
modes of transportation. First, the steam locomotive, then the automobile, next the
propeller airplane, and most recently, the jet transport —all have acted to transform
the structure and style of our lives by providing a wide range of transportation methods.
The transportation industry represents, in total, approximately 14.5 per-
cent of the gross national product in 1970 and employed approximately 13.3 percent
of the total labor force. This major section of the nation's economy is defined, for
this report, as the sum total of the:
• Commercial aircraft and airline industry
• General aviation industry
• Highway vehicle industry
• Recreational vehicle industry
• Railroad and urban mass transit industry
• Commercial shipping industry
The economic structure of this industry and the general division and magni-
tude of the transportation services provided are illustrated in Figure 2-1. ~ The rapid
growth of several segments of the transportation system since 1950 is summarized in
Table 2-1. While there are many important sources of noise which intrude on our
everyday lives, noise from all types of transportation vehicles tends to dominate most
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Transportotion Service and Equipment Product
($145 Billion in 1970)
Transportation Service and Equipment
Capital Investment
($114 Billion in 1970)
Transportation Service and Equipment Employment
(11.3 Million in 1970)
Freight Traffic
0.81 Air Freight
Waterborne ~ Short Tons
1.4
(Passenger and Freight)
Rail Freight
Trucking
i i i i i
I I I I I I M
10
Ton-Mi les~ Bill ions
100
408
765
1000
Buses
Air Carriers
Passenger Traffic
Passenger Trains 11
27
130
Passenger Vehicle Miles (Estimate)
i i i I i i i i I
i i I i i 111
10 100
Passenger Miles~ Billions
1000
1000
Figure 2-1. General Characteristics of the Transportatiori"Industry in 1970
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Table 2-1
Growth in the Transportation System, 1950-1970
Source
Population (in millions)
Passenger Cars (in millions)
Trucks and Buses (in millions)
Motorcycles (in millions)
(Highway)
Motorcycles (in millions)
(Off-road)
Snowmobiles (in millions)
2-3 Engine Turbofan Aircraft
4- Engine Turbofan Aircraft
General Aviation Aircraft
Helicopters
1950
151
40.4
8.8
0.45
-
0
0
0
45,000
85
1960 N .
181
61.7
12.2
0.51
-•
0.002
0
202
76,200
634
1970
204
87.0
19.3
1.2
1.8
1.6
1174
815
136,000
2800
residential areas. In fact, the cumulative effect of the increase in noise intrusion by
transportation vehicles is, to a large extent, responsible for the current concern with
noise pollution. This section briefly describes the general nature of transportation
system noise sources and considers their overall impact in the United States today.
Aircraft, one of the more dominant sources of noise in the transportation industry,
will be considered, first.
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2.1 Commercial Aircraft
2.1.1 Introduction
There has been a significant increase in air travel during the last decade
which is closely related to the introduction and growth of the commercial jet aircraft
fleet. Since 1958, when the first commercial jet aircraft started operating, passenger
air travel has grown at an average annual rate of 13.2 percent, to a total of 132
billion passenger miles in 1970. In 1970, 170 million passengers were flown by the
airlines, producing an operating revenue of $7.6 billion. In addition, 5 billion ton-
miles of air freight were transported for a revenue of $715 million. The scheduled air-
lines employed 300,000 people. The aerospace and related manufacturing industries
employed 765,000 people and had a total of $8.6 billion in commercial aircraft
, 1,2
sales.
i ,
The advantages of jet-powered passenger airplanes — greater speed and
reduced operating cost per passenger-mile —have led to a gradual phasing out of the
older propeller-driven commercial aircraft. Only a small percentage of piston-
powered aircraft now remains in the fleet, and the turboprop aircraft in use are
' i
primarily short range twin-engine types used on light traffic routes.
The original commercial jet aircraft were powered by turbojet engines.
These engines have been largely replaced by quieter and more powerful turbofan
i *
engines. There are two basic types of jet aircraft in the current commercial fleet.
The first type is the 4-engine turbofan aircraft such as the Boeing 707 and 720 and
the McDonnell-Douglas DC-8. These aircraft are used primarily on medium and long
range flights and are almost exclusively powered by first-generation turbofan engines.
The second basic aircraft type is exemplified by the Boeing 727 and 737 and the
McDonnell-Douglas DC-9. These aircraft are powered by two or three more advanced
f
and quieter turbofan engines and are used on short and medium range flights.
Two new types of commercial jet aircraft have recently been introduced
i ,
in the fleet. These are powered by advanced technology turbofan engines that are
much more powerful and quieter than engines used in the previously mentioned aircraft
8
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types. The 4-engine 747 widebody jet, introduced in 1969, is intended for long
range transcontinental and intercontinental flights. The 3-engine widebody aircraft,
DC-10 and L-1011, will be used on high density, medium length flight routes.
Figure 2.1-1 summarizes the category of commercial aircraft in terms of
2-13
type, application, passenger capacity and noise.
\ i
2.1.2 Source Noise Characteristics
' *
The noise associated with jet aircraft is primarily generated by the jet
engines. Noise is an operational by-product of these power plants. The primary pur-
pose of a jet engine is to produce the thrust necessary to push the aircraft through the
air. A jet engine produces thrust by taking in air through the inlet, raising the air
temperature and pressure inside the engine, and then expelling it to the rear with a
high velocity from the jet nozzle. Noise is produced by several of the processes that
•
take place both within and outside the engine. By far the dominant source of noise .
from the early turbojet engines was the broadband jet noise generated in the exhaust
wake. Jet noise is caused by the turbulent mixing that occurs along the boundary
between the high velocity exhaust jet and the ambient air. The sound power generated
increases very rapidly with increasing jet velocity, hence the high noise levels are
associated with the high velocity exhausts of turbojet engines.
The turbofan engines that have replaced the turbojets offer substantial jet
exhaust noise benefits because they take in larger quentities of air and expel this air
at lower jet velocities. This change has been accomplished by the use of a fan section
• \
in the engine that takes in air, raises its pressure, and expels it through a separate
nozzle, thus bypassing the burner and turbine sections of the engine with part of the
total airflow. However, with reduced levels of jet noise and with a noise radiation
path rearward out the fan duct and forward out the inlet, fan whine was elevated from
a secondary noise source to one of dominant importance, particularly at approach powers.
Figure 2.1-2 shows typical noise levels and spectra measured during takeoff
and approach operations for 4-engined aircraft with low bypass engines.^ The engine
t
thrust, and thus the jet exhaust velocity, is higher during takeoff than during approach
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1
2-3 Engine
Turbofan
Commercial Aircraft
1.
4 Engine
Turbofan
4 Engine
Widebody
Turbofan
1 1
3 Engine
Widebody
Turbofan
Propeller
Aircraft
» Short to
Medium Range
» B727, B737
» DC -9, BAC-11
• Medium to
Long; Range
e B707, B720
1 • DC -8
• Long Range
• B747
• Medium Range
• DC-10,
L- 1.0 11
• Short Range
• F-27
• CV340/440
• DC-3
Average Passenger Capacity
100
150
365 1
250
40
Typical Range - Miles
250-1500
1000-4000
1000-2500
Growth of Aircraft Fleet
1960 65 70 I960 65 70 1960 65 70
Typical Noise Levels
I960 65 70
IOUU
1
|l200
<
14-
O
_g 600
E
0
1
209
r
1
174
202
712
815
79
1 — 1
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Figure 2.1-1. Characteristics of Commercial Aircraft
-------
Jet/Turbine
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Total
Contributing Subsources for Typical Aircraft
Figure 2.1-2. Noise Limits and Spectra of 4-Engine
Low Bypass Ratio Turbofan Aircraft
11
-------
and consequently the low frequency jet noise is significantly higher at takeoff than at
approach. However, the high frequency fan noise is relatively insensitive to engine
power setting and thus becomes clearly dominant at approach engine conditions.
Typical noise levels and spectra for the 2- to 3-engine turbofan aircraft,
powered by later model turbofan engines, are shown in Figure 2.1-3. The noise pro-
duced by these aircraft is lower than that shown in Figure 2.1--2. The jet noise is
lower because of slightly reduced jet velocities, and the high frequency fan noise is
' i
considerably reduced due to fundamental improvements in fan design.
'
The 4-engine turbofan widebody aircraft, which are powered by new
technology engines, incorporate several advancements both with respect to propulsion
( t
efficiency and reduced noise generation. These engines pass a high percentage of the
total airflow through the fan section, and are therefore considered high bypass ratio
t . -i '
turbofan'engines in comparison with the earlier low bypass ratio engines. The low jet
exhaust velocity made possible with these new engines has resulted in a significant
reduction in jet noise. This reduction is clearly shown by comparing the noise levels
I /- o'
and spectra presented in Figure 2.1^-4 with those of Figure 2.1-3. The fan noise
dominates both during takeoff and approach operations. Despite the considerable
technological advances that were incorporated in the fan design, the discrete frequency
fan whine forms the major obstacle to achieving significant noise reduction.
The new 3-engine turbofan wTdebody aircraft uses similar engines, but
o I
with additional improvements in fan noise reduction. These improvements will be dis-
cussed in Section 2.1.4 and furfher information on the mechanisms of jet engine noise
generation may be found in Appendix C . i ,
The noise generated by commercial propeller aircraft is dominated by pro-
peller noise. Typical noise spectra and levels for various types of commercial propeller
aircraft are compared with the noise of the original turbojet aircraft in Figures 2.1-5
and 2.1-6. The increase in aircraft noise which occurred with the introduction of
the jets is evident. Because propeller aircraft constitute such a small percentage of
commercial aviation aircraft, especially so with respect to their relative noise impact,
,12
-------
Jet/turbine
Fan
Compressor
no
z
a
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CN
fi'
OQ
-
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-------
Fan/Compressor
Jet/Turbine
CN
z
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O
-------
-------
CN
100
90
I
o
CN
CO
-o
-------
the detailed discussion of propeller noise will be deferred to the General Aviation
Category for which it forms the dominant characteristic noise.
The noise level in the interior of jet aircraft is dominated by a different
noise source. Because these aircraft travel at high speeds, the pressure fluctuations
generated by the turbulent mixing that occurs in the boundary layer between the air-
craft fuselage and the surrounding air become significant. These fluctuations cause
the fuselage walls to vibrate and radiate noise into the aircraft interior. The "boundary
layer" noise dominates at most interior locations except at the aft end of the aircraft,
at which low frequency jet noise impinging on the fuselage and transmitted through to
the interior may becomes the dominant noise source.'0-13
Sonic Boom
'Supersonic aircraft introduce a new element into the aircraft noise problem.
Whereas the noise from subsonic aircraft is primarily a phenomenon associated with the
airport environment, the sonic boom generated by aircraft flying at supersonic speeds
creates a ground impact underneath its entire flight path. Although supersonic flights
by military aircraft over populated and areas within the United States have been pro-
hibited, supersonic military aircraft continue to produce an estimated 6000 sonic booms
annually over sparsely populated areas.'^
When an airplane flies at supersonic speed, it compresses the surrounding
air, pushing a shock wave, much like a boat creates a spreading bow wave. This bow
wave, or cone of increased air pressure, spreads out behind the airplane. Correspond-
ing waves are generated at locations of airflow discontinuities along the length of the
airplane. At great distances, the separate waves or shocks interact with each other
and coalesce into two waves, a bow shock and a tail shock. In this form the pressure
signature is called an N wave. Figure 2.1-7 shows that as the distance from the
airplane is increased, the distance between the bow and tail wave is also increased.'"
The intensity of the sonic boom depends on such factors as speed, altitude, weighted
shape of the airplane, atmospheric conditions, and type of terrain over which the air-
craft is passing.
17
-------
/ —
Atmospheric Pressure
Figure 2.1-7. Nature of the Sonic Boom Phenomenon
18
-------
Community impact studies conducted in anticipation of the United States
supersonic transport aircraft have suggested that the sonic booms generated by a fleet
of this aircraft would produce a clearly unacceptable noise impact on populated areas.
For example, sonic booms generated by the military B-58 aircraft, at a strength of 1.7
pounds per square foot nominal peak overpressure, were judged by residents of a sub-
urban community to be equal in acceptability to the noise from a subsonic jet at about
107 dB(A), which is clearly an unacceptable value. This result, together with the
vigorous complaints, political and legal actions encountered in other sonic boom over-
flights, has led to an administrative decision at the Federal level to prohibit supersonic
military and commercial flights over populated areas. This prohibition in the United
States, and similar prohibitions in other countries, are expected to continue until new
technology developments result in supersonic aircraft concepts that produce acceptably
low sonic boom levels.
2.1.3 Environmental Noise Characteristics
The noise generated by commercial aircraft results in two types of noise
environments that differ in terms of the noise levels and duration of exposure, as well
as in the aircraft operations that generate the noise impact. The participant, or pas-
senger, is exposed to moderately high noise levels throughout the entire history of air-
craft operations from the time of boarding the aircraft, takeoff, cruise to the flight
destination, and landing. Figure 2.1-8 gives time histories of typical cabin noise
levels for the flight duration. If the aircraft makes intermediate stops, the passenger
may be subject to this set of operations several times during a single flight.
Commercial jet aircraft are designed to maintain interior noise levels during
cruise operations which enable passengers to converse at normal voice with good speech
intelligibility. As is shown in Figure 2.1-9, the cruise interior noise levels range
typically from 79 to 88 dB(A), depending on the interior location, with a characteristic
O i o
value of 82 dB(A). During takeoff and landing operations, the noise levels in
aircraft with wing mounted engines are up to 12 dB higher, but only for periods of up
to 1 minute during each operation. The statistical characteristics of the passenger
i o
environment, summarized in Table 2.1-1, refer to 1970 figures.'/z
19
-------
100
90
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g 80
.3
0>
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'5
Z
70
60
Liftoff
Boeing 720B - Seat 22F
McDonnell-Douglas DC-8 - Seat 64A
— Boeing 727 - Seat 19A
Landing
Preparation
Thrust Reversers
Engine
Power
Cutback
Climbout
Cruise
Descent
Cutback
J'j
0 5 10 15 20
Time From Brake Release - Minutes
fi
II
20 15 10 5 0
Time To Touchdown - Minutes
Figure 2.1-8.. Time Histories of Typical Cabin Noise Levels
-------
Jet/Turbine
Fan
Compressor
110
100
90
—Jet Noise-
CM
o
CN
| 80
a>
o
eo
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t>
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70
60
50
Boundary Layer Noise
Typical Spectrum
100
1000
Frequency in Hertz
10000
100
S" 80
TJ
I 60
5
» 40
•- 20
o
-^
z o
—
—
— •
85
>9
Aerodynamic
Noise "
(Fuselage)
<
» .
ft
65
Jet
89
79
Tot
Contributing Subsources
Figure 2.1-9. Interior Cabin Noise Levels and spectra
for Commercial Jet Aircraft
21
-------
Table 2.1-1
Passenger Environment-
Number of Passengers/Day — 465,000
Characteristic Cruise Noise Level — 82 dB(A)
Average Duration of Exposure — 1.4 hours
Characteristic Takeoff and Landing Noise Level — <95 dB(A)
Approximate Duration of Exposure per Operation — ^ 1 minute
With respect to the nonparticipant environment, the noise impact from
commercial air operations is experienced in the vicinity of the airports, and to a
lesser extent further from the airport under the climbout and approach paths. For-
tunately, during cruise operations, current commercial aircraft fly at too high an
altitude to generate a significant noise impact on the ground. However, takeoff and
landing operations generate very high noise levels on the ground that"extend over
large areas, and where the airport is close to a city, large numbers of people may
live within the noise impacted areas.
The growth of the noise impact due to commercial aircraft operations is
very closely related to the introduction of the commercial jet aircraft in 1958 and the
manner of growth of air travel during the following decade. First, as illustrated in
Figures 2.1-5 and 2.1-6, the jet aircraft were approximately 12 to 20 dB noisier on
approach and takeoff than piston-engined aircraft which they replaced.'^ Secondly,
although the number of major airports has increased only slightly since the late 1950's,
the number of commercial aircraft in the fleet has grown many times' over. Finally,
vast new residential communities have been established in the vicinity of nearly all
busy airports. This combination of expanding air travel and'residential growth has
resulted in a serious airport-community noise problem.
In order to assess the impact of aircraft noise on the community, the
Noise Exposure Forecast (NEF) method has been widely used. This method gives a
single number rating of the cumulative noise produced in the vicinity of an airport by
22
-------
aircraft operations, taking into account the total mix of aircraft utilizing the airport,
subje'ctive noise levels generated by each aircraft class, flight paths, number of opera-
tions in day and night periods, et cetera. Figures 2.1-10 and 2.1-11 show an example
of NEF values versus slant range, for takeoff and landing operations, respectively, for
the various types and numbers of commercial aircraft that are expected to utilize a
typical large airport.'/2 It is readily apparent in this example that the 4-engine turbo-
fan aircraft powered by the first-generation low bypass ratio turbofan engines (B707,
B720, DC-8) give the maximum NEF values, primarily because they have the highest
noise levels together with having about 30 percent of the total operations. On the
other hand, the low NEF values of the Boeing 747, shown in this example, primarily
reflect its relatively small percentage of total operations. The NEF 30 contours result-
7 8
ing from this example are shown in Figure 2.1-12. ' For simplicity, the aircraft are
assumed to operate in the same direction on a single runway, and the contour combines
the effects of both takeoffs and landings. Operations by the 4-engine low bypass ratio
turbofan aircraft (Boeing 707 and 720, McDonnell-Douglas DC-8) contribute 69 percent
of the total impact area, despite comprising only 30 percent of the total number of
operations.
Current Federal guidelines for planning recommend that new residential
construction should not be undertaken in areas around airports exposed to values of the
NEF rating of 30 and higher.'" In addition, they state that individuals in existing
private residences may complain about noise, perhaps vigorously, when the NEF is
between 30 and 40. When the NEF exceeds 40, residential use is considered incom-
« •''.-' "
patible with the noise. The community reaction scale ° essentially agrees with this
expected complaint level .when the outdoor residual noise level in the community may
be classified as urban residential, a condition which is generally met in the vicinity
of our major airports. However, if the outdoor residual noise level in the community
has a lower value, such as would be expected for quiet or normal suburban residential,
l ft
it is suggested that the NEF values for equivalent reaction must be lowered accordingly.
However, for simplicity in this report, a constant value of NEF 30 will be used for the
23
-------
80
70 -
B707, B720, DC-8
DC-8 (Stretched)
B727-100, B737, DC-9
B727-200
B747 (Jumbo)
Propeller Aircraft
Total Impact
60
Equivalent Large Airport (1970)
50
CO
TJ
ID
Commercial
Aircraft Type
6727^100, B737, DC-9
B727-200
B707,B720,DC-8
DC-8 (Stretched)
B747 (Jumbo)
Propeller Aircraft
No. of
Ope r./Day
40
30
20
10
TAKEOFF
* Total operations, 86.3 percent of which
occur during daytime (0700-2200 hours) and
13.7 percent of which occur during night time
(2200-0700 hours).
200
400
4,000 7,000 10,000
700 1,000 2,000
Slant Distance to Aircraft - Feet
Figure 2.1-10. Noise Exposure Forecast vs Slant Range (Takeoff)
for an Equivalent Large Airport
24
-------
B707, B720, DC-8
70
DC-8 (Stretched)
B727-100, B737, DC-9
B727-200
B747 (Jumbo)
—- — Propeller Aircraft
— Total Impact
Equivalent Large Airport (1970)
Commercial
Aircraft Type
No. of
Operations/Day
50
B727-100, B737, DC-9
B727-200
B707, B720, DC-8
DC-8 (Stretched)
B747 (Jumbo)
Propeller Aircraft
X
X
X
30
20
10
0
APPROACH
Total operations, 86.3 percent of
which occur during daytime (0700-
2200 hours) and 13.7 percent of which
occur during night time (2200-0700 hours).
700 1,000 , 2,000
Slant Distance to Aircraft — Feet
4,000 7,000 10,000
Figure 2.1-11. Noise Exposure Forecast vs Slant Range (Approach)
for an Equivalent Large Airport
25
-------
20 _
o
10
0)
1§
Ǥ-
o
c
Q
l/>
Q
10
20
Representative Large Airport (1970)
Commercial
Aircraft Type
B727-100, B737, DC-9
B727-200
B707, B720, DC-8
DC-8 (Stretched)
B747 (Jumbo)
Propeller Aircraft
Landing
Number of
Operations/Day
140
40
108
18
6
54
366*
Total operations, 86.3$of which occur during
daytime (0700-2200 hours) and 13.7$ of which
occur during nighttime (2200-0700 hours).
Total No. of Aircraft
4 Engine Low Bypass Turbofan
All Other Aircraft
NEF 30 Contours
Takeoff
50 40 30 20
Distance to Threshold
1000 ft
10
0
1-0 20 30 40
Distance from Brake Release
1000ft
50
Figure 2.1-12. NEF 30 Contours for Representative (Single Runway) Airport
-------
purpose of discussing the noise impact- from aircraft operations. The use of this value
to define the boundary of the noise impact zone is conservative, but it should not
impair any qualitative conclusions, since the majority of the currently impacted area
is in the residential urban ambient noise level category.
Within the United States, the total area within which NEF 30 is exceeded
has grown from approximately 100 square miles in 1958 to approximately 1450
•JO on
square miles in 1970. ' These areas are estimated to contain respective populations
of approximately 500 thousand and 7.5 million people. A considerably larger number
of people are undoubtedly annoyed by aircraft noise, because of the conservatism
indicated above, and because over 30 percent of the population exposed to NEF 30
are expected to be very much annoyed with the noise, and approximately 20 percent
are very much annoyed when exposed to NEF 20.
2.1.4 Industry Efforts In Noise Reduction
The commercial jet airplane and jet engine manufacturers have generally
been involved with the military as well as the civilian aircraft marker. In fact, the
jet engines that were responsible for ushering in the new era in commercial air trans-
portation were originally developed for military purposes, and the first commercial jet
aircraft were based on technology fall-out from the development of large military jet
aircraft.
Noise impact has never been a major design constraint in the majority of
military applications of jet-powered aircraft. It is not surprising, then, that military
jet engines have been, and still are, extremely noisy. The civilian derivatives of
these engines have thus had their basic characteristics designed without any noise
criteria. Both the airframe and engine manufacturers have been aware of the potential
community noise problems due to the excessive noisiness of jet aircraft, and have
carried on research and development work on jet engine noise reduction since well
before the introduction of the first commercial jet airplanes. Unfortunately, the rapid
development of the commercial jet fleet market demanded technological advances in
jet engine performance and noise acceptability faster than the embryonic jet engine
27
-------
noise technology was able to accommodate. The first turbojet engines were made
modera'rely quieter by means of jet noise suppressors mounted on the engine tailpipes,
but still generated unacceptably high noise levels. The introduction of the low bypass
ratio turbofan engines was anticipated to reduce the jet noise problem. However, the
appearance of fan noise as a dominant noise source negated some of the expected
benefits.
The high bypass ratio turbofan engine represented the first commercial jet
engine for which engine noise technology was sufficiently well developed to measurably
influence the basic design. Although these engines did not rely on noise considerations
as the primary basic design input, they did include the most advanced practical con-
cepts of low noise generation. As will be discussed below, later models of these
turbofan engines have incorporated still more noise-reduction features.
Figure 2.1-13 shows the present and a projected composition of the United
~ ' • 71
States commercial jet aircraft fleet/ The low bypass ratio turbofan aircraft form the
great majority of the fleet and will continue to be dominant until 1985. Hence, the
introduction of the quieter high bypass ratio turbofan aircraft will not automatically
.result in a reduction of the community noise problem except on a long-term basis. This
becomes even more apparent on examining the projected growth in commercial aircraft
airport operations, presented in Figure 2.1-14. This figure has been prepared on the
assumption of a 5 percent annual increase in the number of passenger emplanements
and a corresponding annual increase of 3 percent in the number of aircraft operations.
The increased number of operations is sufficient to offset the potential benefits of the
quieter aircraft unless steps are taken to reduce the noise generation by the older
turbofan aircraft.
The commercial jet aircraft industry has been strongly committed to the
reduction of jet engine noise, especially so during the last 7 years, and has carried
out extensive research and development programs both at industry expense and with
the assistance of Federal funding. These efforts have been aimed both at the develop-
ment of advanced noise technology for use in the design of future jet engines, and the
28
-------
2500
2000
E
u
1500
2-3 Engine Low Bypass Turbofan
4-Engine Low Bypass Turbofan
— 4-Engine Wide Body (747) High Bypass Turbofan
— 3-Engine Wide Body High Bypass Turbofan
New Technology CTOL or STOL (125- 150 Passenger)
E
E
o
U
1000
(D
_Q
E
Z
500
0
1970
1975
1980
1985
Year
Figure 2.1-13. Projected Change in Commercial Aircraft Fleet Composition
(Based on Projected Passenger Capacity Demand
Increasing at 5 Percent/Year)
29
-------
700
^
8
u 600
0)
Q.
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C
.2 500
ii
1 400
l_
Annual Increase in Number
Operations of Aircraft Operations
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1975
1980
1985
2000
Figure 2.1-14. Projected Growth of Commercial Aircraft Traffic
30
-------
development of practical concepts and hardware to permit retrofitting of present jet
engines. Although these programs have yet not been singularly successful in reducing
the noise impact, encouraging progress is being made. The adoption of Federal regu-
lations governing the permissible noise impact by new airplanes and their anticipated
extension to coverall commercial aircraft will hopefully spur the implementation of
the technology developments in the aircraft fleet. These regulations will be discussed
in a separate section below.
The anticipated development of large (125 to 150 passenger) STOL com-
mercial aircraft during the next decade will create new demands on the industry's
noise abatement technology. These aircraft will operate out of short field length gen-
eral aviation and new urban airports as well as the large commercial airports, and
must be able to meet stringent noise level standards in order not to impose pollution-
level noise impacts at their operation centers. The concept and technology develop-
ments planned for these future air transports will be discussed in a later section.
Federal Government Regulations of Aircraft Noise
After receiving authority from Congress, the FAA initiated a lengthy and
far-reaching rule-making process which culminated in Federal Aviation Regulation,
Part 36 — Noise Standards: Aircraft Type Certification, published in the Federal
Register of 21 November 1969. The noise limits of this regulation apply only to
subsonic jet aircraft in the following categories:
• Airplanes that have turbofan engines with bypass ratios of 2 or more
(i.e., new technology high bypass engines used by the new wide-
bodied transport aircraft) and for which application for certification
was or is made on or after January 1, 1967.
*
The technical requirements of FAR-36 are reproduced in Appendix A.
31
-------
• For new airplanes that have turbofan engines with bypass ratios of 2
or more, which do not meet FAR-36 noise levels and where applica-
tion for certification was made prior to January 1, 1967, the FAA
will place a time period in the type certificate. At the expiration
of this time period, the type certification will be subject to suspension
unless the type design of aircraft produced under that type certificate
after the end of this time period is modified to comply with the noise
I* • «.
imits.
• . Airplanes that do not have turbofan engines with bypass ratios of 2 or
more (i.e., pure jets or low bypass turbofans as found on most current
aircraft) and for which application for certification was made after
December 1, 1969.
FAR-36 defines noise limits such aircraft must meet at certain locations
with respect to the airport runway, shown in Figure 2.1-15.
Three measurement locations are required in certification. They are:
• Landing — 1 nautical mile from threshold, directly under the aircraft
path,
• Takeoff— 3.5 nautical miles from brake, release, directly under the
aircraft path, and
• Sideline —at the location of maximum noise along a line parallel to
•and at a distance of 0.35 nautical miles from the runway centerline,
for aircraft which have four or more engines; and 0.25 nautical miles
from the runway centerline, for aircraft which have three or fewer
engines.
Additional restrictions are imposed to insure that aircraft become pro-
gressively quieter at flight positions further from the airport.
The noise limits at the three measurement positions are given in terms of
the aircraft's maximum certificated gross weight. The permissible variation with gross
32
-------
CO
CO
Measurement
Point
Assumed position of brake
release for takeoff and
threshold for landing
. Sideline Measurement
Takeoff
Measurement
Point
Figure 2.1-15. FAR-36 Noise Certification Measurement Positions
-------
weight gives implicit recognition to the fact that for a given technology in engine
design, the absolute noise from an airplane must increase with required thrust — which
in turn must increase with gross weight. For many airline flights, the aircraft operate
at less than maximum gross weight, and hence, less noise.
The Effect of FAR-36 on the Noise of Future Aircraft
Most of the turbofan aircraft which .constitute the bulk of the present jet
aircraft fleet exceed the FAR-36 noise limits. Figures 2.1-16 to 2.1-18 make these
comparisons for the landing, takeoff and sideline noise measurement points, respectively.
It is obvious that the noise levels of most current aircraft are significantly higher than
the noise limits of FAR-36, particularly for takeoff and landing.
The comparisons show the amount of noise reduction that will be accom-
plished by designing and producing future aircraft which meet the certification
requirements. Effective perceived noise levels of future aircraft will be reduced by
as much as 14 EPNdB for takeoff and landing, and 5 EPNdB along the sideline.
Noise Reduction Progress
In the previous section, it was noted that the research efforts by the
industry have been directed towards both the development of advanced technology
quiet engines and the development of retrofit concepts for current engines. At this
time, both efforts have yielded results that are in evidence in new aircraft in the
current aircraft fleet. Figure 2.1-19 shows the noise levels generated by the older
turbojet and low bypass ratio turbofan engines compared with the new advanced tech-
nology high bypass ratio turbofans. ' ' It is noted that the second generation turbofan
engines of the older type are up to 8 EPNdB quieter than the first types on the basis of
equal thrust. The JT9D high bypass ratio engine is also quieter, despite producing
250 percent more thrust. The newest engine shown/ the CF6, generates noise levels
up to 16 EPNdB less per unit thrust than the first turbofan engines. This engine
represents a significant advancement in the application of noise reduction technology,
and will be discussed in more detail.
34
-------
125
CD
-a
Z
a.
'5
0)
1
The "%" symbols indicate
levels produced by
existing aircraft
120
115
105
^ftf-
100
95
JL
J_
75,000 150,000 300,000 600,000
Takeoff Gross Weight in Pounds
Figure 2.1-16. Effective Perceived Noise Level for Landing for Today's Aircraft
-------
The "0" symbols indicate
levels produced by
existing aircraft
115
CO
•D
Q-
I
>
110
9)
.—
'5
Z
TJ
0)
'5
105
tj
£
LLJ
100
95
90
150,000
300,000
600,000
Takeoff Gross Weight in Pounds
Figure 2.1-17, Effective Perceived Noise Level for Takeoff for Today's Aircraft
Compared to FAR-36 Limits
-------
120
The "%" symbols indicate
levels produced by
existing aircraft
115
CO
-o
Q_
110
CO
VI
0)
0)
trt
'o
Z
(D
'55
105
.36 \jif
0)
•»-
u
4)
100
95
90
I
I
I
I
75,000
150,000 300,000
Takeoff Gross Weight in Pounds
600,000
Figure 2.1-18. Effective Perceived Noise Level at Sideline for Today's Aircraft-
Compared to FAR-36 Limits
-------
70
Q_
60
_c
I—
o
O)
_0
o
I
Z 50
Q_
40
Turbojet
'JT3C
(13,500 Ib)
Low Bypass
Turbo fan
High Bypass
Turbofan
JT3D
(18,000 Ib)
JT8D
(13,500 Ib)
*9D
(45,000 I
%
\
CF6
(40,000 Ib)
• Takeoff Condition
o Approach Condition
1000 ft from Aircraft
\ -
1960
1965
1970
Year
Figure 2.1-19. Trends in Jet Engine Noise Generation .
38
-------
Three basic features of the CF6 engine are dominantly responsible for its
low noise characteristics. The first is the selection of high bypass ratio in order to
reduce the jet exhaust velocity and hence greatly reduce the jet exhaust noise. The
second is the advanced technology design of the fan section to minimize the genera-
tion of discrete frequency turbomachinery noise. The third, and perhaps the most
significant noise reduction feature, is the use of long inlet and fan discharge ducts
that are lined with sound-absorptive treatment in order to reduce the transmission of
turbomachinery noise out from the interior of the engine. This combination of features
has resulted in noise levels that make the DG-10-10 aircraft, powered by the CF6
engine, much quieter than current aircraft as shown in Table 2.1-2 below:
Table 2.1-2
Maximum Perceived Noise Levels of the DC-10-10 Relative to
Those of Current 4-Engine Jet Transport^
• Current
s Relative
Takeoff
Full Thrust
75 Percent Thrust
Approach
Typical Thrust
Jet Transports Powered by Four JT3D-3B
Levels in PNo'B
1000 Feet
Outdoors
-11.5
-13.5
400 Feet
Outdoors
-10
Engines
3500 Feet
Indoors
-15
-13
1500 Feet
Indoors
-11
39
-------
Figure 2.1-20 shows the noise spectrum of the DC-10-10 compared with
that of a current 4-engine turbofan aircraft. It is apparent that both the jet exhaust
noise and the high frequency turbomachinery noise have been significantly reduced.
NASA has funded several research and development programs aimed at
developing technology for the retrofit of current turbofan engines. The NASA
Acoustically Treated Nacelle Program attempted to reduce the fan noise radiation
from the inlet and discharge ducts of 4-engine low bypass ratio turbofan aircraft by
i " OO
treating the nacelle with sound absorbing lining. Independent studies were carried
out by both Boeing and McDonnell-Douglas on B707-320B and DC-8-55 aircraft.
These programs achieved a significant reduction in approach noise, but only a slight
reduction in takeoff and sideline noise. However, the weight and cost penalties
involved are too severe to be readily accepted by the aircraft operators. The main
results of the programs a re summarized below in Table 2.1-3.
Table 2.1-3
i
NASA Acoustically Treated Nacelle Program^!
Variable
Boeing
McDonnell-Douglas
Reduction' of Approach Path
Noise (3° approach at 1 n.mi.)
Range Effect
Weight Penalty
Cost of Retrofit per Aircraft
(300 to 400 aircraft)
Increase in Direct Operating
Costs
15.5 EPNdB
200 n.mi. loss
3140 pounds
$1,000,000 ,
9.6 percent
10.5 EPNdB
150 n.mi. gain
332 pounds
$655,000
4.2 percent
40
-------
CM
^ 110
O
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% 100
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Figure 2.1-20. Flyover Noise Levels - DC-10-10 Compared to
Current Jet Transports Powered by (4) JT3D-3B Engines' '
41
-------
Another NASA-funded program is aimed at demonstrating the capability
of advanced fan design technology and nacelle acoustic treatment to guide the design
of a new high bypass ratio turbofan engine with takeoff and approach noise levels
significantly lower than have been achieved to date. Carried out by General Electric
and Boeing, this Quiet Engine Program is due to be completed during 1973.23 Integra-
tion studies conducted by McDonnell-Douglas show that substitution of an engine with
the design parameters of the Quiet Engine for the old turbofan engines on DC-8 and
B707 aircraft would result in improved performance as well as dramatically reduced
f\ A
noise levels. However, the high cost of engine replacement, and the fact that only
experimental component hardware will come out of the program, throws doubt on the
prospects of its immediate implementation. Rather, the Quiet Engine Program should .
be viewed as a development of new technology which can be applied in design of new
engines for future aircraft. The expected results of the Quiet Engine Program are
summarized below in Table 2.1-4, with the CF6 engine included for comparison:
Table 2.1-4
NASA Quiet Engine Program
Noise Level Goals Compared with B707/DC-822
Flight Condition
Takeoff
Approach
Noise Reduction - EPNdB at
FAA Measurement Positions
Bare
Quiet Engine
13
17.5
Acoustically Treated
Quiet Engine
23
25.5
CF6 Engine
(DC- 10)
18
11.5
42
-------
An alternative approach to noise reduction for the current fleet of aircraft
is that of altering flight procedures. At some airports, the concept of reduced thrust
takeoff has been adopted. This procedure consists of a full thrust takeoff and initial
climb, after which the aircraft climbs over heavily populated areas at a reduced
thrust for some distance before resuming a normal climb. By this method, maximum
noise reductions at the FAA takeoff measurement position of 6 to 10 EPNdB may be
expected for 2- to 3-engine low bypass ratio turbofan aircraft and 3 to 6 EPNdB for
oc
4-engine low bypass ratio turbofan aircraft. For new aircraft incorporating the CF6
•engine technology, thrust reduction does not appreciably change the noise levels/*
Additional fan noise suppression will be necessary to realize the potential of this
operational procedure for these advanced technology engines.
In order to reduce the noise impact during approach, a "two-segment"
landing procedure has been proposed. This consists of an initial approach glide slope
of 6 degrees down to a yet unspecified distance from the end of the runway, at which
the standard 3 degree approach is resumed. In analytical studies carried out by
NASA, reduction of 10 PNdB or more was achieved at 1.5 nautical miles from the
")f\ 97
runway threshold for profiles with an intercept altitude of 400 feet. °' Figure 2.1-21
illustrates this procedure fora current 4-engine turbofan aircraft and shows the effect of
OQ
retrofit with the NASA Acoustically Treated Nacelle concept/0 However, it must be
realized that feasibility of the steep approach in terms of airplane operational safety
has not been verified. This factor must be thoroughly evaluated and assessed before a
decision on the adoption of this landing procedure can be made.
Plans for Future Suppression of Noise
The commercial jet transport industry, together with several Federal
agencies, is expected to continue and in some areas intensify its research and develop-
ment programs aimed at achieving quieter air transportation systems. These programs
include the development of practical and economical retrofit hardware, research into
quiet engine technology beyond the scope of the Quiet Engine Program, and the
development of STOL transportation concepts.
43
-------
Approach FJight Paths
o
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0
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C O
O O
0^1-
Extend Gear
?> increaseo _-~.>,,.« N_^^I
^ Glide Slope , | p Select 25° Flaps
3000 ft
V
-Select Landing Flaps >~ Vecfor Altitudes
T 1500ft
Normal Glide Slope
4 6 8 10
Distance from Threshold, n. mi.
12' ' 14
16
-------
On the basis of the technology developments resulting from the NASA
Acoustically Lined Nacelle Program, FAA is funding a development program to design
and manufacture noise reduction packages suitable for retrofitting the current low
29
bypass ratio turbofan engines. The possible future implementation of these retrofit
packages in the aircraft fleet will insure compliance with the FAA noise regulations.
NASA is funding several preliminary studies to determine the feasibility of
a future advanced technology transport aircraft. Three separate noise objectives are
being considered: the current FAA noise regulations (FAR-36), FAR-36 minus 10 PNdB,
and FAR-36 minus 20 PNdB. NASA anticipates a 6 to 10 year development program
o/\
for this aircraft, starting in the middle 1970's. The future development of short-
range V/STOL transportation systems is discussed fully in the V/STOL section of this
report. However, the potential large jet-powered STOL aircraft falls logically within
the scope of commercial jet transportation. NASA is currently funding preliminary
development of STOL jet propulsion systems, and has proposed the development of a
i • •
150 passenger STOL airplane, with the concurrent development of a quiet STOL jet
engine. These developments include a primary!emphasis on noise reduction, with the
planned requirement of a maximum noise level of 95 PNdB at'a distance of 500 feet
from the aircraft. The Sryear prototype STOL program is currently scheduled for
completion at the end of 1975.30'31
Table 2.1-5 summarizes some of the major Federally-funded technology
development programs that are exclusively oriented toward jet engine noise reduction
or include noise reductions as a primary requirement (anticipated future programs
included).
2.1.5 Noise Reduction Potential
! ' -^———————r-—————
The potential noise reduction achievable by means of current and poten-
tially available technology, starting with the technology demonstrated in the CF6
engines and those of the Federally-funded research programs, is summarized in Table
2.1-6.4/23 The noise levels are specified in terms of the FAR-36 takeoff measure-
ment posi'tion.
45
3. - 7 9
-------
Table 2.1-5
Federal Noise Abatement Programs^, 29,30,32
Program
NASA Acoustically Lined
Nacelle Program
NASA Quiet Engine Program
DOT Retrofit Program
STOL Noise Reduction
Demonstrator Program
Augmentor Wing STOL
Program
STOL Prototype Program
STOL Quiet Engine Program
Advanced Technology
Program
Approximate Program Cost
Millions of Dollars
15
22
7-15
8
1.5
100
58
250
Scheduled
Completion Date
1968
1973
1973
1972
1972
1975
1975
1983
Table 2.1-6
Estimated Aircraft Noise Reduction Potential for Takeoff
EPNdB at FAA Measurement Position
DC- 10- 10 Technology
Quiet Engine
Quiet Engine with
Optimum Jet Noise
Technology
Further Fan and Core
Noise Reduction:
Optimum Quiet Engine
Jet Noise
EPNdB
95
94
88
88
Fan and Core
Noise Reduction
dB re DC- 10- 10
o
-10
-10
-16
Total
EPNdB
100
95
91
89
46
-------
This analysis suggests that-a noise reduction of 11 EPNdB below the
noise levels generated by the DC-10-10 aircraft is possible. It must be realized,
however, that a high level of investment by Federal Agencies and the aircraft
industry in research and development will be necessary to achieve this goal, partic-
ularly in the area of core noise reduction.
The requirement of 95 PNdB at 500 feet for the 150-passenger STOL
transport must be examined in order to assess whether this noise level is attainable
with current potential jet engine technology. Application of the optimum Quiet
Engine concept discussed above results in a noise level 5 to 10 PNdB higher than the
!
objective. It must be realized,however, that the STOL aircraft will have a some-
what lower critical requirement for cruise efficiency than do conventional jet aircraft.
Hence, the STOL power plant may incorporate a sonic inlet to further reduce forward
radiated fan noise and a geared fan concept that permits still higher bypass ratios with
resulting lower jet velocity. The combined effect of these features may be sufficient
to gain the extra noise reduction, but there may be unavoidable performance
penalties associated with the requirement.
The potential noise reduction discussed above will be examined in light
of the future requirements. In attempting to establish specific noise reduction objec-
tives for the commercial jet aircraft fleet, it is instructive to consider the growth of
.the noise impact during the last decade due to commercial aircraft operations, and
attempt to predict future trends on the basis of current and potential jet engine noise
reduction technology. Obviously, the projected rate of growth of commercial air
traffic will influence these estimates. Extrapolating from the traffic growth during
the 1960's and predicting the impact of the anticipated social and economic changes
during the next decades, FAA and others have arrived at projected annual rates of
growth of up to 12 percent. 3/34 Recent estimates by the commercial aircraft industry
on the future commercial aircraft market, however, are consistent with an annual
growth rate of 5 percent. ' The latter figure, although realistic from the point of
view of the growth in population and gross national product, is sufficiently low that
it may be considered a conservative estimate, or a lower bound.
47
-------
Figure 2.1-22 shows the growth in noise-impacted areas since the intror
duction of commercial jet aircraft, and projects the future trend in noise impact on
the assumption of a 3 percent annual growth in the total number of aircraft operations,
corresponding to the 5 percent annual growth in the number of passenger emplanements
discussed above. The use of this constant ratio assumes that the current trend toward
increased aircraft capacity will continue, and may well cause an underestimate of the
growth of operations beyond 1985 if the trend does not continue.
The following factors were considered in the calculation of the noise-
impacted areas:
• Airport land, surrounding industrial land, and other compatible land
are included in the total noise-impacted areas. The airport land
above is estimated to cover 250 square miles in 1970, and this figure
may increase in the future.
• The growth of air freight is not sufficient to become a controlling
factor.
• ' A 5 dB reduction in the NEF value was assOmed to give a 55 percent
reduction in area.
• The constant mix of daytime-nighttime operation remains unaltered.
• No change in aircraft aerodynamic .performance or flight procedures.
• The trends in the growth or decrease of the,impacted areas are con-
sidered to be reasonably accurate. The expected accuracy of the
actual values,, however, are probably only with ±50 perce.nt.
• NEF 30 was used to define the impact boundary. This is a relatively
high noise exposure criterion, particularly for suburban communities.
Therefore the areas represent minimum estimates of impact.
Figure 2.1-22 shows a great range in the projected impact area depending
on the application of noise reduction technology to the future commercial aircraft fleet.
As an extreme example, maintaining the current aircraft noise levels would result in
an increase in impacted areas to 185 percent of the 1970 figure, by the year 2000.
48
-------
3000
-SJ 25,00
ii
o-
to
2000
o
u
LU
Z
c
• •^
• •^
*
to
1500
5$ Projected Annual Growth
in Passenger Enplanements
and Air Freight Tonnages
New Aircraft Similar to
Current Types of Aircraft
w/o FAR-36 Restriction
1000
z
o
500
Retrofit to FAR-36
(1973 to 1977)
and all New Aircraft
Meet FAR-36
All New Aircraft After
1985 Meet
FAR-36 -lOEPNdB
All New Aircraft After
1980 Meet
FAR-36 -4 EPNdB
1960
1970
1980
1990
2000
2010
Year
Figure 2.1-22. Estimated Noise-Impacted Areas (30 NEF or Higher)
as Function of Jet Engine Noise Reduction Goals
49
-------
The application of retrofit to the existing aircraft fleet to ensure that all commercial
aircraft comply with FAR-36 criteria would result in a significant initial decrease
in impact area in the 1976-1987 time period. This significant decrease demonstrates
the effectiveness of aircraft certification for noise accomplished by the FAA, coupled
with the significant 10+dB reduction in noise between 1958 and 1968 accomplished by
government and industry research and development. However, by year 2000 the land
: area will again have increased measurably due to the projected increased number of
aircraft operations. The two additional curves show the effects of further reduction in
aircraft noise levels. The attainment of aircraft noise levels corresponding to FAR-36
minus 10 EPNdB would result in a 83 percent reduction in impact area below the 1970
value by year 2000.
In order to further illustrate the implications of these noise reduction values,
Figure 2.1-23 shows the dependence of the respective noise impact areas on the choice
of the annual rate of growth in aircraft operations, assuming a constant rate of growth
in the period from 1970 to 2000. The noise reduction effect of changes in operational
procedures has also been included. The lower bound in impact area for which this
effect may be considered reflects the assumption that these procedures may be applied
only above certain critical aircraft altitudes during the takeoff and approach operations,
corresponding to a ground distance of 8000 feet from threshold on approach, and
12,000 feet from aircraft rotation on takeoff.25"*28
The philosophy may be adopted that the tremendous growth in noise impact
since 1960 has been due to the fact that commercial jet aircraft have been excessively
noisy, and hence, the noise reduction objectives should be aimed at reducing the noise
impact areas to the pre-1960 values. This criterion may seem somewhat arbitrary in
view of the considerable expansion in airport areas since 1960. However, it includes
consideration of the fact that whereas the NEF 30 contour lies outside the most vigorous
complaint area for urban residents, it still has a considerable annoyance associated with
its noise levels (more than 30 percent of the populace registers annoyance), and it is
18
in the vigorous complaint area for quiet suburban areas.
50
-------
-------
Referring to Figure 2.1-23, the NEF 30 impact- area may be reduced to,
within its 1960 value of 200 square miles by year 2000, i.e., the airport and sur-
rounding industrial area, for annual aircraft operation growth rates of up to 8 percent
if new aircraft after 1985 comply with a noise criterion of FAR-36 minus 15 EPNdB.
Using the DC-10-10 aircraft as a baseline, this noise reduction objective corresponds
to 89 EPNdB on takeoff and 93 EPNdB on approach at the FAR-36 measurement
positions. This takeoff requirement is equivalent to the potential noise reduction
for an optimum quiet engine, previously discussed together with Table 2.1-6.
52
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2.2 V/STOL Aviation
2.2.1 Introduction
Although current Vertical/Short Takeoff and Landing (V/STOL) aircraft
are inherently part of both the commercial and general aviation fleet, their unique
capability of operating from very small airfields or from urban centers tends to distin-
guish them in terms of noise impact from the remainder of the aviation transportation
industry.
The present V/STOL fleet is predominantly comprised of helicopters (VTOL).
The STOL fleet is not yet a significant reality, but is currently undergoing considerable
Federal and industry study. The principal objective of STOL aircraft is to move much
of the inter-city air transportation (short-haul) away from the congested major-hub
airports and toward the urban community where the public will be better served.
Tentative noise goals have been proposed for aircraft operating from the projected
peripheral STOL ports, but as yet a community-compatible noise goal has not been
defined for the intra-city heliports now in operation, or for those which will serve as
1-4
city-feeder terminals for the STOL ports.
Figure 2.2-1 shows the typical subcategories of the present V/STOL fleet
and their major applications. Of the current total of 3260 vehicles, approximately
1900 are based in counties with population densities in excess of 1000 people per
square mile. The most significant increase of usage in recent years has been by civil
government agencies, with 120 operator agencies in 1971 compared with 80 in 1969.
In particular, the number of city police helicopters is rapidly increasing, with a total
of about 150 vehicles in present use. '
Commercial helicopter service grew until 1967, when a total of 29.7 million
revenue-passenger miles were flown. Since 1967 this service has declined to 11.3 mil-
lion passenger miles for a revenue of $7.6 million in 1970. Cargo traffic has followed
the same trends with 34 thousand ton-miles transported in 1970 for a revenue of
$350 thousand. Manufacturers shipped approximately 500 completed rotor aircraft
in 1970/'&
53
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V/STOL Aviation
1
1 1
Light Utility
Helicopters
(2-7 Seats)
Medium Weight
Helicopters
(10-15 Seats)
1
Heavy Transport
Helicopters
(20-50 Seats)
1 SI
1 A!
' (40-1
I
Air-Taxi
Law Enforcement
Executive Travel
Rescue/Ambulance
Agriculture
Traffic Monitor
Scenic/Survey
• Commercial Charter
• Company Transport
• Executive Travel
• Fire Control
• Coast Guard
Scheduled Transport
Industrial Cargo
Construction Lift
Coast Guard
STOL
Aircraft
)-150_Sea
• Commercial Transport
Number in Service
2900
100
< 90
-a
1 80
_S 70
1 60
50
a>
• ^
i—
. 78
\S/\
//
65
8
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g 2.5
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0) C
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2 n
•
•
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I/)
£
86
///
^^,
73
£
8
lO
100
86
i.
0)
320 | 40 In Planning Stage
Typical Noise Levels
96
88 H
^ ^
/X^ 83
76
h_
§ 1
lO ^
92 94
^85 83
82
(Approx.
Equivalent
to 95 PNdB
A: ~ .£ Proposed
8 'c 8 Limit)
IO »— i U">
Growth of Helicopter Fleet _
D « E coi—
8 'o .2 E'l — i
8 o fc -5 ^ u
C 0) | H- 0) o
05 .3 "* U S . « ^
£ ^ > 8 — « '^ f~~
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0)
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rT
1965 1970
Figure 2.2-1. Characteristics of V/STOL Aircraft
54
-------
Commercial usages are predominantly charter air-service operations, with
only about 15 vehicles on scheduled intra-city air carrier routes. The average route
stage length of the latter services is 20 miles, in 10 minutes flight time, compared
with' a possible 40 minutes (or more) by city road transport. This market potential can
be expected to be more fully exploited with the introduction of urban STOL ports.
Figure 2.2-2 shows one projection (DOT/NASA, 1971) of the expected 1985 V/STOL
fleet.9
2.2.2 Source Noise Characteristics
VTOL Aircraft
The helicopter is unique in that its noise signature is characteristically
different from all other common noise generators. This difference is attributable to
the main (lifting) rotors which rotate at relatively low revolutions per s.econd, but
generate very high amplitude pulsating sound pressures at their blade tip regions.
The resulting noise, observed both at ground level and within the aircraft cabin, is a
distinctive low frequency throbbing sound which often suddenly increases in level
and exhibits more of a slapping nature during descent, maneuver, and high-speed
cruise operations. Due to the predominance of the low frequency content of the
noise, it is extremely difficult to control its intrusion into the passenger cabin or
into ground buildings by sound-insulation methods, which are notably inefficient in
the low frequency range. This problem is further complicated by the fact that low
frequency sound propagates through the atmosphere more efficiently than higher
frequencies. Thus, helicopter noise can be distinguished at greater distances than
can most other sources of equal noise level. Typical noise spectra for two classes of
current commercial helicopters, shown in Figures 2.2-3 and 2.2-4, demonstrate these
frequency characteristics.
The noisiness value of rotor noise is often under-predicted by current
subjectively weighted noise scales such as dB(A) and EPNdB. These scales do not
account for the attention-gathering potential of a helicopter, which results from
55
-------
Intraurban
0-50 mi
Short Haul Interurban
50-500 mi
(4-10 Passengers)
STOL/VTOL Taxi
(80-100)
STOl/VTOL |
1 f I 1
(40-80)
1 1 STOL/VTOL Bus
.1. i i I
200 400 600 800 1000 1200
Number of Aircraft
1400 1600
Figure 2.2-2. Potential 1985 U.S. V/STOL Fleet
56
-------
^ 100
o
CN
90
0)
0>
£ 80
I
C
O
o
CO
o
O
70
60 -
50
\I\lS\
Interior Cabin
\
Ground Level at- 500 feet
100
1000
10000
Frequency in Hertz
Figure 2.2*-3. Typical Noise Spectra of Light Rston-Engined Helicopters
57
-------
Main Rotor
Power Plant
anooannoDD
CM
3
o
CN
0)
CQ
c
0)
I
a_
-o
o
ca
0)
I
u
O
Ground Levels at 1000 feet
60
Frequency in Hertz
Figure 2.2-^4. Typical Noise Spectra of Heavy Helicopters
58
-------
throbbing or slapping noise of the rotor, analogous to a flashing light compared with
a steady one. Most other sources of noise, including propellers, are more analogous
to a steady light due to absence of low frequency modulation, and consequently are
better assessed by the current scales. Other noise sources on the helicopter, notably
the tail (stabilizing) rotor and piston or gas turbine engine, can be particularly
annoying in certain conditions. Additional information relating to the noise generating
mechanisms of helicopter rotors is presented in Appendix C.
In areas close to the takeoff/landing terminal, prolonged periods of
engine-idle operation during the (dis)embarking of passengers are accompanied by
the piercing whine of the gas turbine or the equally disturbing bark of a piston engine
exhaust. As the tail rotor is usually direct-geared to the powerplant, it is also rotating
at a sufficient speed, during these idle operations, to generate an additional noise
nuisance. In some cases, the tail rotor and engine noises exceed the main rotor in
subjective (nuisance) impact during flyover. This problem is more common on light
utility helicopters which have lower main rotor loading and piston engine powerplant,
as shown in Figure 2.2-3.
Other subsources, such as the transmission system between engine and
rotors, can be distinguished in the passenger cabin and at very close external regions.
Their significance is generally low compared to the rotors and powerplant, but in the
few cases where they are notably present, the noise is of an annoying nature if prolonged.
STOL Aircraft
Current design concepts of commercial STOL aircraft are based on a
1 2
projected requirement for operation from 2000-foot length STOL port runways. '
The economic viability of the proposed STOL fleet relies on both its payload capa-
bility and its ability to operate from terminals close to the potential customer — the
urban community. Each of these requirements has a distinct bearing on the propulsion
systems to be incorporated in the fleet aircraft, arid on the noise characteristics to be
expected and allowed of STOL aircraft. A tentative limit of 95 PNdB (approximately
80 dB(A)) has been proposed by the FAA to be applied at a 500-foot distance from
59
-------
each aircraft.'^ The alrframe and propulsion system industries are vigorously pursuing
this noise goal. Consequently, the final flight-ware systems may radically differ from
the basic breadboard systems now under test and development. Of these systems, those
now in development for application to the 40-80 seat category aircraft are:
• Compound (single and twin rotor) helicopters, V/STOL
• Quiet-Propeller, STOL
• Tilt-Rotor, V/STOL
• Prop-Fan, STOL
• Lift-Fan, V/STOL
• Jet-Flap, STOL
Full-scale or model acoustic testing of these concepts has indicated that
the 95 PNdB limit can be met by the 40-80 seat passenger V/STOL systems. The
typical frequency spectra noise characteristics of the propeller, rotor and prop-fan
systems are shown in Figure 2.2-5. Note that these spectra do not include the
engine-noise contribution. The main difference in the spectra are attributable to the
rotational speeds (revolutions per second) and number of blades typical of each system.
The prop-rotor is a 3-blade low speed system. The propeller is also a 3-blade system,
but operates at about three times typical rotor speeds. The ducted prop-fan has about
12 blades operating at speeds slightly higher than the propeller.
Present estimates of the larger (80-150 passenger) STOL system projections
indicate that the proposed 95 PNdB limit at 500 feet will not be met by designs based
on current technology. The sideline distance corresponding to the 95 PNdB level is
projected to be between 3000 and 4000 feet for current designs, and will expectedly
3
converge toward the 500-foot goal as technology is improved.
2.2.3 Environmental Noise Characteristics
The significance of helicopter noise in the community environment is not
immediately apparent from the statistics of total number of helicopters in operation.
As discussed earlier in the report, the present aircraft noise problem primarily involves
60
-------
Prop-rotor
JS. «. r "\ «
Propeller
© © £|D © ©
Prop-fan
100
1000
Frequency in Hertz
10000
Figure 2/2-5. Typical Frequency Characteristics of Propulsion Systems
for 40 to 80 Passenger STOL Aircraft (excluding powerplant) at 500 feet
61
-------
a large number of people living near airports affected by landing and takeoff operations.
For conventional aircraft, the cruise condition flight is usually at high altitude and
therefore does not contribute much to the ground noise exposure. However, helicopters
are most commonly operated at low altitudes due to short stage distances, ground
observation requirements of the service, or simply to provide the added attraction of
a panoramic view to the intra-city passenger. This extended low altitude operation,
most often directly over urban and suburban regions, significantly increases the noise
impact potential of the helicopter. The increasing incidence of police patrol operations
over populated areas further aggravates this situation due to the prolonged noise intrusion
of a hovering or surveying helicopter, operating at low altitude.
Because the helicopter flight route patterns are essentially random at
present, it is practically impossible to define their current impact on the environment
in terms of exposure duration, land area or population. A sustained public reaction
has not materialized, despite the intrusive nature of the sound/probably because of
the irregularity of this usage pattern. However, widespread complaints have arisen
9
due to air taxi services in New York, police operations in Los Angeles, and others.
This is not surprising since the noise levels at 500 feet from a commercial helicopter
are in the 80 - 90 dB(A) range, as are the levels from a police helicopter at 250-foot
u-i. A 10~12
altitudes.
The introduction of the STOL fleet as a convenient commuter mode of
transportation will bring many benefits to the urban resident. However, it will also
bring a new source of noise into his environment, and the total community acceptance
will be dependent on the effectiveness of STOL port planning, aircraft routing, and
noise abatement procedures currently being designed.
Figure 2.2-6 shows a comparison of various V/STOL noise levels with
3
those of the community ambient noise levels (Lg-.). A difference of 25 - 30 dB(A)
or greater between a single-event intruding noise and the ambient (LOf)) will annoy
many people in the community. If the single event at such a level is repeated suffi-
ciently often, an appropriate community reaction may be anticipated. For example,
62
-------
s.
CO
I 80-
Rural
Quiet
Suburban
Urban
Suburban
Residential
Urban
Commercial
Downtown
Commercial
Urban
Commercial
Urban
Suburban
Residential
Rural
Quiet
Suburban
I
Light Piston Heli-
copter at 500 ft.
Heavy
Helicopter
at 1000 ft.
I STOL at 500 ft. T/O I
Light Turbine Heli
copter at 500 ft.
rSTOLat2000ft. T/Oj
Figure 2.2-6. Comparison of Typical V/STOL Aircraft Noise Levels with Community Ambient Noise Levels (I-9Q)
-------
10 overflights per hour during day and evening of a helicopter meeting the 80 dB(A)
noise goal would cause a community noise equivalent level of 60 dB(A). No commu-
nity reaction would be expected in a noisy urban residential community, whereas
"widespread complaints" to "threats of legal action" would be expected in a quiet .
suburban community. To reduce the expected reaction of the quiet suburban commu-
nity to "no reaction", the minimum altitude over the community should be approximately
4000 feet for this assumed frequency of operation and vehicle noise characteristic.
Figure 2.2-6 also illustrates the problems faced by .the city heliport and
urban STOL port planner. The desire for central-city operations must be tempered by
the constraints imposed by the local outdoor noise levels. Solutions being considered
are the use of industrial areas suitable for port locations, and the optimal use of high-
rise, non-residential buildings to shield the noise from residential areas.
From the viewpoint of the potential V/STOL passengers, who are predicted
9
to comprise more than half the total revenue passenger complement, in 1985 ., the
internal noise of rotor and propeller powered vehicles will require significant reduction
from their present levels if the service is to be considered attractive. The noise level
10-14
inside many current helicopters ranges between 90 and 100 dB(A) , representing
a definite risk of hearing damage to the constant traveler, particularly if his exposure
exceeds 1/2 hour per day. .Also, the occasional passenger may accept poor speech
communication during short flights, but the regular-commuter passenger will consider
such features a distinct inconvenience. In such cases, it maybe expected that manu-
facturers will attempt to alleviate the problem from a solely commercial standpoint.
2.2.4 Industry Efforts'in Noise Reduction ' ' •
VTOL Aircraft ,
The helicopter manufacturing industry .is primarily engaged in
military helicopter requirements, which account for approximately 80 percent of the
more than 20 thousand production vehicles produced prior to January 1970. The
j
vulnerability of military helicopters during reconnaisance or evacuati.ve missions
64
-------
has been closely correlated to their excessive noise signature which allows early
detection and consequent retaliatory enemy action. The industry has therefore been
keenly engaged in research and development programs specifically aimed at the
problem of noise reduction. However, much of the work has been directed toward
the development of modification concepts applicable to long-established production
models or economically viable to production lines. As almost all of the civil heli-
copter fleet are direct derivations of military models, later production models have
benefited from the noise suppression developments. Retrofit modifications are generally
not economically feasible for many private operators, although made available by
*u • ^ * 18'19
the industry.
Another approach taken by the industry toward noise alleviation has
been in educating the private operator in particular methods of operation which
avoid prolonged community noise exposure and which circumvent the condition of
19
blade-slap noise during descent maneuvers. These and other facets of the industry's
awareness of the noise problem relate to past and immediate production helicopter
types which will tend to dominate the civil market for the next decade.
The responsibility for developing noise suppression techniques for heli-
copters has been firmly implanted in the manufacturing industry because of the
encompassment of aerodynamic structural design and performance considerations
in the acoustic technology matrix. The emphasis of past and current programs has
been in the specific area of rotor and propeller noise reduction because of its
predominance in the acoustic signature of most V/STOL aircraft, although significant
attention has also been given to engine and transmission system quieting. The latter
is important when it is realized that almost 50 percent of the light utility helicopters
in operation in 1970 were piston-engined and that most of these have unsatisfactory
18 19
exhaust mufflers as original factory-installed equipment. '
An illustration of programs related to helicopter design is presented in
Figure 2.2-7. ' ' ' Examples of the noise reduction benefits attainable by
these approaches are shown in Figures 2.2-8 and 2.2-9, and are indicative of what
65
-------
© Lower revolutions per second
(?) More blades
(D Large blade area
(5) Modified blade tip shapes
©Reduced blade interaction
©Engine inlet suppression
©Engine exhaust muffling
®Cabin insulation improvements
Figure 2.2-7. Current Design Approaches to Helicopter Noise Reduction
100
CO
90
80
; 70
60
TJ
0>
8 50
40
30
Original Design
Modified Version
Redesigned Version
100 200 400 700 1000 2000 4000
Distance to Helicopter — feet
Figure 2.2-8. Effect of Design Changes on a Light
Helicopter Noise Signature (Demonstrated)
66
-------
120
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.5
« 100
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80
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Figure 2.2-9. Demonstrated Noise Reduction
of a Heavy-Helicopter Twin-Rotor System
ea
TJ
o_
a>
a>
5
Z
I
70 -
Unmuffled
Stack-Mounted Muffler
Structure-Mounted Muffler
60 .
200 500 1000 2000
Distance from Helicopter (ft)
Figure 2.2-10. Exhaust Noise Suppression
of Light Piston-Erigined Helicopters
67
-------
can be expected in future helicopter models designed specifically toward noise goal
objectives. Major areas of noise reduction study pursued by the industry are discussed
• *u F ii • u 10-13,19
in the following paragraphs.
• Propeller/Rotor Noise Reduction — The most direct and efficient
methods of propeller and rotor noise reduction are to reduce the
blade tip speed and reduce either the total load on each blade or
the load per unit blade area. There are obvious limits to the
application of these principles in addition to those of aerodynamic
stall (which gives a sudden noise increase) and the weight/
performance requirements for economic operation. Thus/ most
research effort has been aimed at deriving more subtle approaches
to design, whereby the above methods can be implemented and
improved upon with negligible performance penalty. Some of the
more successful of these methods are:
- Larger blade area
- Increased number of blades
- Variable geometry blades (changeable camber in flight)
- Modified blade tip shapes
All of these have either resulted from, or have been made practical
by, combined efforts in acoustic, aerodynamic and materials research.
In particular, the noise reduction potential from increasing the number
of blades and blade area has been known for quite some time, but this
approach has only recently become practical due to the development
of lightweight construction materials and fabrication techniques.
Blade tip shape modifications have undergone extensive investigation
for both aerodynamic and acoustic benefits, including reduction of
blade slap. Helicopter rotor tests indicate that 5 to 8 dB can be
achieved by this approach.
68
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Engine Noise Reduction — At ground-idle and in-flight conditions
with noise abatement procedures in operation, the loudest component
sound of a V/STOL aircraft may be its engine noise. For piston-
powered helicopters, the exhaust noise is extremely noticeable in the
signature. Gas turbine engines are distinguishable by high frequency
whine of their compressor stages and by their exhaust when the jet is
used as q propulsive force. Each of these can be treated by different
types of suppression, from the relatively simple piston-engine muffler
to the more complex jet-exhaust suppressor. However, all methods
cause some degradation of the performance-cost ratio of the vehicle,
and consequently the buyer/operator is often reluctant to include
them in his optional equipment list. The manufacturer/seller is also
reluctant to include them as standard equipment because of the sales
competition within the industry and his desire to provide the most
economically operable item. Nevertheless, the equipment for noise
suppression has been developed, demonstrated, and made available
by the industry and other independent companies in the form of retro-
fit kits composed of factory-installed options. Although much remains
to be done to improve the noise and performance influence of suppres-
sion devices, an immediate improvement can be obtained if their
usage is required.
The noise reduction currently attainable by available mufflers for
helicopter piston-engine exhausts is shown in Figure 2.2-10. '
Stack-mounted units are very lightweight and are designed to fit
directly onto the exhaust port of the engine. The acoustic performance
of these units ranges from relatively poor to moderate, but they are
designed to impose little penalty on operating costs. Structure-
mounted types are heavier and more efficient in noise reduction, but
are more expensive and in particular have the greatest detrimental
effect on operating costs.
69
-------
Gas turbine (jet) powered helicopters are generally quieter than
19
their piston-engine counterparts/ as shown in Figure 2.2-11',
however their market popularity is restricted by a requirement for
specialized fuel. Again, the main noise problems are associated
with their terminal operation rather than their cruise mode. In
this case, both the inlet and exhaust require noise suppression
treatment, and absorptive vanes or lining installed within the
appropriate ducting has been demonstrated to provide a total
reduction of up to 10 dB with a resultant power loss of about 1 to
2 percent.
The past 5 years have seen a most significant advancement in V/STOL
noise control. Methods of noise suppression have been developed which can, if
applied to new production models and the noisier of the older types, allow the full
development of the V/STOL as a community service item. Until recently, little
attention has been given to the design of the actual landing site to alleviate the
noise radiated to nearby residential areas. In fact, the tendency of some operators
is to deliberately aim for line-of-sight pads in order to advertise their service. This
practice is highly undesirable from a noise nuisance viewpoint. Recommended
practices, or even mandatory regulations, should be developed for city heliport
design and construction.
In summary, the industry is acutely aware of the noise problem and its
relationship to the development of an expanding market for their products. It has
been involved in considerable research and development study (at both Federal and
industry expense) to find practical methods of reducing the noise levels of current
and future production line models. The present situation is that these efforts have
been significantly successful, but only in terms of present helicopter usage patterns.
The expected increase in intra-city transport and law-enforcement usage will change
this pattern over the next decade. This change must be accompanied by further noise
reduction built into the helicopter and by more detailed study of urban helicopter
-------
90
<^
CO
I
'5
Z
CO
TO
I
4-
o>
0)
i/i
'o
Z
T3
4)
'5
80
70
100
90
80
2000
Distance = 500 feet
-2-5
5000 10,000
Gross Weight (Ib)
20,000 30,000
-20-40 -
Approximate Seating Capacity
<\
Turbine-Powered
Distance = 500 feet
2000
5000 10,000
Gross Weight (Ib)
20,000 30,000
Figure 2.2-11. Trend of Helicopter Noise Levels with
Gross Weight and Seating Capacity
71
-------
structures/ location and operating procedures, to ensure that the helicopter fleet
will not impose an unacceptable noise burden on the community it serves.
STOL Aircraft
The STOL industry has a tentatively-defined noise goal to meet to
ensure its command of the commercial aviation market by 1985. This goal is
being approached by intensive research and development of suitable propulsion and
lift concepts, some of which have been described in Section 2.2.2. The main
difference between the VTOL and STOL industries is that the latter must include
noise as a major parameter in their conceptual design studies, whereas the pre-
dominant objective of the VTOL (helicopter) industry is to reduce the noise of their
established design models.
2.2.5 Noise Reduction Potential
VTOL Aircraft
The most immediate problem for the VTOL industry is to further develop its
noise suppression technology to make it economically acceptable to the commercial
and private operator. With the increasing usage of helicopters within the urban
service system, it can be expected that community reaction to the noise intrusion
will also increase and force legislation of operational characteristics to be developed
and imposed. It has been demonstrated that significant noise suppression can be
installed on current design concepts and therefore it is practical to consider that the
helicopter can become compatible with community usage. However, the result can
only be achieved by incorporating noise reduction methodology into vehicles produced
for the urban-user market as a standard procedure. The potential for future helicopter
(VTOL) noise reduction is summarized in Table 2.2-1.
STOL Aircraft
The long term future of the interurban STOL aviation economy depends on
the development of the larger (80 to 150 passenger) STOL bus. Current projections
indicate that with present technology the 95 PNdB goal will not be met at the 500 foot
72
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Table 2.2-1
Estimated Noise Reduction Potential for Helicopters
Time Period
Short Term Potential
Utilizing Available
Production Methods
Long Term Potential
Utilizing Current
Industry Trends
Long Term Potential
Utilizing Demonstrated
or Advanced Technology
Noise Reduction, dB^1)
Heavy
Transport
Helicopters
0
10
10
Light and
Medium
Turbi ne- Powered
Helicopters
' 5 '
15 '
17
*
Light Piston-
Powered
Helicopters
10
10
20
Noise reduction relative to typical current noise levels in dB(A) <
at 1000 feet.
o
distance. This would mean that a large section of residential area around STQL ports
would be subjected to unsatisfactory noise intrusion levels. Further, many quiet sub-
urban communities under the STOL flight path would be exposed to excessive noise
unless the aircraft cruise altitude were increased enough to achieve compatible ground
noise levels. The economic tradeoffs between source noise reduction and higher than
optimum airspace altitudes must receive careful study.
73
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2.3 General Aviation Aircraft
2.3.1 Introduction
The term "general aviation" refers to all civilian aviation activity other
than that of the commercial air carriers. Within this broad definition, general aviation
includes a wide variety of aircraft uses. The following use categories may be con-
., ,1, 2
sidered:
• Business Aviation — This is the largest single category of general
aviation in terms of total aircraft hours flown. It includes all aircraft
used by corporations and individuals for business transportation.
About one-third of the total hours flown by general aviation aircraft
fall into this category and these hours are flown by about one-fourth
of the registered general aviation aircraft.
• Personal Flying — This covers over half of general aviation aircraft
registered in the United States. This category is generally made up
of smaller and less expensive aircraft than those in the business
aviation group.
• Air Taxi, Charter and Contract Usage — These aircraft are generally
considered part of the general aviation fleet. Also included in this
category are small charter aircraft contracted with a flight crew.
• Instructional Usage — This category accounts for about one-fourth of
the total general aviation aircraft hours flown. However, in numbers
of aircraft, instructional aircraft comprise only about 11 percent of
the total fleet. Most of these are smaller single-engine types.
• Aerial Application, Industrial and Special Use — This includes air-
craft used for agricultural spraying purposes, patrolling, advertising
photography, aerial surveying and equipment testing. This category
is relatively small both in terms of numbers of aircraft and hours flown.
74
-------
The use of general aviation aircraft has grown in the past 10 years from
12 million flight hours to a total of 25.5 million aircraft hours flown in 1970. Equally v
significant, the composition of the general aviation fleet has changed from a pre-
dominance of small, single-engine propeller types to a much more complex fleet mix.
Figure 2.3-1 summarizes this fleet mix and provides information on the number of air-
craft operations and typical noise levels produced.
A conservative picture of the economic impact of general aviation is
obtained from the fact that manufacturers of airframes, power plants and avionics
employ 23 thousand people and had gross sales in 1970 of about $375 million. In addi-
3
tion, $240 million of gasoline was utilized by the general aviation fleet.
2.3.2 Source Noise Characteristics
The noise associated with general aviation propeller aircraft with both
piston and turbine engines is produced principally by the propellers. This noise con-
tains a harmonic series of discrete frequency tones, with the dominant fundamental
4
tone typically in the range from 50 to 250 Hz. Depending on the propeller blade
shape and the propeller operating environment, the second and third harmonic tones
may also have significant levels. Figure 2.3-2 shows typical noise levels and spectra
4
measured during propeller aircraft operations. The broadband and discrete frequency
noise above approximately 250 Hz consists of higher propeller noise harmonics, dis-
crete frequency noise from the engine and exhaust, and exhaust broadband noise. The
latter noise sources may contribute measurably to the total noise generation by some
types of general aviation aircraft, but are generally masked by the propeller noise.
Additional information on the noise generation mechanisms of propellers is contained
in Appendix C.
The noise characteristics of. jet-powered general aviation aircraft, or
executive jets, are shown in Figure 2.3-3. Their characteristics are similar to those
of commercial jet aircraft. Most business jets are powered by pure turbojet or low by-
pass ratio turbofan engines; thus, the jet exhaust is the dominant source of noise.
Since these engines are much smaller than those used to power commercial jet aircraft,
75
-------
Single-Engine
Propeller
• Pleasure
• Instructional
• Business
General Aviation Aircraft
Multi-Engine
Propeller
• Pleasure
• Business •
• Commercial
Executive Jet
Corporate Aircraft
Business
o
o
| 7°
Z 60
110,500
Numbers in Service (1970)
105
90
77
67
76
85
17,500
Growth of Aircraft Fleet
no,
68,
1
1
*
81,
300
i
300
50C
7,
n,
250
17,
800
500
60 65 70 60 65 70
Typical Noise Levels
93
80
70
79
%
900
900
loor—]
o .—I I
60 65 70
97
87
^
[Approach 2
//^
93
[Takeoff
95
%
80
c
15
o
U
Approach and Takeoff Levels Measured at '1000 feet
Figure 2.3-1. Characteristics of General Aviation Aircraft
76
-------
Exhaust
Propeller
Engine
Z
3.
8
CO
T3
a>
I
§
l/>
I
T3
o
O
CO
a>
I
u
O
I I II I I
Propeller Noise
Engine/Exhaust
Ground Noise Level at 1000 Feet
—Typical Operation
50
1201—
81
Frequency in Hertz
76.5
82.5
80
°<
"w .»' 60
> -o
a>
« 40
V)
| 20
Z
0
: —
—
"••
__
Propeller
Engine/
Exhaust
r
Total
Contributing Subsources for Typical Aircraft
Figure 2^3-2. Noise Levels and Spectra of General Aviation Propeller Aircraft
77
-------
Fan/Compressor
Jet/Turbine
CO
I
o
50
120
100
80
60
40
20
0
1000
Frequency in Hertz
89
89
—
••••
—
—
—3
77
o
1
Q.
E
o
U
\
o
u_
a
"o
Contributing Subsources for Typical Aircraft
Figure --'.3-3. Noise Levels and Spectra of Executive Jet Aircraft
78
-------
the characteristic frequencies in the jet noise are higher, and also the noise levels are
lower than for the big turbofan engines.
2.3.3 Environmental Noise Characteristics
The operator or passenger in a general aviation aircraft is subjected to
noise levels of about 90 dB(A), which is 5 to 15 dB higher than in a commercial jet
aircraft. Figure 2.3-4 shows a typical interior cabin noise'level for a general avia-
tion propeller aircraft. This high noise environment is caused by several factors:
t
N
• The engine is mounted close to the cabin, hence the cabin walls are
exposed to the highest sound pressures generated by the propeller
without any benefit of attenuation from distance. This situation is
aggravated in conventional twin-engine aircraft.
• The dominantly low frequency content of the propeller noise makes
i
conventional fuselage noise insulation techniques rather ineffective.
• The small volume within the cabin limits the effect of interior wall
sound absorption.
The airport noise impact due to general aviation aircraft noise is quite
small when compared to the impact of commercial aircraft operations. Figures 2.3-5
and 2.3-6 show NEF values versus slant range, respectively for takeoff and landing
operations, for the average national mix and the number of aircraft that are expected
to utilize a typical general aviation airport. The lack of significant impact is evident
on noting that the NEF values stay below 30 even at very close ranges and below 20
for relatively short ranges. Consequently, the vast majority of general aviation air-
ports do not have a serious community noise problem.
The low level of impact associated with executive jet aircraft in this
example is due to their relatively small number of operations, despite their high noise
V.
levels. However, at several general aviation airports that have a significantly higher
rate of operation for executive jets than the national average mix, these aircraft tend
to dominate the airport noise picture. This effect is illustrated by the additional
79
-------
Propeller
o
CN
£
CO
-o
o>
I
§
to
I
0>
I
o
o
-—Typical Operation
100
1000
10000
120
•
£ 100
8
8 80
^S 60
^ ~O
-1 40
o
(A
!o 20
n
1 l«««i|WII^«T III | « VI 1 A
—
"
91
Propeller
85
Engine and
Exhaust
92
.. ,
Total
Contributing Subsources for Typical Aircraft
ly^itt z.J-4. Interior Cabin Noise Levels and Spectra for
General Aviation Propeller Aircraft
80
-------
0>
I
20 -
10 -
-10
-20
-30
Operations per Day
Executive Jet (national average) 1
Executive Jet (higher proportion)
Single-Engine Propeller Aircraft
Multi-Engine Propeller Aircraft
.i.l , ,
10
240
23
200
400 700 1,000 2,000
Distance to Aircraft — Feet
4,000
7,000 10,000
Figure 2.3-5. Noise Exposure Forecast Values for an Example of a Representative
General Aviation Airport with Daytime Use Only
81
-------
Operations Per Day
Executive Jet-(National Average) 1
Executive Jet (Higher Proportion) 10
Single-Engine Propeller Aircraft 240
— Multi-Engine Propeller Aircraft 23
APPROACH
i , , . ,
400
4000
7000 10,000
700 1000 2000
Distance to Aircraft — Feet
Figure 2.3-6. Noise Exposure Forecast Values for an Example of a Representative
General Aviation Airport with Daytime Use Only
82
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business jet curves in Figures 2.3-5 and 2.3-6. In the future, the proportion of execu-
tive jets in the general aviation aircraft fleet is expected to increase considerably.
Hence, these aircraft may become major noise sources around typical general aviation
airports unless their noise levels are reduced.
An additional source of aircraft noise at some general aviation airports
consists of the operations of fighter and trainer aircraft of World War II vintage. These
airplanes are generally very noisy and tend to create noise problems wherever they are
based. The eventual retirement of these aircraft appears to offer the most satisfactory
means of alleviating this problem.
2.3.4 Industry Efforts in Noise Reduction
The great majority of all general aviation aircraft are owned by private
individuals. More than one-half of these aircraft are used for personal and recreational
flying. Therefore, the general aviation aircraft industry deals predominantly with a
consumer market similar to that for automobiles or motorcycles. Competitive confor-
mance requires maximum capacity and performance within the particular price class,
coupled with economy of operation. The exploitation of technologies such as noise
reduction that bear only indirectly on product desirability are consequently relegated
to secondary levels of importance. Thus, the consideration of noise in general aviation
aircraft is geared to competitive objectives within the industry, rather than to any
desired standards.
The industry noise objectives have been aimed at quieting the aircraft
interior in order to provide more comfort to the operator or passenger. The approach
.has been rather cautious and straightforward. Existing quiet engine and quiet propeller
technology have been utilized within the constraints of performance, but the main
efforts have been directed at cabin noise insulation. Again, the progress has not been
spectacular due to the weight penalties associated with noise-insulating materials and
the governing performance constraints.
General aviation aircraft are not at the present time a major source of
noise pollution. At the hub airports, at which approximately one-half of the aircraft
83
-------
operate, their noise characteristics are masked by the much noisier commercial aircraft.
The remainder of the aircraft are distributed over more than 11 thousand airports within
2
the United States. With some exceptions, the noise levels at the general aviation
airports have not reached a magnitude at which the environment is severely affected.
Thus, the general aviation industry has not, until very recently, considered aircraft
noise in terms of the non-participant environment.
The general aviation fleet has grown rapidly during the last 15 years and
will continue to grow an an accelerated rate until at least 1985. As is indicated in
Figure 2.3-7, what is more important than the total growth in the fleet from noise
considerations is the growing number of multi-engine piston, turboprop and turbojet
8
aircraft in the projected fleet. Hence, the typical general aviation aircraft will
become noisier. This factor, in addition to the increase in the number of aircraft
operations, will lead to an increasing noise pollution potential.
Noise Reduction Programs
As discussed above, the main effort in noise reduction by the general
aviation industry has been directed toward lowering the interior cabin noise levels.
This objective has been achieved by combining reduced noise generation at the source
and improved transmission loss through the cabin walls. Propeller and engine noise
reduction have not been actively pursued. However, as discussed in the V/STOL
Section, the propeller and engine manufacturers have been engaged in the develop-
ment of quiet concepts for military and V/STOL commercial applications, and some of
the results have fed back to the general aviation industry. As an example, current
aircraft models generally have three-bladed propellers rather than the old two-bladed
9
propellers, with a resulting noise reduction of 3 to 5 dB. This result has been made
possible through materials technology development by the propeller manufacturers
whereby the new propellers weigh less than the older types, despite the increased
\
number of blades.
Reduction of the interior noise levels by means of cabin wall insulation
has been the subject of more active participation by the industry. The typical interior
84
-------
150
S
.t ^ 100
< c
•si
t- O
(0 _C
_D I—
E ^-
1 50
Single Engine, Over 3-Place
Single Engine, 1-3 Place
Multi Engine
Turbine/Jet
Other
1955
1960
1967
1975
1980
1985
Figure 2.3-7. Number of General Aviation Aircraft
with Projections for 1975, 1980 and 1985
85
-------
noise levels of general aviation aircraft lie in the range of 90 to 105 dB(A). Some
9 "•-
of the new models on the market have corresponding noise levels down to 85 dB(A),
a reduction of 5 to 20 dB of which 5 to 12 dB is due to improved cabin wall insulation.
The executive jet aircraft are typically much noisier than propeller-driven
airplanes, but they constitute such a small percentage of the total general aviation
fleet that their noise impact has generally been kept within bounds except at some air-
ports which have a much higher than average proportion of jet operations. However,
r
with the projected future growth in the number of executive jets, they may be expected
to cause noise problems at an increasing number of airports unless their noise levels are
reduced. The jet engines in use by the executive jet aircraft fleet have been developed
for military purposes, or as smaller versions of early jet engines for the commercial fleet.
Hence, they tend to be objectionably noisy. Only very recently has the general
aviation industry actively sought more advanced and quieter jet engines for the business
jets. An example of the noise reduction achieved by substituting an advanced tech-
nology engine (AiResearch TFE-731 turbofan) for an older type jet engine is presented
in Figure 2.3-8. This change will reduce the noise level generated by the Lear Jet
at the FAA certification position on takeoff from 96 EPNdB to less than 86 EPNdB.
Another example is provided by the Cessna Citation business jet, powered by Pratt &
Whitney JT15D turbofan engines. FAA certification figures for this aircraft show noise
levels of 76 EPNdB on takeoff and 88 EPNdB on approach at the FAR-36 measurement
positions.il These figures lie 17 and 14 EPNdB respectively, below the noise levels
stipulated by FAR-36. An equivalent noise reduction throughout the business jet fleet
would strongly reduce the potential noise impact of these aircraft.
With respect to the suppression of the sources of noise in general aviation
aircraft, the industry will, at least in the near future, continue to rely on the power-
plant and propeller manufacturers for further developments. These programs are dis-
cussed elsewhere in this report; propellers and the associated powerplants are evaluated
in the V/STOL Section, and the jet engine programs are discussed under Commercial
Aircraft.
86
-------
The general aviation industry's plans for further reduction in the interior
noise levels are formulated in terms of what the expected achievements are, rather
than as desirable objectives. Disregarding any possible significant reduction in the
i.
powerplant noise levels, an interior noise level of 75 dB(A) is considered possible
: 9
within the next 10 years. This will be achieved by means of improved cabin wall
lining materials and a more sophisticated evaluation of the critical noise transmission
paths. This level would represent a considerable improvement over the typical noise
levels in the current general aviation fleet, as shown in Table 2.3-1.
Table 2.3-1
9
Interior Noise Level Objectives
Interior Noise Levels - dB(A) Year
Typical Older Aircraft
in Current Fleet 90 - 105
Current' Production
Aircraft . 83-85 1971
Objective for Future
Aircraft Design 75 1981
2.3.5 Noise Reduction Potential
In order to assess the potential noise reduction in the general aviation
fleet, it is appropriate to establish specific noise reduction objectives. Figure 2.3-7
shows that by 1985 there may be 316 thousand general aviation aircraft operating
'8
within the United States. However, 58 percent of these are expected to be con-
centrated within the population hubs, where in many cases their noise characteristics
will be masked by commercial aircraft operations. The remainder will be distributed
throughout the suburban and rural areas served by approximately 11 thousand general
aviation airports. In the low population density rural and outer suburban areas, the
87
-------
Static Test at Takeoff Power
Noise Levels at 400 Feet
30'
90
60
120C
150
140 130 12i
110 '110 120 130 140
Exhaust
Approximate
Sound Pressure Level
in dB(A)
CJ610-6
TFE 731-2
Figure 2.3-8. Comparison of Noise Levels at Various Angles from
Engine at Approximately 3000 Ibs Thrust
-------
general aviation airports are generally located sufficiently far away from population
centers that no significant noise impact is expected, even with the noise levels generated
by the current type aircraft. The potential noise problem is thus predominantly associated
with the growth of aircraft operations at major suburban general aviation airports. Assum-
ing a normal suburban residential area, median daytime outdoor noise level (Leo) of
49 dB(A) and a typical minimum slant range of 500 feet, a single event maximum noise
level of 74 to 79 dB(A) may generally be considered acceptable. Figure 2.3-9 compares
this range of levels, extrapolated to 1000 feet distance, with the noise levels generated
4
by a variety of current general aviation propeller aircraft. Some light, single-engine
airplanes fall within the desired range, but generally a suppression of 5 to 15 dB will be
required to meet the suggested criterion. For the business jet aircraft, a suppression of
at least 15 dB will be required over that achieved with the current state-of-the-art, as
demonstrated by the Lear Jet with advanced technplogy turbofan engines (discussed in
Section 2.3.4).
In order to establish whether these noise reduction objectives are realistic,
t
propeller aircraft will first be considered. As discussed in the V/STOL Section, a
reduction in engine/exhaust noise of 13 dB is achievable with current technology.
Similarly, a realistic objective for propeller noise reduction is approximately 10 dB
over the next 5 years. Extrapolating these values to the 1980's, it appears that a maxi-
mum noise level objective of 68 to 73 dB(A) at 1000 feet for general aviation propeller
aircraft is achievable.
For business jet aircraft, the potential quiet airplane is evaluated by con-
sideration of the expected possible noise reduction in commercial jet aircraft. Extrapo-
lation of the potential noise levels of the commercial quiet jet engine to the size and
thrust required for the business jet aircraft powerplant yields a level of approximately
75 dB(A) at 1000 feet during takeoff operations, which is within 2 dB of the desired
result.
It must be emphasized that these noise reduction values refer to new air-
craft only. The future potential noise reductions are summarized in Table 2.3-2.
89
-------
Table 2.3-2
Potential General Aviation Aircraft Noise Reduction
Future Noise Levels
Noise Reduction at 1000 Feet
indB dB(A)
Propeller Aircraft 5-15 68-73
Executive Jet Aircraft
Near Term 13 85
Long Term 23 75
90
-------
95
90
85
CO
1
0 80
O
"s 75
'o
Z
70
65
60
/ Indicates Flyover
\ Indicates Landing Unprimed symbols
indicate takeoff
Symbol Aircraft Type
O 1 Eng. Piston
• 2 Eng . Piston
^ 1 Eng. Turboprop * * /^
A 2 Eng. Turboprop * /
- 00 / A f -
. •«>
O
•X.
0 • *'
Desirable
x Maximum
O 0 Noise Level -
Range
-
-
100
200 400 1000
Total Engine Horsepower — HP
2000
Figure 2.3-9. Noise Levels Generated by General Aviation Aircraft
91
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2.4 Highway Vehicles
2.4.1 Introduction
Highway vehicles include automobiles, trucks, buses, and maintenance
and utility vehicles. Motorcycles are treated in the section on Recreation Vehicles.
Traffic studies of highway vehicle usage in typical urban areas show that about 1600 to
2300 trips are made by automobile drivers and passengers every day for every 1000
people, while 200 to 400 truck trips are made for every 1000 people. Approximately
40 percent to 45 percent of the latter terminate in residential areas. This urban travel
represents about 52 percent of the estimated 3 billion highway vehicle miles traveled in
1970. The general characteristics, numbers, growth patterns, and typical noise levels
for highway vehicles are summarized in Figure 2.4-1. Significant factors relative to
each type of highway vehicle are summarized in the following paragraphs.
• Automobiles — Automobiles are the primary mode of transportation in
the United States and constitute the largest number of highway vehicles.
From 1950 to 1970, the number of automobiles in use has increased
from 36 million to 87 million; passenger cars traveled 1000 billion
miles in 1970. Automobile sales, including vehicles, equipment and
service, reached $92 billion in 1970. Approximately 5 million people
were employed by this industry.
• Trucks— The total number of trucks in use has increased from 8.2
million in 1950 to almost 19 million in 1970. Total truck miles in-
creased to 206.7 billion in 1969 from 90.5 billion in 1950. The
average annual mileage for all trucks is over 11,000 miles. A majority
of the total truck operating hours (194 billion) was in population
centers, 86 percent of the time in pickup and delivery service, and
the remainder in long haul service. Thirty-nine (39) percent of all
truck miles were on urban streets.
• Buses — Highway and city buses accounted for about 27 billion passen-
ger miles in 1970. Mileage has been on a slight decline for a number
92
-------
1
Automobiles
1
• Passenger Cars
• Sports & High
Performance
• Economy & Compact
• Imported
Highway
1
Trucks
1
• Light Trucks
• Medium
• Heavy Duty
Vehicles
1
Utility &
Maintenance
1
1
Buses
1
• Street Sweepers • Highway
• Garbage Compactors • City
• Tree Chippers • School
87,000,000
Numbers in Operation
19,000,000
Estimated 75,000
400,000
Growth of Number of Highway Vehicles
100
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50ft.
50ft.
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Figure 2.4-1. Characteristics of Highway Vehicles
93
-------
of years, and bus passengers now const!tute 4.2 percent of the com-
mercial total. Around 74 percent of the total of 400 thousand buses
are school buses and account for about one-half of the total mileage.
The combination of local and intercity bus lines have carried 5.8
billion passengers in 1970, for a passenger revenue of $2 billion, and
have employed 150 thousand people.
• Utility and Maintenance Vehicles — The three major types of vehicles
in this category selected for study are garbage compactors, street
sweepers, and brush and tree chippers. It is estimated that there are
approximately 75 thousand garbage compactors, street sweepers, and
tree and brush chippers in use in the major cities of the United States.
Garbage compactors and street sweepers generally operate 40 hours per
week. They usually begin operation by 6:00 a.m. and often extend to
Saturdays to meet pickup requirements.
2.4.2 Source Noise Characteristics
The noise levels produced by highway vehicles can be attributed to the
following three major noise generating systems:
• rolling stock; tires and gearing
• propulsion system: engine and related accessories
• aerodynamic and body
The noise levels produced by highway vehicles are generally dependent
upon vehicle speed, as illustrated for a number of different vehicle types in Figure
-) A 96-8
2.4-z.
Figure 2.4-3 illustrates the relative contribution of tire and engine noise
9 10
to the overall noise levels of automobiles and trucks at highway speeds. ' The
small difference between the 65 mph coast and cruise conditions for the automobile
indicates that its noise is generated primarily by the tires. In fact, tire noise for auto-
mobiles becomes a significant contribution to overall levels at around 35 mph. The
94
-------
90
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70
C
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50
Heavy Trucks
Highway Buses
Rangs
Passenger Cars
____ Mean Levels
i i i i i i i
0 To 20 30 40 50 60 70 80
Speed - Miles per Hour
Figure 2.4-2. Single Vehicle Noise Output as a Function of Vehicle Speed
95
-------
Diesel Truck
55 mph Cruise
Truck - Coasting
i i i
Auto
65 mph
50
IOO
1000
Frequency in Hertz
IOOOO
figure 2.4-3. Diesel Truck and Automobile Noise at Highway Speeds/
Cruise and Coasting (at 50 Feet)
-------
tire noise for trucks begins to become important in the high frequency portion of the
spectrum at speeds of 45 to 50 mph, although even at 55 mph, engine noise controls
the low frequency spectrum.
Tire noise levels vary by 7 to 10 dB, depending upon road surface compo-
sition and roughness. Another 5 to 7 dB variation may be expected for truck tires as a
function of axle load. In addition, significant variations in noise are found to be a
function of tread design and state of wear. At constant speed, these variations may
12-14
result in a 20 dB range in noise levels.
Figure 2.4-3 also identifies the segmenf of the noise spectra contributed
by the propulsion system. This contribution is further defined in Figure 2.4-4, which
compares the typical noise spectra produced by a heavy diesel truck and by an auto-
mobile, both under maximum acceleration at 35 mph. The noise characteristics of
propulsion systems may be classified as either acoustic noise radiating directly out of
the engine openings, or as noise produced by internal engine processes which then
radiate from the engine structure. Figure 2.4-5 illustrates the relative effect of
silencing on overall engine-generated noise attributable to these two classifications.'"
The unsilenced exhaust noise is seen to overshadow the total of the other noises by 10
to 15 dB in each octave over the entire audible range. With the exhaust silenced, in-
duction noise is observed to prevail at frequencies below 1000 Hz, whereas noises
radiated from the engine structure control the spectrum above 1000 Hz.
The third principal source of noise in highway vehicles includes aero-
dynamic turbulence and body rattles. It is generally felt that streamlined designs do
much to reduce the noise contributions of automobiles and buses at highway speeds;
however, application of aerodynamic styling to trucks is not considered practical due
to servicing requirements. Body rattles generally reflect the care and maintenance
the vehicle has received. These are mainly an annoying factor at low speeds in resi-
dential areas and can be controlled only by routine servicing of the vehicle and
careful loading of the truck and cargo space.
The following paragraphs provide a discussion of the characteristics of the
noise generated in trucks, automobiles, buses and maintenance vehicles. An analysis
97
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Figure 2i4-4. Typical Octave Band Spectra for Diesel-Truck
and Automobile (Full Throttle Acceleration at 35 mph
at 50 Feet)
98
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Figure 2.4-5. Octave Band Spectra of Diesel Engine Inlet and Exhaust Noise
Illustrating the Effects of Silencing for the Exhaust and the Intake
99
-------
of the major noise sources for trucks will be provided first, as the characteristics of
these sources are relevant to all types of highway vehicles. Additional detail on the
most significant of these noise sources is presented in Appendix C.
Trucks
Gasoline engines power 97.5 percent of the trucks in operation, and the
remaining 2.5 percent are powered by diesel engines. Diesel trucks are generally 8
17 18
to 10 dB noisier than gasoline powered trucks and 12 to 18 dB noisier than automobiles. '
The noise output of trucks increases with age, and about 60 percent of operating trucks
are more than 5 years old. This increase of noise with age is aggravated by the tendency
to overhaul trucks with replacement mufflers or recapped tires which generate higher
noise levels than the original equipment.
The major contributing subsources of truck noise include the exhaust,
cooling fan, engine mechanical noise, intake noise and tire/roadway noise. Figure
2.4-6 and Table 2.4-1 depict the relative contribution of these subsources to overall
noise levels, and Figure 2.4-6 presents a range of octave band spectra for typical
19-21
operating modes. Following is a discussion of each of these major
, 12-14,16, 18, 22-32
subsources.
• Exhaust — The noise levels generated by truck exhaust systems are
dependent on factors such as engine type, timing and valve duration,
induction system, muffler type, muffler size and location in the
exhaust system, pipe diameter, dual or single system, and engine
back-pressure. The actual noise-generating mechanism is created by
vibrating columns of gas at high pressure amplitudes which are pro-
duced by the opening of the exhaust valve. This noise is communi-
cated directly to the atmosphere. Additional exhaust noise is created
by the direct impingement of these released gases on the pipes and
muffler shell. The fundamental and harmonics of engine firing fre-
quency are the principal components of exhaust noise. At high engine
100
-------
Exhaust-
Aerodynamic Noise
Cooling Fan
Typical Range of
Spectra for
Normal Operating
Modes
SAE Maximum Noise
Acceleration to 35 mph
92 dB(A)
Highway Speed
(84 dB(A))
Cruise 75 dB(A)
50
100
1000
10000
Frequency in Hertz
90 .
IO
60
89
85
-
o
u
«• o
C-C
•— o
c •
-------
Table 2.4-1
DIESEL TRUCK NOISE COMPONENT CONTRIBUTIONS TO MAXIMUM
NOISE LEVELS AT 50 FEET FROM VEHICLE
Truck Examples
#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
#11
Contributing Subsource
Engine
Mechanical
81
85.5
83
85
83
81
82.5
85
.83
81
83.5
Exhaust
84
81
86
82
83
77
86
82
83
77
82.5
Intake
75
74
80
80
72
70
79
80
72
70
74
Cooling
Fan
82
81
81
83
78.5
82
82
83
78.5
82
78
Total Vehicle
Noise Level
dB(A)
88
87.5
89
89
87
85.5
89.5
89
87
85.5
87
102
-------
speeds, these individual frequency components are masked by a more
continuous spectrum created by the turbulence noise produced by the
flow of high velocity gases through the exhaust valve.
Cooling Fan— In nearly all applications involving water-cooled
engines, an axial flow type fan is used to draw cooling air through a
forward-mounted radiator. In many designs, fan noise approaches the
level of exhaust system noise and is generally considered an important
factor in reducing overall vehicle levels.
Generally, fan noise is directly related to fan speed. It has been
shown that fan noise increases at a rate of 2 dB per 100 rpm at speeds
between 1000 and 1500 rpm and at a rate of 1 dB per 100 rpm between
1500 and 2000 rpm. The noise output is also dependent upon tip speed
and configuration, blade design and spacing, and proximity of acces-
sories and other objects which affect airflow.
Intake— Induction system noise is created by the opening and closing
of the inlet valve, starting and stopping the air flow into the cylinders.
It is also markedly affected by the flow properties of the exhaust valve
and the exhaust system due to the fact that during the duration of in-
take and exhaust valve overlap, some exhaust noise is transmitted
through the intake. Intake noise of supercharged,blower-scavenged
and turbocharged engines is created by the air-compressing process.
It may be modified by resonant induction systems which can, under
certain conditions of engine speed and system length, amplify intake
noise levels. The intake noise increases with increasing load. For
diesel engines between no-load and full-load, this increase may range
from 10 to 15 dB, while gasoline engine intake noise may increase
from 20 to 25 dB.
Engine Noise— Engine-associated noise in internal combustion engines
is produced by the compression and subsequent combustion process
103
-------
which gives rise to severe gas forces on the pistons and to forces of
mechanical origin, such as those produced by piston-crank operation,
the valve-gear mechanism, arid various auxiliaries and their drives.
Both types of fluctuating forces produce mechanical vibrations of the
engine structure which in turn cause all components attached to the
engine to resdnate and radiate noise.
As previously noted, diesel engines are typically about 10 dB noisier
than gasoline engines. This difference results mainly from their different
mechanisms of ignition. Gasoline engines initiate combustion with a
spark from which the flame front gradually spreads throughout the com-
bustion chamber until the entire fuel/air charge is burnt. This yields a
smooth blending with the compression. The diesel engine, however,
relies on a much higher compression ratio to produce spontaneous com-
bustion which burns a large volume of fuel/air mixture rapidly. This
yields a much more severe and more rapid pressure rise in the cylinder,
causing more engine vibration for the diesel engine in comparison with
the gasoline engine.
Many efforts at quieting diesel engines are aimed at smoothing out this
abrupt pressure rise, either through prechamber combustion chamber
designs or turbocharging (which tends to reduce these abrupt pressure
rises). However, efforts at reducing diesel engine noise by smoothing
out cylinder pressure rises are only effective when combustion-excited
noise is greater than mechanical noise.
At constant speed, diesel engines show only slight reduction in noise,
with reduction in load due to the high compression pressure even under
no-load. Gasoline engines, however, show a substantial decrease in
noise output with decreasing load, due to throttling of the inlet which
yields a large reduction of compression pressure. Therefore, the '
change in noise level between no-load and full-load conditions is
104
-------
rarely more than 3 dB for a diesel engine, but can be as high as 10 dB
for gasoline engines. In addition, compression ignition in diesel
engines produces their characteristic ''knock" which is associated w,ith
a broad peak of noise in the frequency range from 800 to 2000 Hz.
Engine speed also affects engine noise output. At low speeds under
full load, the gasoline engine is quieter than the diesel; however, the
noise from, gasoline engines increases much more rapidly with increasing
•engine speed than from diesels (45 dB per tenfold increase in engine
speed versus 30 dB for diesels). Hence at high speed, the noise levels
of both diesel and gasoline engines are of the same order of magnitude
for the same horsepower.
Tires — Truck tire noise presents the major obstacle in limiting overall
vehicle noise at speeds above 50 mph, since at this speed tire noise
often becomes the dominant noise-producing source on heavy duty trucks.
Typical noise levels from truck tires at 50 mph range from 75 dB(A) for
"low noise" tread designs to over 90 dB(A) for "high noise, level" tires.
Figure 2.4-7 illustrates the noise output of various truck tire tread con-
figurations over the normal speed range of interest. The major offender
is the standard cross-bar design used by the vast majority of trucks on
their drive wheels. These tires may produce levels in the 80 to
85 dB(A) range when new, but their noise increases with wear as much
as 10 dB in the half-worn condition. This increase is attributable to a
change in the tread curvature resulting from wear. Cross-bar retreads
pose an even greater problem as their noise level can be as much as
95 dB(A) at 50 feet when operated at 55 mph in,the half-worn condition.
Despite their noise, cross-bar retreads are very popular for economical
reasons and each tire is recapped an average of two to three times.
They wear roughly twice as long as the continual rib automobile type
design tires and exhibit superior dry and wet traction performance.
105
-------
100
CN
8
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CO
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90
80
o
10
TO
0>
.
s
Q_
X
30
X^:
X
.-n
.
40 50
Speed, mph
60
New Retread
1/2 Worn Retread
1/2 Worn Crossbar
New Crossbar
New Rib
35535655
•i«^«0^M^0
New Rib
Passenger Car
(Typical)
Figure 2.4-7. Effect of Various Tires Mounted on the Drive Axle.
Loaded Single Chassis Vehicle Operating on
Concrete Road Surface (Levels at 50 Feet)
106
-------
Tire manufacturers state that recapped tires are generally much noisier
than are new tires because of tread design. Current new tread designs
are optimized on the basis of both traction and noise output. However,
most recap tire molds are 5 years old or more and do not reflect the
newer thinking in quiet tread designs such as randomized tread element
size and spacing variations. These older molds become a critical noise
factor when one considers that well over half the truck tires on the road
today are retreads.
Axle loading is also a major factor in the amount of noise generated by
tires. Retread tires exhibit the most predominant dependence upon load.
One example indicates a decrease of 15 dB resulting when load per tire
was reduced from 4500 to 1240 Ibs. The explanation is that with the
tire unloaded, the sides of the retread do not contact the road surface,
hence the cups in the tread cannot seal against the road surface and
compress small pockets of air.
New and half-worn cross-bar tires also produce more noise with in-
creasing load. The explanation follows that with increasing load, the
tread pattern is compressed, hence more of the load is carried on the
outer sections of the tread.
The rib type tire designs are generally independent of loading due to
their uniform tread design accross the tire cross-section.
Variations in road surface also significantly affect tire noise generation.
Here again, retread tires exhibit the most dependence on this variable,
with the most noise generated on smooth road surfaces. Differences
have been observed experimentally to be of the order of 8 dB at speeds
of40to50mph.
Automobiles
While not as noisy as trucks, buses and motorcycles, the total contribution
of automobiles to the noise environment is significant due to the very large number in
107
-------
operation. Approximately 70 percent of automobiles on the road in 1970 were over 3
years old, the average age being about 5-1/2 years. Vehicles over 2 years old tend
to produce higher noise levels (2 to 3 dB) under most operating conditions, due to
deterioration of exhaust silencer performance and the response of the vehicle to pave-
Q
ment roughness. Like trucks, the noise produced by individual automobiles is a
function of several subsource contributions — exhaust, cooling fan, intake, tires,
!
engine and transmission noise and aerodynamic noise.
Figure 2.4-8 illustrates the relative contributions of these major subsources
of noise to the overall noise levels and shows typical octave band spectra for various
f f\ J OO
automobile operating modes. ' ' Following is a discussion of each of these
subsources.12-14,21,24,34-36
I • , Exhaust — For most automobiles, exhaust noise constitutes the pre-
i I dominant noise source for normal operation at speeds below about
35 tq 45 mph, depending upon the condition and design of the
exhaust system. Above this speed range, in many cases tire noise
becomes equally significant. While exhaust noise does not create a
significant interior noise problem, certain objectionable periodic
I :
tones may be audible inside the car. '
• I Intake — Intake noise in automobiles constitutes a minor problem in
achieving current and projected automobile noise requirements, arid
the noise control, principles are well understood by automotive
engineers. Underhood space is sufficient to allow air cleaners large
enough to achieve adequate silencing with minimal air restriction.
• Fan Noise — In some cases, the intensity of fan noise is almost equal
with exhaust noise. The parameters which govern fan noise
generation are essentially the same as those related to trucks. More
work has been done in the passenger car area to reduce noise in the
passenger compartment, hence quiet fans have been desirable for some
time.
108
-------
Aerodynamic
Noise
Intake
Exhaust
100
I
V
0)
-------
• Tire Noise — Tire noise in passenger cars presents much less of a
problem than in trucks. The principal reason for this is that standard
automobile tires do not employ the cross-bar tread design. For com-
parative purposes, Figure 2.4-7 includes the noise characteristics of
a typical rib-type passenger car tire. As can be observed, its noise
level at 50 to 60 mph can be as much as 25 dB less than the worst truck
tires. Snow tires on automobiles are similar in design to truck tires
and produce high noise levels on the order of 85 dB(A) at highway
speeds. At highway speeds and at rated load, current automobile
tires produ'ce levels on the order of 65 to 75 dB(A). In most new auto-
mobiles, these are the controlling noise sources at the higher speeds.
• Engine Noise— Nearly all passenger cars utilize four-cycle gasoline
powered engines which for the most part (imported and compact vehicles
excepted) normally operate at a fraction of their rated horsepower out-
put. Consequently, engine mechanical noise is a minor problem to the
observer. In addition, automobile engines are well shielded on all
sides; therefore little noise is radiated directly out to the observer.
Most attention to engine/transmission noise is focused on reduction of
interior noise levels. Extensive noise attenuation treatment work is
i
conducted on the majority of U.S. cars to reduce engine noise trans-
mission into the passenger compartment.
Buses
Although trucks and buses share many basic design characteristics and some
common components, buses are generally quieter due to their increased packaging space,
which allows larger mufflers, and their enclosed engine compartment. Typical noise
spectra for buses at highway speeds are shown in Figure 2.4-9. At highway speeds,
passenger buses exhibit noise levels primarily in the range of 80 to 87 dB(A) at
50 feet, principally due to tire noise. One of the most annoying noises produced by
city buses is heard by the person standing at the curb while a bus pulls away. As the
110
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o
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0)
£
3
90
80
-o
I *>
1/1
c
o
CO
u
O
60
50
..-»'---
Standing Start Drive Away
Measured from Curb (4-6 feet).
(94 dB(A»
60-69 mph
(at 50 feet)
50-59 mph
(at 50 feet)
100
1000
5 I 0 000
Frequency in Hertz
Figure 2,4-9. Typical Octave Band Spectra for Highway Buses
111
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bus passes the person, its noise level increases until it reaches a maximum of well above
90 dB(A) as the engine intake grille passes. This noise has a startling effect because of
37
its sudden onset and very high level.
Utility and Maintenance Vehicles
Utility and maintenance vehicles share many common elements with trucks.
The chassis elements are essentially identical to heavy and medium trucks, hence the
noise output at most speeds is quite similar. The major distinction lies in the type of
auxiliary functions these vehicles perform. A typical octave band spectra is presented
in Figure 2.4-10 for a garbage compactor during the compacting operation. °
2.4.3 Environmental Noise Characteristics
Noise from vehicular traffic generally establishes the residual noise levels
(defined in Section 2.1) in most urban and suburban communities. This residual noise
level varies throughout the day, based on the average density of noise sources in a given
39
community. In the immediate vicinity of a major arterial or freeway, the noise level
is much higher. Its actual value is dependent upon traffic flow rate, average vehicle
speed, distance to the traffic lane and the ratio of trucks to automobiles on the highway.
For a typical 4-lane freeway, average daytime traffic flow rates can be of the order of
6 to 10 thousand vehicles per hour. For this condition, the median noise level beyond
100 feet from the flowing traffic is equivalent to a continuous line of noise sources.
Under this condition, the average noise level varies in the manner shown
in Figure 2.4-11. This level increases 3 dB for every doubling of traffic flow rate,
6 dB for every doubling of vehicle velocity, and decreases approximately 3 dB for
every doubling of distance from the freeway centerline. At distances of the order
of 500 to 1000 feet from the freeway, the decrease in noise level with distance
generally ceases, as the freeway traffic noise becomes equal to ambient level in the
neighborhood.
Superimposed on this median traffic noise level are the intrusive or single-
event noises from individual noisy trucks, cars and motorcycles. These are normally
112
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90
80
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15 to 25 dB above the residual noise levels on neighborhood streets. However, at the
high traffic flow rates typical for freeways, these individual single events are barely
distinguishable from the overall roar of the total traffic flow. During nighttime hours
on major interstate freeways, the percentage of trucks is often much higher than on
typical freeway systems, and truck noise dominates the traffic noise levels.
In a rural or "quiet" suburban community located well away from major
highways, the normal ambient is 10 to 15 dB lower than in urban areas, and the passby
of a noisy car will momentarily increase the noise level by as much as 40 dB above
39
the ambient (L0(~). A noise intrusion of similar magnitude can also be created by
garbage compactors and street sweepers that begin their rounds at 6:00 a.m.
Interior Noise Levels
Because most noise reduction in current automobiles has been created for
passenger comfort, a special discussion is warranted on the subject of interior noise
levels. Figure 2.4-12 shows a representative range of automobile interior noise
40 41
spectra at highway speeds. ' At the upper end of the range is a popular import.
while the lower end represents a medium-size standard domestic passenger car. The
noise levels in the smaller import tend to be higher because of less sound treatment in
the body, less resilient tires, and stiffer suspension systems.
Generally, the interior noise levels increase with speed, with the noise
of domestic passenger cars increasing at about 2.5 dB per 10 mph, while the noise in
sports cars and small imports increases at a higher rate— up to 5 dB per 10 mph. At
35 mph on an asphalt road, the typical interior noise levels range from 64 to 73 dB(A).
Typical noise levels at 60 mph inside automobiles at highway speeds range from 63 to
82 dB(A) on concrete with windows closed. Air conditioners add at least 5 dB to the
40 42 43
overall interior noise level, depending on operating mode and vehicle speed. ' '
Open windows generally increase noise levels 5 to 15 dB, depending on
the "closed window" noise level, aerodynamic design and the combination of windows
which are opened. A particularly annoying Helmholtz resonant condition can be
created in some vehicles by opening just one side window. Noise levels at this
115
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2
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C
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Domestic Sedan at 70 mph
(72 dB(A))
Popular Economy Import- at-
70 mph (82 dB(A))
Range of 15 cars
60
50
40
100
1000
Frequency in Hertz
I 0 000
Figure 2.4-12. Range of Typical Interior Noise Levels for Domestic
and Imported Passenger Cars at Highway Speeds
116
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resonance may be well in excess of 100 dB(A). This resonance usually occurs at a
specific speed and often may be stopped by opening an opposite window a very small
40
amount.
Buses, by virtue of their rear engine design and adequate allowance for
interior sound package treatment, provide generally acceptable interior noise levels
in the range of 72 to 80 dB(A). However, the interior noise in trucks ranges up to
100 dB(A) for the largest and noisiest trucks. These higher levels may be excessive
in terms of a potential hazzard of hearing loss.
2.4.4 Industry Efforts Toward Noise Reduction
The highway vehicle industry is strongly committed to the development of
vehicles intended for specific segments of the consumer public. Each vehicle model
is manufactured with a particular performance goal or overall image in mind. This
image ranges from a luxury vehicle, wherein a quiet car is desired by the consumer, to
a performance vehicle which generally exhibits as high a noise level as is legally
allowed to provide the consumer with a sense of power. Considerable technical effort
has been expended for many years to obtain the "proper sound" for each automobile
design. ,
At its infancy in the early 1900's, the automotive industry found it neces-
sary to equip its engines with mufflers when the noise of the "horseless carriage".
frightened horses on the road. Cities and towns began to require mufflers on cars in
the 1920's and the automobile muffler has improved significantly since that time.
Trucks, utility and maintenance vehicles, and buses are generally manu-
factured to individual customer specifications which place major emphasis on perfor-
mance, operating economy and initial cost. The customers in this industry often
associate noise with better economy and more power; hence there has been little cus-
tomer pressure to reduce truck noise, although individual cities and towns have begun
to demand quieter maintenance vehicles and buses. In the late 1950's, recognition
that exterior truck noise was causing problems led the Society of Automotive Engineers
(SAE) to develop a truck noise measurement standard and to recommend a maximum
117
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exterior loudness level of 125 sones at 50 feet. This standard, including the recom-
mended maximum level, was voluntarily adopted by the major producers of trucks and
resulted in a reduction in the noise of the larger trucks. More recently, this standard
has formed the basis for the measurement of truck noise by new state legislation and
regulations. The manufacturers are committed to meet the exterior noise goals of this
new state noise legislation. However, the accomplishment of this commitment is
greatly complicated by the fact that the new vehicle manufacturer faces a number of
differing noise laws and measurement standards throughout the country, and different
time deadlines for achieving various amounts of noise reduction. In general, manu-
facturers have been faced with very short time constraints and have been essentially
forced to exploit the "band-aid" type of problem solution, without having adequate
time to incorporate the new requirements into a basic redesign. This approach is
generally wasteful of effort and costly to the consumer. It is preferable for the manu-
facturer to have a single set of regulations which are technically and economically
achievable and which contain a time schedule compatible with the basic design, proto-
type, test and production tooling timeframe. This approach generally will achieve the
best overall design in respect to both vehicle performance and ultimate cost to the
consumer.
An additional factor which influences the industry commitment is pending
legislation in other areas of concern to manufacturers which include safety, emissions
and, of late in the trucking industry, horsepower/ton considerations which may greatly
affect powerplant and chassis designs.
The industry employs qualified noise control engineers who haye extensive
experience in solving all types of vehicle noise problems to satisfy market requirements.
They are geared to solve problems in new models within very tight schedule constraints
prior to start of production. Many companies incorporate large noise control staffs
which have at their disposal sophisticated laboratory facilities and computer assisted
analysis equipment. The analyses are highly refined and are geared toward problem
area definition and comparison of relative improvements in problem areas.
118
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Though most of the principles of noise generation in highway vehicles are
w«ll understood, incorporation of advanced acoustic technology proceeds slowly for a
number of reasons, the foremost being that the engineers are almost always dealing
with a basic design which is in production. Any new refinement to a specific model
may require modification to the original basic design and must be compatible with all
design; and production constraints.
A further consideration in the application of acoustic technology is that a
majority of the components in a motor vehicle are supplied by outside specialty product
vendors who do not have direct responsibility for the performance of the total end
product. The net result of this aspect is that many manufacturers are now compelled
to supervise the design of these auxiliary components or to produce many of them to
insure that the total system will be compatible in terms of function and desired acousti-
cal performance. A good example of this is the cooling system on heavy trucks, where-
in the entire cooling system must now be engineered by the vehicle manufacturer to
achieve adequate engine cooling, together with reduced transmission of engine
24 25 44
mechanical noise and reduced cooling fan noise. ' ' The increased require-
quirements for system design which tend to exceed the technical scope and capability
of the specialist vendors may lead to major changes in the historical purchasing
pattern of the entire industry.
One final aspect which impedes application of advanced acoustic tech-
nology is the high use factor associated with highway vehicles and the.very severe
economic/durability constraints on the manufacturer. Extensive and time-consuming
highway durability test programs always precede introduction of any modifications to
today's vehicles, as illustrated by the typical engineering/development/production
timing schedule shown in Figure 2.4-13.
2.4.5 Noise Reduction Potential
Figure 2.4-14 illustrates the present ranges of noise levels for highway
vehicles under both maximum noise conditions (SAE test method) and highway cruise
conditions. It summarizes noise reduction potentials deemed achievable in the near
119
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A Typical New Diesel Engine Design
and Development Program*
£
'>
Engineering Acti
Detail
Design
Single
Cylinder
Lab Tests
1
Further Lab
Development
Testing
Toolina
i
1
Engineering
Prototype
Development
and Test
i
_j
•™i
Durability
' Fleet Usage
i i
12
24
36
Months
48
60
Based essentially on minor modifications (such as displacement increase)
of an existing engine series.
Figure 2.4-13,. Typical Industry Production/Timing Schedule
120
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Current
Levels
I
Short-term
Potential
Long-term
Potential
I
Cfc
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D
90
80
70
60
SAE
Test Method
Highway
Speed
35 mph
Accel.
Highway
Speed
35 mph
Accel.
Highway
Speed
Normal
Operation
Trucks
Automobiles
City & Highway
Buses
Utility
- and
Maintenance
Figure 2.4-14. Effect of Potential Noise Reduction for Highway Vehicles
-------
future for existing vehicle concepts with current technology, and long term potentials
which should result from further research and development efforts. These noise reduction
potentials are based on an extensive analysis of the subsources of vehicle noise, and '
assume continuing advancement in the applicable noise reduction technology. For most
vehicles at highway speeds, the long term potential is limited by tire noise which is
inadequately understood at present. Further noise reduction, particularly at high speeds,
requires successful research and development efforts in tires. At low speeds, further
reduction may require considerable effort in advancement in engine design and muffler
technology, and for large vehicles possibly a change from the conventional reciprocating
engine to new devices such as the gas turbine for propulsive power. The following para-
graphs discuss current and projected noise reduction activities of the various segments of
the highway vehicle industry.
Trucks
Historically, many new trucks were sold without mufflers in their exhaust
system and with little or no attempt to minimize cooling fan and engine noise levels.
Such noise reduction simply was not in keeping with the customer's request for maximum
performance and economy of operation. However, heavy diesel trucks are now recog-
nized as the loudest single category of highway vehicles. A recent statistical study on
traffic noise shows the average noise level at highway speeds of tractor trailers to fall
in the 85 to 90 dB(A) range. Considerable effort has been expended on the part of
industry in attempting to quiet these machines. One particular program currently under-
way involves a joint effort between the California Division of Highways and the Inter-
44
national Harvester Corporation. Their goal is to silence a standard heavy-duty diesel-
powered vehicle as much as is feasible through application of current acoustic technology.
Their stated goal is 83 dB(A) at 50 feet, but they are attempting to achieve lower
levels. While program costs are not available, the project has been in progress for the
past 6 months and is expected to continue for another 3 to 6 months.
The average heavy diesel truck will probably run over 500,000 miles in its'
lifetime. Over this time period, many of the components will be replaced due to wear
122
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or be modified to meet individual operator needs. The net result of this long-term
usage is that after a year or two, the noise characteristics of many heavy trucks is
altered significantly. The widespread usage of retread tires and modified exhaust
13 35
systems contribute to even higher overall truck levels. '
Figure 2.4-15 illustrates the potential noise reduction of the major sub-
sources of truck noise. The potential for reduction of noise generated by these sub-
12-14, 16, 18, 22-24, 27, 29-31, 33, 44-47
sources is discussed below.
• Exhaust System — In achieving reductions in the noise produced by
heavy trucks, a foremost consideration must be the exhaust system.
The effect of adequate exhaust silencing treatment alone, under maxi-
mum noise output conditions, can provide a gross overall noise reduc-
tion of at least 10 to 15 dB, bringing the over 100 dB(A) unmuffled
offenders down to the 90 dB(A) range. It is considered that a
feasible goal for the near-term in exhaust noise for all trucks appears
to be in the range of 80 dB(A) measured at 50 feet, (In some instances
a power loss may result.)
The current state-of-the-art in muffler technology, which relies on
large muffler volumes to obtain adequate silencing with low back-
pressures, will allow approximately 18 to 20 dB attenuation through a
muffler alone. When greater reduction values are sought, noise
radiation from the pipe and muffler casing becomes a significant
factor. In one program, where greater exhaust noise reduction was
required, the exhaust pipe diameter was reduced from 4 inches to
3-1/2 inches, yielding a noise reduction of the order of 25 dB with
a typical diesel engine and muffler. This reduction in diameter, how-
ever, could lead to an increase in back-pressure of approximately
40 percent. Some turbo-charged diesel engines (exhaust turbine-
driven supercharger) may meet current legal noise restrictions without
the use of mufflers. These devices, like mufflers, extract energy
123
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90.
o
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70.
8
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SAE TEST METHOD_
at 35 mph
HIGHWAY
SPEED '
Current
Levels
Short-term
Potential
Levels
Long-term
Potential
Levels
Figure 2.4-15. Effect of Potential Noise Reduction for Diesel Trucks
-------
from the stream of exhaust gases. Further research into exhaust
system designs, or allowance for more muffler space in new truck
designs, could produce additional exhaust noise reduction for future
vehicles without drastically increasing engine back-pressure, although
in the interim some increased back-pressure and the associated power
loss may have to be accepted to achieve significantly reduced levels.
It would appear that exhaust levels in the 70 to 75 dB(A) range should
be feasible in the longer term.
Cooling Fan — The standard method of reducing fan noise is to utilize
a larger fan running at a slower speed to produce essentially the same
air flow. In many cases, this solution necessitates a larger radiator at
a definite cost and weight penalty. The extent to which this technique
may be applied is, of course, limited by the overall radiator size which
is of concern for driver visibility. Thermostatically-controlled fan
release clutches are also successful in greatly reducing fan noise, but
are only effective at highway cruising speeds where a sufficient cooling
air flow is provided by the vehicle speed.
A major consideration in the design of engine cooling fan systems is to
minimize the horsepower requirements of the fan itself, which consumes
from 5 to 11 percent of total engine horsepower. Larger fans, or
increased cooling capacity requirements resulting from application of/:,
engine shielding and enclosure will have a marked effect on fan horse-
power losses and hence performance and economy.
Substantial development is required in the area of total engine system
cooling and related heat transfer in order to provide a more refined
solution to this problem. Acoustic technology for reduction of fan
noise developed for the noise control of aircraft can be implemented;
however, additional applied research and experimentation will be
required before estimates of expected performance are possible.
125
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Certain manufacturers are now instituting internal research activities
aimed at development of new concepts in engine cooling. Based on
analysis of existing programs, fan noise levels in the low 70 dB(A)
range are a reasonable future expectation for low speed truck operation.
Intake — Silencers are readily available which achieve reduced levels by
utilization of a design which incorporates an expansion or plenum
chamber to reflect noise back toward the engine. The amount of
silencing achieved by these devices is a function of air cleaner size
and location in the induction system, the optimum being the center of
the air intake system. The frequency range of attenuation generally
depends on location and air cleaner length; larger air cleaners atten-
uate lower frequencies. The most effective air cleaner/silencer
designs currently available utilize an absorbent packed construction
for high frequency absorption and incorporate a Helmholtz resonator
into their designs for attenuation of frequencies below 600 Hz. Feasible
near-term potential for intake noise levels fall in the 70 to 75 dB(A)
range.
The major considerations in implementing these designs are packaging
the silencer and minimizing the amount of performance loss due to
increased restriction in air flow. One manufacturer suggests that
approximately 2.5, 3 and 8 percent power losses result from each
additional inch of mercury restriction for two-cycle blower scavenged
diesel, four-cycle naturally aspirated diesel and gasoline engines,
respectively. It is believed that further overall engine development
in this area will aid in reducing intake contribution to the 68 to
70 dB(A) range in the long term.
Engine Noise — Reducing the total mechanical and engine-generated
noise output is a critical problem facing truck manufacturers. Most
current efforts by U.S. industry in reducing engine noise have involved
126
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acoustic shielding and encapsulation of the engine and transmission.
These methods have met with little success, primarily due to engine
cooling problems and increased servicing costs. Reduction in the
diesel engine mechanical noise output appears limited to the general
range of 81 to 84 dB(A) measured at 50 feet (see Table 2.4-1).
Further, the current trend in engine design is to make power plants
lighter and to extract more power; this exaggerates the noise
problem.
Substantial research has been conducted by Priede in England on the
subject of engine design. His work has established that by certain
radical changes in design of the engine structure, engine noise levels
can be reduced by 10 dB. The effect of these changes has been
demonstrated in a research engine with resultant 7 to 8 dB noise
reduction. The techniques involved adding more crankshaft main
bearings to reduce crank vibration amplitudes and stiffen the engine
structure, reducing maximum combustion pressure, closer tolerance
to reduce piston slap and remounting accessories on the cylinder
head (because of its stiffness, the cylinder head exhibits low vibration
amplitudes and hence transmits little vibratory energy to accessories).
In addition, all valve covers and engine cover plates were heavily
damped and an isolated crankshaft pulley was used which incorporated
damping rubber between the hub and rim to reduce noise radiation.
The American manufacturers generally support Priede's work, but feel
at the present time these techniques are only minimally effective and
are presently impractical from cost and servicing standpoints. The
basic problem in implementing these concepts is one of proving
durability.
The research efforts of Priede and others could be the basis for a long-
term goal in engine noise levels to be in the 72 to 76 dB(A) range
127
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(based on reduction levels achieved with experimental engines).
The combined result of these noise reduction efforts for all component
sources is a potential reduction of total truck noise, measured under
current SAE maximum noise tests at 50 feet, to the 74 to 80 dB(A)
range in the longer term.
A consideration in assessing long-term goals for truck noise reduction
lies in the realm of incorporating power plants other than conventional
reciprocating internal combustion engines. Much effort has been
expended on the part of industry in attempting to utilize the gas turbine
engine effectively in heavy duty truck applications. The technology
exists for quieting turbine engines to a high degree of efficiency,
although widespread application of turbines in the next 10 years is
not anticipated unless significant breakthroughs in certain key design
areas occur. However, the gas turbine may eventually provide a
major breakthrough in truck engine noise reduction.
Tires — At speeds greater than 45 to 50 mph, total truck noise levels
are affected by tire noise. Obviously (from Figure 2.2.4-7), one
way to reduce these levels is to outlaw the present design cross-bar
design tires and not allow retreads of this design. This action would
probably reduce truck tire noise levels at highway speeds by as much
as 12 to 15 dB. However, as the cross-bar tires exhibit superior .
wear and traction characteristics over the alternative automobile-
type rib tire designs, this change might have a significant impact on
operating cost and safety. Therefore, it is reasonable to assume that
current levels reflect the maximum reduction that can be achieved
within economical and safety constraints with current technology.
Further research into tire noise generation and the parameters of tire
design is needed to achieve levels of 74 to 76 dB(A) at highway speed.
In addition, as has been pointed out in Section 2, tire/roadway noise
128
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is greatly influenced by pavement surface characteristics; consequently,
the burden of reducing tire noise levels should be studied jointly by
the tire manufacturers and those responsible for highway design.
Automobiles
Substantial noise reduction is currently incorporated into the majority of
automobiles. Much of this noise reduction is directed at reducing interior noise levels,
and successful industry efforts have been rewarded by increased sales of those vehicles
which emphasize quiet ride and passenger comfort. One automobile manufacturer has
advertised that —
"In the last 5 years, the noise level in American cities has risen over
20 percent. In the last 5 years, sales of the very quiet (manufacturer's
48
brand name) have risen over 160 percent."
This passenger car model, and other American passenger cars in the $3000 and up
category, typically exhibit interior noise levels at highway speeds on the order of 63
to 70 dB(A) with the windows up and the air-conditioner off, With the air-conditioner
on, the levels are usually increased by at least 5 dB. The automotive engineers who
develop air conditioning systems feel the customer associates air conditioning fan noise
with cooling — quiet air conditioning fans are not popular.
Studies of the exterior noise levels of passenger cars, measured under
various normal operating conditions along freeways, city streets and rural roads, show
the noise of the newest vehicles is slightly less than that of older vehicles. For
example, a recent statistical study, conducted by the California Highway Patrol,
obtained extensive noise data listed by manufacturer for models "1964 and earlier,"
and "1965 and later." In nearly all operational modes, the newer and older vehicles
exhibited the same statistical average noise level fora given operational mode. Also,
the vehicles of the various manufacturers exhibited identical average exterior levels.
An exception were Volkswagens of "1965 and later" which were 1 dB noisier than the
rest. (Volkswagen represents 55 percent of all imports on the road.) Subsequent
studies have been conducted which distinguish between "1968 and older" vehicles,
129
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and "1969 and newer." The newer cars in these studies average around 2 to 3 dB
o
quieter than earlier models under most operational modes.
Further silencing efforts in passenger cars, as in trucks, must be accom-
plished in the exhaust system. In general, incorporation of a dual muffler exhaust
system will yield more noise reduction than the more economical single exhaust system.
This is largely due to the principle relating exhaust silencing to muffler volume. Many
current 1971 model passenger cars now produce levels approaching 80 dB(A) at 50 feet
in the maximum noise tests when the test vehicle is fitted with a dual muffler system.
For one manufacturer, this system raises the price of the car by an estimated $30.00
33
over that of a car with a single exhaust system.
However, most major automobile manufacturers have stated that they will
be incorporating catalytic conversion muffler systems to meet the 1975 emissions
standards. It is anticipated that these systems will increase gas temperatures in the
exhaust system by a significant amount in many applications and hence necessitate
larger muffler volumes to achieve current noise levels. The use of the dual exhaust
systems mentioned above will now become considerably more expensive due to the
requirement of dual converters. Also, packaging of muffler units is a critical con-
sideration in automobile design, and in most cases the addition of extra mufflers would
.necessitate redesign of the vehicle underbody. This change will undoubtedly also
require the use of larger radiators, fan shrouding and larger fans. The net result will
probably be a requirement for a great amount of effort to maintain current fan and
,,.,', 24,33,34
exhaust noise levels.
As has been stated earlier, under normal operating modes the automobile
probably sets the majority of the ambient noise levels in communities. Hence, any
major.reduction in automobile noise will have a significant effect on the ambient noise
environment. It would appear that levels around 68 to 70 dB(A) under cruise con-
ditions at all legal speeds are potentially possible for automobiles. However, at 60
to 70 mph, the levels are highly influenced by tire noise, and hence cannot be achieved
without further research and development. Thus, less potential noise reduction is
130
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anticipated at highway speeds in comparison with that expected for 35 mph maximum
acceleration, as shown in Figure 2.4-14.
It is questionable whether or not the current SAE new car noise certification
49
test for vehicle noise is a totally reliable measure of automobile noise output, since
a very small percentage of actual driving time is spent at full throttle acceleration. It
is felt that to further reduce new vehicle noise levels, more attention must be paid
their normal operating modes and future noise legislation must be geared in this direction.
Buses
Most noise reduction in buses has resulted from the desire to provide more
passenger comfort. Buses utilize essentially the same propulsion systems as heavy trucks,
but by virtue of their designs, which allow for larger mufflers, quieter tires and
enclosed engines, are much less a noise problem.
As an example of silencing existing highway vehicles, a major manu-
facturer has developed a "retrofit" exhaust and noise emission reduction package for
diesel-powered buses. The package includes modified fuel injectors and a large and
rerouted exhaust muffler which now incorporates a reactor to provide further odor and
emission control. In addition, the package includes a more effective air-cleaner/
silencer unit and a modified engine mounting system which reduces noise by isolating
the engine from the bus chassis. This system will provide up to 10 dB reduction in noise
levels as well as providing significant reduction in exhaust emissions, smoke and odor.
The cost of this conversion is $373.00 when installed on new coaches; however, to
convert a used bus runs up to $1300.00 for materials, with an average of 160 manhours
33 50 51
required for installation. ' ' Clearly from this example, further effort into the
area of developing economical "retrofit" noise reduction packages for long-life vehicles
would appear to be feasible and warranted and not solely limited to bus applications
but heavy trucks as well.
It is believed that further efforts toward aerodynamic styling will aid in
reducing aerodynamic noise at highway speeds. Further reduction at highway speeds
will be dependent upon newly-designed "quiet" tires. It is estimated that the noise at
131
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50 feet from both city and highway buses can be reduced to levels of 74 to 76 dB(A)
under both acceleration and highway speed conditions in the long term.
Utility and Maintenance Vehicles
Utility and maintenance vehicles are a breed apart from the rest of high-
way vehicle types. The only common elements are their chassis and propulsion systems.
These vehicles are most often operated at low road speeds in lower gear ranges. As
many of these vehicles are diesel powered, they tend as a group to produce high noise
levels even at low speed. These vehicles are normally muffled, but little attention
has been paid to noise associated with the auxiliary functions they perform.
Certain manufacturers have developed quiet utility vehicles and market
them on a limited basis. One excellent example is the "quiet refuse truck" developed
by General Motors for the State of New York. In addition to larger mufflers and a
silenced air cleaner, numerous additional engine seals were utilized along with a
"quiet" cooling fan and "quiet" tread tires. The refuse packer itself was quieted by
isolating the hydraulic valves and lines, cushioning certain components and damping
the body panels. Typical noise levels at 50 feet were reduced from approximately
87 dB(A) during the packing cycle to 80 dB(A). It is estimated that these modifications
33 50 51 52
added about $3000 to the price of the complete unit.
Thus, auxiliary functions performed by these vehicles are amenable to
noise reduction treatments. It is estimated that the refuse packing function can be
reduced in noise level to the 76 to 78 dB(A) range in the medium to long term. The
noise levels of street sweepers and other similar function vehicles should also be able
to be reduced to a level of 70 to 75 dB(A) in the long term.
132
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2.5 Rail Systems
2.5.1 Introduction
Rail systems are used for a variety of applications, including long distance
freight and passenger trains, commuter trains and rapid transit trains. These applications
have required development of specialized vehicle systems which differ significantly in
their noise characteristics. In discussing the problem of noise in rail systems, it is con-
1-3
venient to consider the two following groups:
• Railroads — including locomotive-propelled freight, long distance
passenger and commuter trains, as well as high-speed intercity trains.
This industry reported $12 billion in operating revenues in 1970, and
employed 566 thousand trained personnel. Railroad passenger traffic
has steadily declined during the past 20 years to a figure of 283 million
passengers carried 11 billion revenue passenger miles in 1970, approxi-
mately one-third of those traveled in 1950. However, freight ton-
mi ies have increased during this period from 590 billion to 776 billion.
Manufacturers of railroad equipment made $2 billion worth of shipments
and employed 50 thousand people in 1970.
• Rail Rapid Transit Systems — including subway and elevated systems,
surface stretching railways and trolley lines. Intracity rail transit has
declined since 1950 from 907 million to 480 million revenue passenger
miles. This segment of the rail industry reported $1.7 billion in
operating revenue for 1970 and employed 138 thousand trained per-
sonnel. This system transported approximately 2.1 billion passengers
in 1970.
The characteristics of rail systems are summarized in Figure 2.5-1.
133
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Rai I Systems
1 1
Rai 1 roads
Rail Transit
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Shunting Operations
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Figure 2.5-1. Characteristics of Rail Systems
134
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2.5.2 Source Noise Characteristics
Railroads
Noise in railroad systems can be separated into the contributions of two
basic sources, the locomotives and the train vehicles which the locomotives haul. '
• Locomotives— The total number of locomotives in service in the
United States was slightly over 27 thousand at the beginning of 1971.
Of these, 99 percent were diesel-electric locomotives, and the
majority of the remainder were electric. Approximately one-half of
the locomotives are used for main line haulage and are generally
powered by engines of 1800 horsepower and greater. Lower powered
locomotives are used for short-haul trains and as switchers in the
railroad yards.
The major source of noise in this group is the diesel-electric loco-
motive. Typical noise levels under various load conditions and speeds
are shown in Figure 2.5-2. The propulsion system includes a diesel
engine, usually 8- to 16-cylinder, that drives an electrical generator.
This generator in turn provides power to traction motors on each axle
of the locomotive. The diesel engine is water-cooled and thus requires
a radiator and associated cooling fans, situated in the roof of;the loco-
motive. Dynamic braking is used to slow the locomotive and train at
higher speeds, and is accomplished by disconnecting the traction motors
from the main generator, using them as generators. The high electrical
currents that result are passed through heavy duty resistors which are
cooled with the use of separate fans in the roof of the locomotive.
The sources of noise in a moving diesel-electric locomotive are, in
approximate order of contribution to the overall noise level:
- diesel exhaust muffler
- diesel engine and surrounding casing, including the air intake and
turbocharger (if any)
- cooling fans
135
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Figure 2.5-2. Wayside Noise Levels and Spectra of Railroad Equipment
136
-------
- wheel/rail interaction
- electrical generator
An additional source of noise is the siren or horn, which produces noise
levels 10 to 20 dB greater than that 'from the other sources. This is not
a source that is operated continuously, however (30 times per hour on a
typical run), and is a necessary operational safety feature causing it to
be excluded from the above list.
The electric locomotive draws electrical power from a catenary. This
electrical power is converted for application to the traction motors by
means of transformer rectifiers and smoothing reactors. The braking is
similar to that described for the diesel-electric locomotive, with the
exception that blowers are used in place of fans. The major noise
sources from the electric locomotive are as follows:
; I
- cooling blowers
- wheel/rail interaction
- electric traction motors
The electric locomotive produces most noise when braking from high
speeds, the increase in noise over that of normal operation being due to
the operation of the dynamic brake resistor cooling blowers. Braking
from high speeds is normally an operation that is confined to rural
areas, so the noise impact is not severe. If this operation is ignored,
the electric locomotive is considerably quieter than its diesel-electric
counterpart, as shown in Figure 2.5-2.
Train Vehicles— The other main noise source associated with railroad
trains is that of the vehicles being hauled. Typical wayside noise
t
levels for freight and passenger cars are shown in Figure 2.5-2. Freight
and passenger cars have no propulsion system of their own, so that the
exterior noise produced is due mainly to the interaction between the
wheels and the rails. The magnitude of the noise depends heavily on
137
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the condition of the wheels and track, whether or not the track is
welded, and on the type of vehicle suspension. Modern passenger
vehicles with auxiliary hydraulic suspension systems in addition to the
normal springs can be 5 to 10 dB quieter than the older type with
springs alone. However, most freight cars have the simple spring
suspension. Additional noise can be produced by empty boxcars with
loose chains and vibrating sections.
The noise inside passenger vehicles is also partly due to the wheel/rail
interaction. Typical interior noise levels are shown in Figure 2.5-3.
This noise is produced in two ways. First, there is broadband noise due
to the inherent roughness both in the wheels and the rails. At high
speeds, variations on the order of a few thousandths of an inch are suf-
ficient to-produce high noise levels. Secondly, there is the impact of
the wheels as they pass over the rail joints, producing the familiar
"clickety-clack." There are two paths by which this track noise
reaches the passenger. First, there is the direct mechanical path from
the wheels through the suspension and hence to the car body. The
resulting vibration of the body radiates sound to the interior of the car.
Secondly, there is the airborne path from the track through the car
body and windows. This latter path becomes more important when the
train is passing through cuttings and tunnels. The introduction of the
welded track eliminates impact noise, leaving the broadband track
noise. At present, only about 10 percent of the nation's railroad tracks
are of the welded type, but the amount of welded track is being
increased at the rate of 3000 miles per year as the older sectional type
requires replacement. In addition to the track noise, interior passenger
car noise is created by the air conditioning system. This is the typical
broadband."rushing" noise emanating from the exit and return grilles,
usually in the roof of the car.
138
-------
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Passenger Cars
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Figure 2.5-3. Train Vehicle Interior Noise Levels and Spectra
139
-------
In suburban areas, manyiof the commuter trains consist of multiple-' -
unit electric car systems in which the motors on all cars in the train
may be operated from the lead car. Many of these systems consist of
modern, high-speed equipment in which noise level criteria were con-
sidered during the design and construction. If the wheels and track are
in good condition, the interior noise levels of these vehicles is often
dependent on the air conditioning system. Figure 2.5-3 shows the con-
1 tribution of track and air conditioning noise to the total noise level of •
a modern high-speed suburban rail car.
i One other major source of noise from railroad operations is produced
in retarder yards where freight trains are assembled. The individual
freight cars are allowed to roll along the selected track and are braked
automatically or manually before they strike the remainder of the train.
The braking mechanism consists of a steel rail that is pressed against
the wheel flange, producing a high-pitched sound at a level that can
exceed 120 dB(A) at 50 feet.
Rapid Transit Systems
At rhe beginning of 197], there were 15 rail rapid transit systems in the
United States. Of these, 7 were subway and elevated, 4 were solely surface and 4
provided inter-urban surface transportation. All of these rapid transit systems use
electric multiple-unit rail cars, designed for fast loading and unloading of passengers.
A minimum amount of seating is provided since the average trip length is between 3 and
5 miles. Consequently, in rush hours the number of passengers standing can easily
exceed those seated by a factor of three or greater. Ease of entrance and exit requires
many doors which are wide enough for these operations to be conducted simultaneously.
In addition, to obtain good general visibility, large-sized windows are utilized.
Efficient operation of a transit train also requires that the cars be lightweight so as to
reduce the overall load to be hauled, the time required for acceleration, and the
motor size and power. All these factors result in vehicles that are inferior to railroad
140
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passenger cars as far as acoustic insulation is concerned. Suspension systems universally
contain steel springs, additional cushioning being provided by either rubber pads or air
cushioning systems.
There presently exists a wide mix of vehicles in operation in terms of age
and condition. The older type of vehicles that still operate on all existing systems in
general,have a poorer suspension system than those more recently introduced. There is
also a definite requirement to use air-conditioned vehicles that allow all windows to be
permanently sealed; These improvements have enabled the modern vehicle to be a
significant improvement over the older type as far as noise and comfort are concerned.
The electrical power for rapid transit trains is collected by means of a shoe
from a third rail and is applied to traction motors, one for each axle of the vehicle.
The motors drive the axles through a gearing system. Most systems use compressed air
braking systems, the exception being the Chicago Transit Authority which uses all
electric braking.
In addition to the electrical power required for propulsion, power is also
required for door operation, lights, fans, heaters and a host of other utilities. Since
the power required for those utilities differs from the type picked up externally,
it is usual to include batteries together with a motor alternator to provide ac power
and a motor generator set to charge the batteries. The motor alternator,is used con-
tinuously, whereas the motor generator and air compressor work only when required.
Air conditioning is provided by means of fans and cooling systems. The lack of space
under the vehicle dictates that this system be small. This means a high pressure system
is required to obtain the necessary air flow, which in turn results in high interior noise
levels as the air passes through the vents. All the electrical motor systems are situated
underneath the vehicle and require a passage of forced air over them both for cooling
and dirt removal. Air fans or blowers are therefore required to provide the necessary
air flow, and these are often operated continuously.
The major noise sources associated with rapid transit systems are, in order
of their contribution to the overall level, as follows:
141
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• wheel/rail interaction
• propulsion system
• auxiliary equipment
Typical ranges of wayside noise levels from rapid transit vehicles, together with the
contribution from the various individual noise sources, are shown in Figure 2.5-4. '
The main source of noise is the interaction between the wheels and rails.
This source is more serious in rapid transit systems than in rail systems because the tracks
are subject to a much higher amount of wear. Unevenness in the track is produced by flat
spots in the vehicle wheels and by heavy braking as the train enters the station. Once this
unevenness is initiated, the track continues to deteriorate with further passage of trains.
;
Another wheel/rail interaction occurs at small radius curves in the track,
where the difference in speeds between wheels on the same axle and the rubbing action
of the wheel flange on the rail can produce a severe squeal. This source may increase
the normal track noise level by 10 dB or greater, the increase occurring mainly at dis-
* r - 15~]7
crete frequencies.
The noise from a rapid transit system is complicated, however, because of
the effect of elements not totally connected with the vehicles. First, there is the pro-
nounced effect of tunnels in subway systems. The surfaces of tunnels are hard and
acoustically very reflective. Hence, the noise from the sources outlined above is now
effectively being radiated into a reverberant enclosure. It is thus possible to obtain
much higher noise levels (as much as 10 dB greater) than those out of tunnels. This
effect is also found in below-ground subway stations which tend to be fairly reverberant.
Noise levels inside rapid transit rail vehicles above and below ground are shown in
_. 0 _ - 18-23
Figure 2.5-5.
Secondly, there is the effect of aerial structures where the track is supported
by concrete and/or steel frameworks above the surrounding city. The track on these
structures is less rigid than it would be at grade level on a solid foundation. Therefore,
noise levels 2 to 5 dB higher can be expected due to increased vibration not only of
the track but sometimes of the structure itself. In some aerial structures, there is a
142
-------
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Figure 2.5-5. Subway Noise Levels and Spectra
144
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direct airborne path from the underside of the train to the ground below. In these cases,
extremely high noise levels can be experienced.
Finally, there is the effect of different types of track systems. Although
reports vary on this subject, it appears that both the type of rail fastener used and the
type of trackbed are significant as far as wayside and interior noise are concerned.
For example, the highly reflective concrete trackbed produces higher exterior and
interior vehicle noise levels than does the tie and ballast which is less reflective.
Similarly, variations of up to 5 dB can be obtained by the use of different rail
12 24
fasteners. '
Street and trolley cars still operate in Boston, San Francisco and
Philadelphia and other cities, in some cases in a dual operation with subway systems.
External noise levels vary in the case of streetcars between the old and the new type
25
of PCC cars, the range being approximately 68 to 80 dB(A) at 50 feet under varying
operating conditions, as shown in Figure 2.5-4. Trolley cars are significantly quieter
in the absence of the wheel/rail noise, producing external levels in the order of
68 dB(A). Internal noise levels are similar in trolley cars and in the newer PCC type
of street cars, 77 to 80 dB(A), whereas in the few remaining old street cars the levels
are approximately 5 dB greater.
2.5.3 Environmental Noise Characteristics
The noise levels experienced by people who live in communities adjacent
to these systems depend upon the distance from the tracks as depicted in Figure 2.5-6
for various types of trains. In this figure, the majority of train types are included in a
single band of estimated noise levels varying with distance from the train. Rapid transit
trains tend to be in the lower half of this band, whereas locomotive-hauled trains
(diesel-electric) are in the upper half. The length of trains varies from as little as
150 feet in transit systems to over 3000 feet for freight trains. Consequently, the
duration of the noise for a single passby varies considerably from a few seconds up to
one minute and perhaps longer.
145
-------
100
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20
100
200
500
1000
2000
5000 10,000
Distance from Train — Feet
Figure 2.5-6. Noise Level as a Function of Distance
from Train for Railroad and Rapid Transit
146
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The noise levels experienced by people on-board the train or by persons
waiting at the station for the train to arrive are in the range 60 to 75 dB(A) on long
distance and intercity passenger trains, and 72 to 93 dB(A) on rapid transit systems.
Noise levels in subway stations are higher on some systems, lying within the range 76
to 96 dB(A). The range of levels in transit systems encompasses trains both above and
below ground under many varied conditions of operation.
Over 80 percent of the passengers using rail transit systems are carried on
the subway and elevated lines. The number of passengers in 1970 averaged 4.3 million
per day, the average trip length being 3 to 4 miles and the trip duration 0.2 hours.
On railroad systems — including commuters — 780 thousand passengers were carried per
day over an average trip length of 38 miles. The trip duration varies widely from
0.5 hours for commuter trains to several hours for intercity trains.
2.5.4 Industry Efforts in Noise Reduction
Rai I roads
The incorporation of noise-limiting requirements in the specifications for
new rail vehicles has only recently caused industry to initiate noise abatement programs.
Therefore, the majority of vehicles in operation today are not affected by these
programs. The only requirements that manufacturers must meet in the specifications
for locomotives concern the noise levels existing in the driver's cab. As far as wayside
noise from railroad equipment is concerned, a small number of programs have been
started and are at present in progress. These mainly concern the noise from diesel-
electric locomotives, but detailed information as to the possible outcome of the pro-
gram is not available at this time.
Diesel-electric locomotives have had little noise control applications other
than to the interior of the cab. The exhaust system has no muffler, and the spark
arrestor provides little attenuation. Since the exhaust is probably the major source of
noise, it is possible that mufflers could be designed that would reduce the overall
sound level. In addition, more substantial or modified casing around the diesel engine,
147
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together with acoustical absorbent material, may well be effective in reducing the
noise from this source.
More attention has been paid to the noise produced by the passenger
vehicles, both exterior and interior. The luxury-type railroad cars as well as the
more modern commuter cars hauled by locomotives are equipped with rubber isolation
pads and shock absorbers, in addition to the spring suspension systems common in the
older stock. The reduction in wayside noise level is on the order of 10 dB or greater.
As far as freight cars are concerned, improvements in functional performance over the
years has had the effect of reducing the noise level as a by-produ'ct. There are, how-
ever, no programs in existence for the control of noise from freight cars.
The modern high-speed, intercity trains such as the Metroliner and the
TurboTrain that travel at speeds around 100 mph have been designed to achieve interior
levels in the region of 70 to 74 dB(A) with air-conditioning equipment running. These
trains have extensive carpeting, improved door seals, smaller windows (Metroliner) and
acoustic insulation in th'e ceiling and wall structures. Wayside noise from the Turbo-
Train propulsion unit at operational power with the train stationary is 82 dB(A) at
26
50 feet. In addition, the modern suspension system incorporated in the TurboTrain
should result in lower interior noise levels'than in the conventional passenger train.
Rapid Transit
The development of specifications for rapid transit vehicles is complicated
by the division of responsibilities between the cognizant transit authority and the
manufacturer. For example, a typical present-day specification concerns noise levels
produced by propulsion units and auxiliary equipment with the vehicle stationary. It
does not include the noise produced by the wheel/rail interaction which in most cases
is the major contribution to the overall noise level. Nor does it take into account the
effect of tunnels upon the interior noise levels in the vehicles. These factors are the
responsibility of the Rapid Transit Authorities. Consequently, vehicles built to the
specifications but operated on tracks that are not maintained in a good condition may
therefore generate interior and exterior noise levels well in excess of those stated in
148
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the specifications. As a result, both the manufacturer and the customer (in this case
the Rapid Transit Authority) are required to pursue-separate programs to reduce the noise
levels.
Much of the work that has been conducted by transit authorities has been
on systems outside the United States. The result is that the transit systems in this
country tend to be amongst the noisiest in the world, as shown in Figure 2.5-7.
The quietest systems in the world are in Berlin, Hamburg and Toronto. It
is true, of course, that European countries in particular have placed and still do place
more reliance on rail transportation. It is therefore natural that research and develop-
ment would be of greater importance in these countries than in the United States,
where rail passenger travel is on the decline.
However, investigations have not been neglected in this country. The
Chicago Transit Authority (CTA) has conducted many experiments in an attempt to
achieve some reduction in noise levels. More recently, New York, San Francisco
and Washington, D.C. also have been particularly concerned with this problem, and
do plan improved systems for the future.
A number of noise abatement programs have been conducted in the past,
both by the equipment manufacturers and by the transit authorities. It was shown in
Section 2.5.2 that there is a wide range of noise levels associated with transit systems,
and that this exists because of the equipment used, the type of surroundings, and the
degree of track and vehicle maintenance. The programs conducted by the transit
authorities have been directed naturally enough toward the noise sources most important
for their individual systems. The conclusions that will be drawn will therefore reflect
what could be done now, using current technology, to reduce noise levels in rapid
transit systems. It is difficult, however, to state overall quantitative conclusions as
to the results of these programs because of the differences existing between systems.
The following review of noise abatement programs will treat each major source of
. , t .... .,, 8, 12-16, 20,27-29
noise separately, as tar as this is possible.
149
-------
O-J
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95
90
85
80
75
70
65
10
20
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nQn
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I I Foreign Subways
40
50
60
Transit Vehicle Speed — mph
Figure 2.5-7. Comparison of Interior Noise
Levels in Subway Transit Vehicles
150
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Wheel/Rail Interaction — The noise produced by the impact of the
wheels on the joints of sectional rail is the dominant noise problem
for almost all rail systems. The most successful approach to reducing
this noise has been the use of continuous welded rail. Reductions
on the order of 5 dB or greater can be obtained by this method. More
systems are now incorporating this type of rail during rail replacement.
The unevenness in the track in the form of corrugations that are the
major source of noise in rapid transit systems can be removed by
grinding. However, the track of some systems appears to be more
susceptible to corrugations than others due to the differences in rail
used and the variability in vehicle wheels and suspension.
In order to reduce the vibration of the wheels and car body when the
vehicle is operating on rough track, the possibility of resilient wheels
has been studied. The results have not been conclusive due partly to
the varying condition of the track in the different systems on which
resilient wheels have been tried. Of the three systems in the world —
Berlin, Hamburg and Toronto — that are considered to be the quietest,
one (Hamburg) incorporates resilient wheels, the others use the con-
ventional solid wheels. It has been confirmed, however, that the use
of resilient wheels does result in a reduction of low frequency ground
vibration. Other wheel treatments include the use of vibration damping
material, sometimes constrained, applied to the truck wheels. Measure-
ments of track noise from vehicles negotiating curves of various radii
have shown noise level reductions of the wheel squeal ranging from 5
to greater than 15 dB. The higher values of noise reduction are
usually determined by the noise levels in a narrow frequency band
covering the main frequency of squeal. On a straight track, the
reduction in wayside levels gt 50 feet are on the order of 2 dB. In
this case, wheel squeal is not evident and the small reduction in noise
151
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levels is obtained over a wide frequency range. At curves'of small
radius, attempts have been made to reduce the severe wheel squeal
by lubricating the track with water, oil or graphite. Such systems
have been fairly successful and have been installed at New York,
Cleveland, Chicago and elsewhere.
An interesting program conducted by the Chicago Transit Authority
involved the use of an experimental rubber rail head. This was quite
a successful program in reducing track noise, but was accompanied by
many practical complications and so was abandoned.
One method of reducing track noise that has been tried in a few cities
(Paris, Montreal and Mexico City) is to use rubber tired vehicles on a
concrete road bed. Reports on the effectiveness of these systems vary,
but the general opinion is that the reduction in noise levels is not
significant when compared with the noise of the more common steel
wheels on steel rails if these are in good condition. It may be con-
cluded, therefore, that in the absence of regular maintenance, rubber
tires may result in lower noise levels, although it is reported that they
require a great deal of maintenance effort. With welded track and
regular maintenance, there is little evidence to indicate the advantage
of rubber tires. Economically, rubber tire systems tend to be expensive,
since a separate guidance system is required as well as a backup system
of conventional steel wheels and track which is reverted to in the event
of a tire failure. There is, however, one exception to the above
generalization. The recently opened system in Mexico City is reported
to be one of the quietest in the world, and is considered to be better
than that of the other two rubber tire systems. One reason put forward
for the lower noise levels is the use of a ballasted track bed as opposed
to the concrete used in Paris and Montreal, but a final opinion will have
to wait until a noise measurement program has been conducted.
152
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There appears to be substantive noise data to support the use of
ballast between the rails. The alternative that is often employed is
a concrete slab which forms a good reflector of sound emanating from
the underfloor equipment of the vehicle and the wheel/rail inter-
action. Ballast provides more absorption and has been shown to
reduce interior noise levels by 3 to 4 dB, if structure-borne noise is
adequately controlled. A similar reduction in exterior noise level
may be expected if it is dominated by noise from the propulsion system
or auxiliaries.
Tunnels — The high reflectivity of tunnel surfaces coupled with the
enclosed space results in higher noise levels for a given source sound
power than it does in open space. The sound energy is confined to a
small volume instead of being able to propagate away in all directions.
A method of reducing the noise levels in tunnels is to apply acoustical
material on the surfaces of the tunnel so as to reduce the reflectivity.
This has been tried in Toronto with the result that the interior vehicle
noise levels were reduced by approximately 10 dB. Although this is
a solution for reducing noise, it is not necessarily feasible from an
economic point of view. For example, there are over 100 million
square feet of tunnel surface area in the New York subway system
which is estimated would cost over $150 million to coat with an
acpustic absorbent material. However, the cost is much less for under-
ground subway stations, which are extremely reverberant, and the use
of absorbent material can result in noise level reductions in the order
of 10 dB or more.
Vehicle Body — Noise reaches the vehicle interior by the transmission
of external airborne noise through the body work and by the trans-
mission of structure-borne vibration to the body work and its subsequent
radiation. An integrated approach is thus required if interior noise
153
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levels are to be reduced. Above ground, with no nearby reflecting
objects, most of the interior noise is radiated from the floor structure,
which provides a noise reduction in the range 20 to 30 dB in existing
vehicles. Eliminating and sealing holes and cracks in the floor and
installing a layer of damping material has been shown on New York
transit cars to reduce the interior levels in prototype cars by approxi-
mately 10 dB. The amount of reduction obviously is dependent on the
original condition of the floor.
A recent trend that substantially reduces interior noise levels is the
introduction of air conditioning systems in modern transit vehicles.
The older systems in general rely on open windows for ventilation,
resulting in interior noise levels as high as 95 dB(A) in some subway
trains. Closing the windows can result in a reduction of 10 to 15 dB
in interior noise levels, depending upon the situation.
Propulsion and Auxiliary Systems — The propulsion system in a rapid
transit car ranks second in the list of sources contributing to the over-
all noise level. This ranking, however, assumes that the wheels and
track are in fair to rough condition. If ground-welded track and
wheels are used, it is possible for the propulsion system noise to be of
greatest significance. Under these conditions, it is possible to achieve
lower wayside noise levels by using an acoustically treated electric
propulsion system with skewed armature slots and a force ventilated
cooling system. The reduction in noise level compared to existing
propulsion units having little noise control treatment is shown in
13
Figure 2.5-8. This figure applies to vehicles traveling close to
their maximum design speed. At lower speeds, the noise levels may
be lower than those indicated. Again, it must be emphasized that
the track should be welded and maintained in good condition for these
noise reductions to apply.
154
-------
95
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£ 85
80
75
70
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Propulsion
System
Modified
Propulsion
System
65
20 40 60 80
Transit Vehicle Speed — mph
100
Figure 2.5-8. Noise Levels at 50 Feet from
Steel Wheel Transit Vehicles on Tie and
Ballast Trackbed for Maximum
Vehicle Speed
155
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Electric propulsion units drive the wheels through gears, the gear
ratio varying from system to system, depending upon the power require-
ment. For a given vehicle speed, the resulting variation in motor rpm
among the various systems gives rise to wayside levels that vary as
much as 10 dB. High gear ratios are thus important as far as noise
from the propulsion system is concerned. The application of improved
or additional motor covers plus sound absorbing material, together
with acoustic treatment of the motor cooling fan ducts, can result in
a 6 dB reduction in noise level from motor units. The noise from the
cooling fans contains pure tones associated with the blade passage
frequency. Variable spacing of the fan blades makes these pure tones
less distinct and produces a subjectively less annoying sound, even
though the reduction in noise level is only 1 dB or so.
There are two main types of motor cooling systems — one that sucks air
(self-ventilating), and one that blows air through the motor. The latter
is preferable from a noise point of view, since noise control techniques
can be applied in the blower ducts. It does have the disadvantage,
however, that it remains in continual operation,, whereas the self-
ventilating type runs off the motor and hence is not operative in
stations. Because of the lack of space under the vehicles, it is not
usually possible to increase the size of the fans and have a lower flow.,
velocity with an accompanying reduction in noise level. The s.ame
comments apply to the cooling systems for the auxiliary equipment on
the vehicle.
Barriers — Since the major noise sources in a rapid transit vehicle are
situated underneath the vehicle body, one method of reducing the
wayside noise level that has been tried has been the installation of a
side barrier. The requirement for the design of the barrier is that it
should prevent a line-of-sight to the underside of the vehicle from
156
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locations where the noise reduction is required. A simple barrier of
thjs type, placed alongside the track and overlapping the vehicle
floor by about 6 inches, can provide a 10 to 12 dB reduction in noise
level at 50 feet.
An alternative to the installation of a barrier alongside the track,
which could be extremely expensive, is to place skirts on the sides of
the vehicles. However, there must be a clearance of a few inches at
the bottom so as to clear the track; so the noise reduction is only about
6 dB in this case. A combination of both types of barrier could result
in noise reductions in excess of the 10 to 12 dB for the wayside barrier
alone.
Even greater noise reductions (in the order of 15 dB at ground level)
can be obtained by placing the track in a cutting. The amount of the
reduction depends upon the depth of the cutting and the angle of
elevation of the sides.
2.5.5 Noise Reduction Potential
A summary of the effect that the application of current technology could
have on the noise levels produced by the various sources is given in Table 2.5-1. The
railroad and rapid transit authorities, together with the manufacturers of rail equipment,
are becoming increasingly aware of the noise problems associated with rail systems and
are planning a number of future programs for noise reduction. In most cases, however,
the programs are not defined in terms of final goals, but more to determine what
reductions can be achieved using current technology. The following programs are
among those that are planned:
Railroads
• A study of the noise characteristics of diesel-electric locomotives
with a view toward eventual noise reduction.
• The development of a new type of auxiliary generator of electrical
power or suburban, locomotive-propelled, commuter trains.
157
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Table 2.5-1
SUMMARY OF THE NOISE REDUCTION POTENTIAL BY APPLYING
CURRENT TECHNOLOGY TO EXISTING TRANSIT VEHICLES
Existing Condition
Standard track, not
regularly maintained
Concrete trackbed
Bare concrete tunnel
surfaces
Bare concrete station
surfaces
Old type vehicles
using open windows
or vents for
ventilation
Standard doors and
body
Standard steel
wheels
Standard type
vehicles
Standard, noisy pro-
pulsion unit
Modified Condition
Welded track, ground
Ballast trackbed
Strips of absorbent
material at wheel height
Limited absorbent
material on wall sur-
faces and under plat-
form overhang
New type cars with air-
conditioning
Improved door seals,
body gasket holes
plugged, et cetera
Steel wheels with con-
strained damping layer
Installation of a 4 ft.
barrier alongside track
Installation of a skirt on
side of vehicles
Modified unit with
skewed armature slots,
random blower fan
blade spacing, acousti-
cally treated fan ducts
Estimated Noise
Reduction dB
Car
Interior
5-15
0-5
5-10
-
10-15
0-5
5-15
: - ' •••
-
0-5
Car
Exterior
5-15
0
-
5-10
-
-
5-15
10-15
6
5
Note: The values of noise reduction are estimated for the particular source alone,
assuming no contributions from other sources. The values therefore cannot
be added to obtain an overall noise reduction.
158
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• Improved suspension system for the TurboTrain which, it is estimated,
30
may reduce interior noise levels from 74 dB(A) to 60 to 65 dB(A).
Due to the noise from the air-conditioning system, the noise reduction
obtained may be less than this. The final levels may be in the range
of 65 to 70 dB(A), depending on the position in the car, unless the
air-conditioning equipment noise is reduced.
• The replacement of old track by welded track. About 3 thousand
miles of track per year are renewed in this manner.
Transit Systems
• The application of spray-on acoustic absorbent material on the
ceilings and under the platform edges, together with noise barriers
between tracks at a New York subway station. This is intended as a
demonstration program that is estimated to provide 6 to 7 dB noise
reduction. The total cost of this experiment will be about $75 thousand.
• The replacement of old transit cars with more modern types incor-
porating air-conditioning, door and window seals, rubber suspension
mounts and vibration damping materials on the body. It is estimated
that a 10 dB reduction in interior noise levels will result. This is a
definite program in New York, Chicago and San Francisco, and is a
trend that is being followed by most transit authorities.
• The replacement of old track with welded track in many transit
systems.
• The New York City Transit Authority is replacing old track with a new
type incorporating a rubber rail pad. Previous tests have shown that
this provides a more comfortable ride and reduces interior noise levels.
• A study to determine whether improved sound insulation of transit
cars can be achieved without increasing the mass of the car body.
Along with this is a study to improve door seals.
159
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Design of an integrated heat transfer system for air-conditioning
equipment that uses cooling coils or fans that are operated while
the train is out of the station area.
160
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2.6 Ships
2.6.1 Introduction
The United States merchant fleet consists of approximately 2000 active
vessels of 1000 gross tons or greater. Of these vessels, about 180 are combination
passenger/cargo type, their average age being over 20 years. The number of ships
capable of transporting passengers has been decreasing since 1950, and in this time
only about seven new passenger/cargo ships have been completed by American ship-
yards. In 1971 the total number of passengers transported by sea from the United
States to foreign countries was 1 .7 million. Not all these people, however, traveled
on U.S. ships.
In recent years, the trend toward larger merchant ships constructed of
lighter materials has resulted in an increasing number of excessive shipboard noise
and vibration problems. Specifications for the construction of ships tend to be rather
loosely written, without specific performance requirements for the levels of noise
and vibration. This practice allows the delivery of ships without adequate noise
control, and often makes it difficult to determine the responsibility for any such
problems that arise.
2.6.2 Source Noise Characteristics
Of all the sources of noise in transportation systems, ships are probably
the least important in terms of an environmental impact on the community in general,
although noise problems may occur on board ship. There are three principal reasons
why ship noise does not impact the community:
• The major sources of noise on a ship are the engine, gears, and
propeller. This equipment is all below the water level and/or is
enclosed by the structure of the ship, and most of the sound energy
generated is radiated into the water.
161
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.•• As far as airborne noise radiation is concerned, the sources of noise
are the vibrating structure of the ship, the ventilation blowers, and
the engine exhaust (funnel) where applicable. However, the hull
vibrations are primarily at very low frequencies, and the noise from
air moving devices is generally controlled sufficiently to make the
noise levels on the deck acceptable for speech communication.
• The only time that a ship produces an appreciable wayside noise
level is when it is under full power which occurs only when the
vessel is out at sea. In ports, ships rarely exceed 5 knots, so wayside
noise is negligible except for horn blasts which are generally well
received by people living in port towns and cities.
345
The principal sources of shipboard noise are: ' '
• Propulsion System and Auxiliary Machinery — This includes gearboxes,
turbogenerators, stabilizers, etcetera. The propulsion motors operate
at a very low rotational speed compared to that of other transportation
systems and consequently, the noise produced by the majority of the
equipment is predominantly at the low frequencies. Gearboxes and
turbines produce noise at the higher frequencies due to gear-tooth
impact, and are audible in many of the cabins, particularly those
located inboard in the vicinity of the engine rooms.
• Ventilation Systems— This equipment produces broadband noise
typical of air conditioning and ventilating units, and is usually
more obtrusive in tourist sections than in first class.
• Movement of People — This is mainly impact noise produced by
people's footsteps on the deck above the observer. It is possible
for such impacts to propagate considerable distances as structure-
borne vibration.
162
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• Plumbing Noise — This is due to the passage of water through pipes
and faucets.
• Bulkhead Noise — The creaking of bulkheads with the movement of
the ship, perhaps caused by wave impact. The noise is due to relative
motion of the bulkhead panels and their supports.
In addition to these sources of noise, there are a number of sources of
structural vibration that can be radiated as airborne noise from walls and floors, '
including:
• Propeller — This is primarily a source of very low frequency vibration
that can produce rattles in loose objects in the aft part of the ship.
• Propulsion System — As discussed above.
• Wave Impact — This is more a random than periodic occurrence and
can be transmitted throughout the ship's structure.
The noise levels existing in a passenger ship (20 thousand to 25 thousand
3 6
gross tonnage) at normal cruise speed are given in Figure 2.6-1 . ' These vessels
are capable of carrying approximately 1000 passengers. There is a fairly wide spread
of levels corresponding to first and tourist class accommodations in various areas of
the ship. In general, the levels are higher on the lower decks than on the upper decks.
Little has been done toward changing the noise levels in cabins, except
for installing ventilation systems which have high speed airflow. There is, in fact,
a scarcity of data on the individual noise sources and the levels that they produce
throughout a typical commercial ship. Some of the problems, such as impact, plumbing
and bulkhead noise, could be reduced in magnitude by using similar techniques to
those used in buildings. Although it is possible to reduce the noise from air conditioning
systems using present technology, in many cases this steady state noise masks the inter-
mittent rattling and creaking of the structure which might be otherwise disturbing. In
addition, further reduction of the noise level might lead to a new requirement for
better transmission loss between cabins to recover adequate privacy.
163
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120
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Tourist- Class Cabins
100
65
1000
Frequency in Hertz
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10000
60
40
20
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Figure 2.6-1. Noise Levels and Spectra on Ships
164
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2.6.3 Environmental Noise Characteristics
As previously stated, the only environment which is significant in an
analysis of shipboard noise is the area within the ship itself. These levels, as shown
in Figure 2.6-1, are generally lower than 65 dB(A), and appear to have found general
passenger acceptance over the years.
165
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2.7 Recreation Vehicles
2.7.1 Introduction
Recreation vehicles, as defined herein, include pleasure boats, snow-
mobiles, all-terrain vehicles and motorcycles. There has been a remarkable growth
in the number of these vehicles in.the last 20 years. This growth is a reflection of
the greater amount of leisure time and availability of these vehicles at attractive
prices. Figure 2.7-1 summarizes the general characteristics of this category in terms
of growth patterns and typical noise levels. The following paragraphs discuss pertinent
1-4
aspects of the major vehicles in this category.
• Pleasure Boats — The pleasure boating industry has enjoyed a
relatively steady increase in sales over the past 20 years, from
2.8 million outboard motors in use in 1950, to around 7.2 million
in use in 1970. There are currently over 8.8 million recreational
boats in use in the United States. Of this number, 627 thousand
are inboard motorboats and 5.2 are outboard motorboats. The
boating industry estimates that over 44 million persons participated
in recreational boating in 1970, and that $3.4 billion were spent
on retail sales and services.
• Motorcycles— Motorcycles have experienced a remarkable increase
in popularity over the last 10 years. Over 90 percent of the
2.6 million motorcycles in the United States today are used
primarily for pleasure and are operated in many residential
and recreational areas. The number in use is expected to increase
to 9 million by 1985. Estimates for retail sales of new motorcycles
in 1970 reached $440 million and used motorcycle sales reached
$142 million. Parts and accessory sales amounted to $155 million
for an aggregate of $737 million in sales. More than 8 thousand
people were employed by motorcycle and parts manufacturers in 1970.
166
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Motorcycles
• Highway < 350 cc
• Highway > 350 cc
• Off Road
• Minicycles
2,600,000
Recreation Vehicles
Snowmobiles
Stock
Modified
Numbers in Service
1,600,000
Pleasure Boats
• Outboard
• Inboard
5,850,000
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Growth of Recreation Vehicles
1.6
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1950-60 -70
Typical Noise Levels
80
86
70
85
115
E'
SL:
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50 ft I
5.8
4.7
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Figure 2.7-1. Characteristics of Recreation Vehicles
167
-------
• Snowmobiles — This is one of the faster growing industries in the
leisure field. Over 600 thousand snowmobiles were sold in the
1970-71 season in the United States and Canada, as compared with
fewer than 10 thousand in the 1962-63 season. There are currently
about 1 .6 million snowmobiles in operation, the majority of which
are recreation vehicles. Persons who live on farms own 28.5 per-
cent of the snowmobiles. Many farmers and ranchers in the west
and midwest rely on snowmobiles for feeding and rescuing storm- t
stranded cattle. In addition, foresters and utility servicemen often
use these vehicles to make their rounds. Almost 80 percent of (the
people who own snowmobiles live in rural communities of 25 thousand
population or less. The average enthusiastic snowmobile owner rides
about 13 hours per week during the snow season. Approximate dollar
volume for the 1970-71 sales season has been estimated at $600 million.
2.7.2 Source Noise Characteristics
The noise output" of leisure vehicles, although dependent upon speed, is
primarily a function of the way they are operated. Though many off-road motorcycles
and some snowmobiles are capable of speeds of 80 to 100 mph, they are most often
operated .in the lower gears at medium to high engine output. Hence, except when
cruising at constant speeds or coasting downhill, these vehicles are operated at high
throttle settings and near their maximum noise output.
The major contributing source of noise from these vehicles is the exhaust.
A high percentage of these vehicles operate solely off-the-road and hence are not
licensed for highway use; therefore, many of the vehicles' exhaust systems are not
silenced. As a result, these vehicles may create noise levels as high as 100 to
110 dB(A) at 50 feet. ' Pending state legislation to regulate the noise produced
by off-road machines has caused manufacturers to reduce the noise of vehicles in
current production to 92 dB(A) at 50 feet. The noise radiated from intakes and engine
walls is also significant in these vehicles. Intakes are not generally silenced, and
engines are either partially or totally unshielded.
168
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The following discussions relate to the various types of vehicles that have
been categorized as recreation vehicles.
Pleasure Boats
In a recent survey, the maximum noise levels measured for a large number
8
of inboard and outboard powered pleasure boats ranged from 65 to 95 dB(A). The
lower limits of this'range are created by small outboard powered craft (usually 6 to
,-. 9 .
10 horsepower). In a different series of tests, levels exceeding 110 dB(A) at 50 feet
were produced by inboard powered ski boats with unmuffled (dry stack) exhausts. '
The typical range for noise levels produced by pleasure boats (by engine size and type)
is illustrated in Figure 2.7-2.
Exhausts are generally the principal source of noise for pleasure boats.
On the fdrger-engined ski boats, whose design incorporates a completely exposed
engine, intake noise and engine mechanical noise also provide a significant contri-
bution. As engine size is reduced, noise levels are typically lowered; however, in
most cases, even though exhaust is exited under water, it is still ;the major noise source.
In the medium and smaller outboard engine sizes, engine mechanical noise and intake
(though acoustically shielded),provide noise output almost equal to the exhaust.
Motorcycles
The noise produced by motorcycles operating under cruise conditions is
highly dependent on speed. ; Figure 2.7-3 depicts typical noise levels for various
operating modes. Figure 2.7-4 illustrates a typical range of octave band frequency
spectra for motorcycles under a variety of operating conditions. The relative contri-
butions of the various subsources'to the overall levels are also shown for a typical
example. The contribution of these subsources to the total noise levels are:
• Exhaust — The exhaust controls the noise levels of motorcycles. In
discussing exhaust system noise, a distinction must be made between
2-cycle (primarily imported) and 4-cycle machines. The noise
spectra are of somewhat different character, with the 2-cycle
169
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Engine
Noise
Intake
Exhaust-
Hull
120
110
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o
90
80
70
60
o
Q.
3
O
3
8
iS
Outboards
Inboard
Ski Boats
Other
Inboards
Type of Pleasure Craft
Figure 2,7-2. Typical Ranges of Noise Levels Produced by
Various Pleasure Boat Types (dB(A) at 50 Feet)
170
-------
100
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Figure 2.7-3. Motorcycle Noise Levels
for Various Operating Modes
171
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Intake
Tires
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Typical Operating Conditions
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Frequency in Hertz
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Figure 2.7-4. Motorcyle Noise
172
-------
machines exhibiting more high frequency spectra energy content
and the 4-cycle machines more low frequency content.
A major consideration in engine performance for 4-cycle motorcycle
engines, over a specific rpm range, is exhaust pipe length. These
machines must, by virtue of their design constraints, emphasize
lightweight, compact construction. These requirements are not
directly compatible with the basic principle of 4-cycle muffler
tuning which equates the degree of silencing to gross muffler volume.
i i
Performance and economy are directly affected by silencing, as these
machines rely on low backpressure to achieve competitive horsepower/
weight ratios.
Two-stroke machines present less of an exhaust silencing problem.
They are designed to incorporate an expansion chamber system (which
i
is considered mandatory for 2-cycle performance), in which much of
the acoustic energy is reflected back into the engine. This principle
is used to advantage in achieving a supercharging effect on the com-
bustion mixture as well as exhaust scavenging of the burned gases.
A well-designed 2-cycle exhaust muffler system will actually increase
power while at the same time reducing noise levels. This effect
is found in the majority of 2-cycle engine applications with the
exception of maximum output racing models.
Intake — Noise radiated through the intake system is almost equal to
the noise radiated through the exhaust system. Here again, performance
and packaging considerations have minimized any silencing efforts in
this area since both 2-cycle and 4-cycle designs rely on low intake
restriction to achieve their power output requirements.
173
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Engine Mechanical Noise — Engine mechanical noise is the source
of greatest concern in future reduction of overall levels. On current
machines, engine noise is approximately the same order of magnitude
as intake noise. The concept of acoustic engine enclosures and
shielding has been considered almost totally impractical for light-
weight air cooled motorcycles.
Drive Chain and Tire Noise — Noise levels from these sources appear
to be low enough to be considered of secondary importance. However,
refinements to drive chain design may be warranted when contributions
from other sources are reduced by at least 10 dB.
Snowmobiles
The noise produced by snowmobiles is highly dependent upon theirage.
Current production models produce noise levels in the range of 77 to 86 dB(A) under
maximum noise conditions measured at 50 feet and 105 to 111 dB(A) at the operator
916
position. ' i The noise levels from poorly muffled machines generally range from
90 to 95 dB(A) at 50 feet with racing machines causing levels as high as 105 to
110 dB(A). ' The operator, on a number of machines surveyed, experienced levels
t
in the range of 108 dB(A) under normal cruise conditions. Figure 2.7-5 shows typical
octave band spectra for snowmobiles for a variety of operating modes, and presents a
bar chart summary of those components which contribute to the overall noise levels. ' '
The major contributors are: i
• Exhaust — A dominant source of snowmobile noise is the engine
exhaust. Design constraints which minimize space and emphasize
lightweight construction, and customer demands for maximum power
have restricted the usage of adequate silencing devices.
• Engine Mechanical Noise — Another major factor in overall noise
output of snowmobiles is engine mechanical noise. The lightweight,
2-cycle, high power design of the snowmobile power plants restricts
174
-------
Engine
Intake/ Track NJogies
Range of Spectra
Over
Typical Operating Conditions
at 50 ft
1000
Frequency in Hertz
90 ,-
85
88
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Figure 2.7-5. Snowmobile Noise
175
-------
the application of quieting techniques to the internal engine structure,
and cowling enclosures provide the only suitable and practical means
for reducing engine noise.
• Intake — Most current snowmobile manufacturers do not silence the
engine intake. Unfortunately, the intake is usually directed ahead
of the operator and contributes significantly to his noise exposure.
Some sacrifice in engine performance may be required to silence the
intake system. However, little work has been done in this area,
although some manufacturers are now producing accessory air-cleaner
units which aid in reducing this problem.
Dune Buggies, ATV's (All Terrain Vehicles) and Other Off-Road Vehicles
The principal noise output of the remainder of those vehicles considered
under the "recreation" classification is predominantly from the exhaust. Because of
the unregulated nature of these vehicles and their use, the owners tend to attempt to
achieve maximum power output through the use of tuned and straight-through exhaust
(unmuffled) systems. An example of typical spectra for a VW-powered dune buggy
20
with a tuned "megaphone" exhaust system is presented in Figure 2.7-6. Engine and
intake noise are also quite apparent in these vehicles,.but are on the order of 15 to
20 dB less significant than the exhaust.
2.7.3 Environmental Noise Characteristics
Except when several recreational vehicles are operating semi-continuously
around motor recreation parks and high usage lakes, they provide only a minor contri-
bution to the steady-state residual noise levels in the areas in which they operate.
However, since the majority of these vehicles are operated in remote areas which have
low residual noise levels, they can be heard as intrusive noises at much greater distances
21
than would be expected in an urban area.
Power boats are operated (by law) at least 100 feet from shore and usually
well away from other boats, hence minimizing the levels at the shore and local communrty.
176
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z
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-------
Mini-bikes/ a particularly annoying noise source in residential communi-
ties, are normally produced with a muffler which reduces their noise levels at 50 feet
1 ^ 99
to the 75 to 80 dB(A) range. ' The problems that arise from the usage of these
machines (primarily by youngsters not old enough to obtain driver's licenses) stem
mainly from their operation in the proximity of residential dwellings. The problem is
further aggravated when the stock muffler is removed and replaced by an "expansion
chamber" exhaust system which the owners feel contributes to the power. The modi-
fied machines are then capable of levels of 85 to 90 dB(A) at 50 feet.2
The operator of most types of recreation vehicles is usually exposed to
high noise levels for the duration of his ride. Typical levels for snowmobiles range to
as high as 115 dB(A) under full throttle acceleration. Under cruise condition, the
5918
operator's noise level is often in the vicinity of 108 dB(A). ' ' It is estimated that
the average enthusiastic snowmobile owner uses his vehicle about 13 hours per week
3
during the snow season. The average duration per ride will probably range from 3 to
4 hours. It is assumed that this usage pattern is fairly typical for other types of recre-
ation vehicles, including watercraft and motorcycles (90 percent of which are estimated
to be pleasure vehicles).
The noise levels in outboard motorboats are also generally high. Typical
levels range from 84 dB(A) for 6 horsepower units to 98 to 105 dB(A) for 125 horsepower
9
units measured at the driver position under accelerating conditions. At cruising speeds,
23
operator levels on all boat types (inboard and outboard) range from 73 to 96 dB(A).
Operator levels on motorcycles also follow this trend of typically high levels with
13
115 dB(A) occurring on some unmuffled off-road cycles.
A factor which should be considered in discussing operator noise exposure
is the use of safety helmets. When properly fitted and used, they provide a significant
reduction in noise levels at the operator's ear, as well as providing accident protection.
There is no question that snowmobiles, many motorcycles, and some boats present a
risk of permanent hearing damage to both operator and any passenger. Ear protective
devices should be worn in these cases.
178
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2.7.4 Noise Reduction r- industry Efforts and Potential
Figure 2.7-7 illustrates the present ranges of noise levels for recreation
vehicles at both the observer at 50 feet and the operator positions. Also summarized
in this figure are the near-term noise reduction potentials deemed achievable with
current technology and the long-term noise reduction potentials which must result
from further research and development efforts.
The recreation vehicle industries have incorporated some rather refined
concepts into their products to achieve current noise levels. The greatest noise reduc-
tion has been accomplished through exhaust system treatment. Because nearly all
snowmobiles, outboard engines, and a good percentage of motorcycles are powered by
2-stroke engines, a good deal of development and research has been done in quieting
the exhaust systems on these devices. The expansion chamber exhaust system, which is
considered essential for 2-stroke performance, has been muffled to a high degree with
little loss of horsepower. ' Engine shielding and isolation have been developed to
a great extent on outboard motors and this technology is gradually being applied to
snowmobiles. Excluding motorcycles, the industry as a whole has nearly reached the
stage where exhaust treatment has been fully exploited, leaving further reduc-
tion efforts to be aimed towards intake silencing and engine noise itself. However,
the motorcycle has yet to overcome its design constraints in packaging exhaust systems
of sufficient size to provide greatly improved silencing; therefore, further research is
required to achieve adquate silencing without imposing severe weight and size restrictions.
In the following paragraphs, current industry efforts in noise reduction and
noise reduction potential will be discussed separately for pleasure boats, motorcycles,
and snowmobiles.
Pleasure Boats
The outboard motorboat has the longest history of any of the products in
the leisure vehicle field. The annoyance caused by noise from outboard motors was
recognized by industry long before any legislative bodies began to act to control its
effect. In the"late 1920's and early 1930's, manufacturers motivated by public pressure
179
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Figure 2.7-7. Potential Noise Reduction for Recreational Vehicles
-------
began experimenting with underwater exhaust systems to reduce the noise output of
these devices. Their success in the late 1940's was one of the factors which led to a
dramatic growth market for motorboats. By the mid-1950's, more sophisticated quieting
techniques were being incorporated, such as extensive vibration isolation within the
25 26
engine and acoustically treated cowling on the engine. ' The outboard engine
has been continually refined up to its present state. The current outboard probably
represents the quietest application of a 2-stroke engine for its power output on the
market today.
The largest manufacturer of outboard engines produces a top-of-the-line
125-horsepower engine that produces maximum noise levels at 50 feet of 81.5 dB(A).
The quietest model is rated at 6 horsepower and produces maximum noise levels of
9
64.5 dB(A) at 50 feet. This same manufacturer feels that because of the company's
efforts in producing quiet outboards, its percentage of the market has increased
27
substantially until it is now the leader in outboard sales. The major areas of
complaint concerning pleasure boat noise are created by the large inboard-drive ski
boats which incorporate dry stack exhausts (unmuffled and not exited under water).
In addition, many inboard ski boats also incorporate the automobile "hot rod" tech-
niques in achieving maximum horsepower from their engines. The engine is fully
exposed, and in addition to unsilenced exhausts, usually has unsilenced carburetor
intake as well. These machines produce noise levels at 50 feet of up to 112 dB(A).
Noise output from the same configuration, with underwater exhausts, has been reduced
to around 97 dB(A). Many states are now moving to prohibit operation of these dry
stack boats.
More refined inboard designs incorporate a silenced intake system and an
acoustically treated full engine enclosure along with the underwater exhaust mentioned
above. This type of ski boat will exhibit noise levels in the 85 to 90 dB(A) range at
50 feet. Smaller engined inboard boats will fall in the 75 to 80 dB(A) noise level
8,9
category.
181
-------
For pleasure boats, significant future noise reduction efforts should be
primarily aimed at further reducing operator noise exposure levels. Crash helmets
are seldom used by participants, except during race events, hence the noise levels
in these pleasure craft must receive more attention.
Significant noise reduction can be accomplished in inboard designs due
to the rather advanced state of acoustic enclosure design for items of this size. It
is felt that for the majority of inboard designs, a long-term goal of 76 to 82 dB(A)
is reasonable. Outboard engines (whose reduction potential is indicated in Figure 2.7-7
for models over 25 horsepower) pose a more difficult problem due to their design con-
straints which emphasize high power-to-weight ratios. It is expected that lower
operator levels for outboard powered craft will only come through further efforts in
intake silencing and either through revised internal engine design or bulkier engine
enclosures. For outboard powered boats, an examination of current abatement tech-
nology indicates that operator noise levels in the range of 78 to 86 dB(A) constitute
a reasonable long-term potential. Further, as a result of efforts to reduce operator
noise exposure, non-participant levels at 50 feet should eventually be reduced to the
range of 70 to 76 dB(A).
Motorcycles
The motorcycle also has a long history in the leisure field. Motorcycles,
due to their design constraints of lightweight construction and maximum power output
for a given displacement engine, have long been criticized for their excessive noise.
The average motorcycle rider tends to associate noise with power and performance,
and generally feels it fits the motorcycle "image". The major manufacturers have
only recently taken steps to change these beliefs. Now all current production motor-
cycles intended for highway use must comply with state noise legislation. In addition,
most major manufacturers, under the guidance of the Motorcycle Industry Council,
have agreed to place mufflers on all their off-road motorcycles to limit their noise
output to 92 dB(A) at 50 feet. The industry is currently in the process of trying to
convince the consumer that noise does not necessarily mean power. It feels that
182
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this is an essential step in preparing the consumer to accept the quieter, new generation
machines that will, necessarily, weigh somewhat more and deliver less horsepower per
cubic inch displacement.
The noise levels of current production motorcycles cover a fairly wide
range among different manufacturers and among vehicles of varying engine displace-
28 29
ment. ' The majority of motorcycles are now meeting the 88 dB(A) maximum noise
specification of various states; however, a number of the large displacement machines
29
are unable to meet this criteria in their present designs. Although the technology
exists to produce quieter motorcycles, achieving further noise reductions will necessitate
some design compromises on a majority of the models.
The exhaust system is the major contributor to overall noise levels. Although
exhaust systems .can be designed to reduce this component's contribution to the 75 dB(A)
14
range, significant packaging and weight limitations must be overcome. Also, current
motorcycles do very little to silence their intake systems, although almost all provide
air cleaner devices. Silencing on the order of 10 dB is feasible if moderate restriction
of intake air flow can be tolerated.
The most critical area yet to be tackled in motorcycle silencing is the
engine and mechanical noise. Acoustic enclosures have not been found to be practical
solutions on air-cooled engines. A number of attempts have been made at silencing
individual engine noise sources, such as adding damping compound to timing gears,
stiffening primary chain covers, positive oil feed lubrication of cam shaft bearings,
and adding cross ties to the engine cooling fins. This attempt by one manufacturer
12
yielded only an average reduction of 1.2 dB.
Achieving the more restrictive noise level requirements for motorcycles
that are forecast for the next 5 years will require major redesign of numerous compo-
nents. Specific examples of solutions that may yield beneficial results include
incorporation of journal rather than roller or ball bearings, timing chains rather
than gears, more lubrication, stiffer structures and nonresonating materials for non-
functional components. With these changes will undoubtedly come an unwelcome
183
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power loss. For example/ one manufacturer reduced engine noise levels in laboratory
12
experiments to 75 dB(A)/ but with a 15 percent power loss. Cost and weight penalty
figures are not available for this example.
Figure 2.7-8 gives the spectra of two 750 cc 4-cycle motorcycles of
different manufacture, but tested under identical conditions. The difference in the
29 30
noise levels produced by the two vehicles is 11 dB. ' The price of the quiet motor-
cycle is $1848 as compared to $1595 for the noisy machine. The quiet vehicle weighs
i
440 pounds versus 480 pounds for the noisy model. The relative horsepower ratings are
57 horsepower at 6400 rpm for the quiet machine as compared to 67 horsepower at
8000 rpm for the noisier vehicle. This example illustrates the compromises with which
the industry and the consumer are faced in achieving reduced noise levels with current
technology.
Motorcycles potentially face severe design modifications if their intruding
effect upon the ambient noise environment is to be significantly reduced. Redesign of
internal engine structure to provide the noise reduction achieved in laboratory experi-
ments may be required to achieve a long-term potential of 75 to 80 dB(A) at 50 feet
under maximum noise conditions. Additional attention must be given the engine intake
system to reach these levels. It is assumed that technology will advance sufficiently
to provide quieter intake and exhaust systems with minimized power loss and reduced
package space requirements.
Operator levels should be reduced to the 85 to 90 dB(A) range as a result
of the modifications listed above. Here again, the use of protective crash helmets
would serve to greatly reduce the risk of high operator noise exposure.
Snowmobiles
Snowmobiles are a relative newcomer on the leisure vehicle scene. Since
their introduction in 1958 as a low powered, lightweight, go-anywhere-?n-the-snow-
type vehicle, they have evolved into a more refined family-type recreation vehicle.
The original concept called for a minimum of weight coupled with maximum perform-
ance for the engine size. Hence, the original snowmobiles possessed unshrouded
184
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Comparison of Two Current Production 750 cc Four-Stroke Motorcycles
Tested Under Identical Operating Conditions
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Figure 2;7-8. Examples of Demonstrated Further Noise Reduction
185
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engines and unmuffled (or poorly muffled) exhausts. Their rapid rise in popularity
led to numerous consumer complaints about their excessive noise. As more vehicles
were produced, consumers demanded higher and higher horsepower outputs until today,
some snowmobiles are capable of nearly 100 mph. Their effect on the noise environ-
ment has been compounded in many cases by the fact that some owners remove the
factory-installed muffler systems in an attempt to achieve more power. In most cases,
this actually results in less power and considerably greater noise.
The noise levels of 1971 models at 50 feet generally range from 15 to 20 dB
less than the noise of 1961 models. This reduction clearly indicates the manufacturers'
concern for the problem, and is impressive, particularly since prior to June 30, 1970,
there were no effective snowmobile noise regulations in effect. Minnesota was the
first state to require that the noise level of snowmobiles not exceed 86 dB(A) at 50 feet.
Most of this reduction has resulted from improved exhaust systems which actually
. ,., , , 14
improve engine lite and pertormance.
Exhaust treatments are currently available which utilize an expansion
24
chamber incorporated into a tuned silencer system. With this design, much of the
acoustic energy is reflected back to the exhaust port, where it acts to supercharge the
mixture. This configuration also creates a negative pressure pulse at the exhaust port
to scavenge the spent gases. Such systems are more effective than straight pipes or
mufflers alone, both for noise suppression and power output.
Another consideration in muffler design is to place the exhaust exit away
from the operator to reduce his noise exposure. Exhaust exits may be directed down
into the snow or beneath the driver; however, care must be taken to avoid icing up the
tracks and suspension by the blast of hot exhaust gases.
Other major considerations in achieving these levels have been in the
areas of intake silencing and engine enclosures. The cowling configurations on the
different brands of snowmobiles vary quite markedly. The lighter weight, price-
competitive units generally use a minimal engine shielding, while the more luxurious
multicylinder units are provided with much better shielding. The need for adequate
186
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engine cooling is a legitimate design constraint and the main argument against engine
enclosure for most vehicle types. However, the snowmobile, by virtue of the environ-
ment in which it operates, is most ideally suited to a well-ventilated acoustic
enclosure. In addition to reducing noise levels to the distant observer, the engine
enclosure is perhaps the most significant factor in reducing the high noise levels
experienced by the operator.
Further reduction is undoubtedly obtainable through more refined engine
cooling methods which would allow more complete engine enclosure, some design
modifications to allow rerouted intake through silencing devices, and more space
for large volume mufflers. ' '
The major problem area left to be fully assessed is the operator noise
environment. While earlier noise levels of 120 dB(A) and greater have been substan-
tially reduced, current models still produce levels at operator position of 105 to
59.
115 dB(A). ' It is felt that the additional work on intake silencers and engine
enclosures will do much to alleviate this problem. It is estimated that the current
snow vehicles reflect a cost increase of about 15 percent to obtain their present
noise levels.
There are currently pending a number of noise laws which, if enacted,
will attempt to limit the noise output of snowmobiles at 50 feet to 73 dB(A) in 2 to
3 years. One manufacturer is currently attempting to develop a machine to comply
with this requirement. While specific details are not available concerning the tech-
niques involved in achieving these levels, he has estimated that such reduction will
carry with it a 15 to 30 percent increase in vehicle weight, and a corresponding
9
30 percent increase in price. A number of the smaller manufacturers with limited
or no research and engineering facilities may be unable to meet these requirements.
One of the major suppliers of mufflers for the snowmobile industry
expressed the opinion that there exist currently available exhaust treatments which
14
provide 30 to 35 dB attenuation. This means a reduction in the contribution of
the exhaust .system from approximately 105 dB(A) unmuffled to the 70 dB(A) range.
187
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This reduction can be accomplished with minor power loss but at the expense of some
additional weight and space required for the muffler.
On the majority of current production snowmobiles, no intake air cleaners
or silencers are used. It has been shown experimentally that a simple air cleaner
assembly will reduce intake noise by 7 dB without impairing performance. 'It would
appear that further reduction in this area is possible, and reductions of 12 to 15 dB •
would be feasible with some power loss, thus reducing the intake contribution to
approximately 70 dB(A) at 50 feet.
14
An example cited by one manufacturer is shown in Table 2.7-1.
It is felt that further overall reduction into the 75 dB(A) range is feasible with
improved engine enclosures.
Table 2.7-1
Example of Further Noise Reduction Using Existing Technology, ;
Noise Producing Component
Exhaust
Intake (stock range 77 to
87 dB(A) )
Cooling fan
Track & suspension
Engine/mechanical*
Unmodified
OVERALL
1971 Model
As Produced
(dB(A) at 50 feet)
82
85
(bare stack)
80
72
76
86 dB(A)
With Intake and Exhaust
Treatment
(dB(A) at 50 feet)
70
78
(with silencer)
80
72
76
82 dB(A)
Test vehicle had production engine cowling in place.
188
-------
Future snowmobile noise output levels at 50 feet could be reduced to the
70 to 73 dB(A) range by 1980. This figure assumes significant .advancement in noise
reduction technology in a number of areas. The first step is to utilize existing exhaust
14
systems, which reduce exhaust noise levels to the 70 to 75 dB(A) range. Further
refinement will be required to produce systems that are of reduced size and do not
drastically affect power output. Intake system silencing should be advanced suffi-
ciently by that time to also provide maximum intake noise levels in the 70 dB(A) range
without significantly affecting engine performance. A key area of attenuation will be
in more refined engine cooling and air ducting techniques that will allow the use of
full engine enclosures, hence reducing this system's contribution to the 70 dB(A) range.
The last significant system that must be further refined would be the drive track and
suspension system. Current contribution from these elements is now estimated at
14
around 72 dB(A). It would appear that component isolation and slightly refined
design will achieve adequate noise reduction in this region.
It is believed that these noise reduction techniques will greatly aid in
reducing operator noise exposure levels. Rerouting the intake and shielding the
20
engine should reduce these levels down to the 88 to 92 dB(A) range.
189
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3.0 Devices Powered by Small Internal Combustion Engines
3.1 Introduction
The noise emanating from equipment powered by small internal combustion
engines is well known to millions of people, particularly those who maintain gardens
or lawns. The total United States production of these engines was about 10.9 million
units in 1969. This total includes all engines below 11 horsepower except those used
for boating, automotive and aircraft applications.
Over 95 percent of these engines are air cooled, single cylinder models.
The vast majority are 4-cycle, while the 2-cycle version comprises most of the remaining
market. More than half of the single cylinder engines power the estimated 17 million
lawnmowers in use today. The majority of the remaining engines are used in other lawn
and garden equipment such as leaf blowers, mulchers, tillers, edge trimmers, garden
tractors and snowblowers. In addition, about 750 thousand chain saw engines and
100 thousand engines for small loaders, tractors, et cetera, were produced in 1970,
while agricultural and industrial usage together account for another 1 .5 million engines.
Generator sets, while not presently employing as large a number of engines, are an
1 2
important consideration because of their growing numbers. '
The categorization of these devices by usage and typical noise levels is
summarized in Figure 3-1.
3.2 Source Noise Characteristics
Generators
Of the 100,000 generator sets sold each year in the United States, most
are used in mobile homes, campers, and large boats, where their electrical output is
used to power air conditioning, lighting, and other equipment. These sets generally
have 3 to 5 kilowatt capacity with a few units producing 8 kilowatts or more. Engine
size is of the order of 2.2 horsepower per kilowatt, often with considerable derating of
the engine for quiet operation so that the generator's noise may be tolerated by users
o
and their neighbors over long periods of use.
190
-------
Generators
Battery Chargers
Air Conditioners
Auxiliary Power
Internal Combustion
Engines
Lawn Care
• Mowers
• Edgers
• Tillers
• Leaf Blowers
• Snow Blowers
Other Types
• Chain Saws
• Model Aircraft
120
110
_ 100
I 90
T 80
0)
70
60
0)
•5 50
40
30
20
Number in Service
550,000
30,100,000
2,500,000
Typical Noise Levels
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Figure 3-2 illustrates a typical one-third octave spectrum radiated by
small generators of the 3 to 5 kilowatt size. The spectrum is characterized by two
peaks, one occurring at the firing frequency, around 40 Hz, and a second peak about
1000 Hz. This spectrum is characteristic of most types of internal combustion engines.
The low frequency peak is associated with the fundamental firing frequency of the
engine. However, the high frequency peak is generally the most annoying portion of
the spectrum since it occurs at a frequency where human hearing is most sensitive.
This peak may be attributed to acoustic radiation by the hot gas bubble leaving the exhaust
with each firing, and to mechanical noise in the engine. In the example given, the
high frequency noise has been heavily suppressed in comparison with other equipment
having less stringent noise requirements.
Lawn-Care Equipment
Lawn-care apparatus built in the United States is predominantly equipped
with engines running at 3000 to 36000 rpm. The characteristic noise spectrum, as
shown in Figure 3-3, has a double peak, the lower frequency peak corresponding to
the engine firing frequency and the higher peak occurring from 2 to 3 octaves above
the firing frequency. Additional high noise levels are radiated by the rotating bjade.
In the case of a rotary mower driven by a 4-cycle engine, the blade passage will be
4 times the firing frequency and will merge with the high frequency engine noise.
Equipment without a rotating blade will generally have other machinery noise of the
same approximate level.
It can be shown that "A" scale measurements of engine noise from this
class of engine is generally 2 or3 decibels below an A scale measurement of the
machinery noise. However, the modulation of the high frequency engine noise by the
lower firing frequency makes the engine noise more audible than the noise of a rotating
blade or other machinery. Thus, even heavy muffling on lawn-care equipment does
not totally eliminate the audibility - or characteristic "putt-putt" - associated with
this modulation.
192
-------
40
100
1000
Frequency in Hertz
10000
dB(A)
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Total
Figure 3-2.. Typical Noise Characteristics of Generators
193
-------
Exhaust
100
1000
10000
Frequency in Hertz
Exhaust Intake Other Blades
(Engine)
92
dB(A)
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Figure 3-3. Typical Noise Characteristics of Rotary Lawnmpwers
194
-------
Chain Saws
A typical chain saw, designed for casual use, weighs from 6 to 20 pounds
and has a blade from 1 to 3 feet long.4 The engine produces 5 to 8 horsepower and has
a life expectancy of 1000 to 2000 hours. In order that a device this powerful may be
made portable, the engine must have a high power-to-weight ratio.
Fuel consumption, muffling, and durability are secondary considerations
even in the industrial machines, as design criteria dictate the use of high speed. A
typical engine may operate at 9000 rpm at a firing frequency of the same rate, or 150
times per second. The engine incorporates a muffler, typically weighing less than a
pound, which includes a spark arrestor to prevent fire. The very high firing frequency
4
brings the direct exhaust noise well within the audible range, as shown in Figure 3-4.
The broad peak, characteristically found in these engines two octaves above the firing
frequency, occurs around 1000 Hz, the region of greatest audibility in humans.
Thus, the requirement for a small but powerful device has resulted in
designs in which the engine noise is in the frequency range of greatest audibility, and
the muffler structure is as light and small as possible. This combination results in equip-
ment which produces levels as high as 115 dB(A) at the operator's position, with levels
4 7
of 83 dB(A) common at a 50-foot distance. ' • •
Model Airplane Engines
Model airplane engines are normally rated by displacement in cubic inches
and few figures are published in terms of horsepower. These engines range from 0.029
to 0.20 cubic inch displacement, and may exhibit up to 1 .5 horsepower per cubic inch.
The noise spectra shown in Figure 3-5 were measured on 0.049 cubic: inch displace-
ment engines which would probably produce 0..06 to 0.08 horsepower. Model airplane
engines are 2-cycle types, turning at very high rotational speeds, typically 12 to 18
4
thousand rpm, resulting in a firing frequency above 200 Hz.
Manufacturers have only recently incorporated any type of muffling.
Figure 3-5 illustrates data taken on two identical engines of 0.049 cubic inch
displacement. One was equipped with a muffler and the other was not. The 200-Hz
195
-------
Chain
Engine
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Frequency in Hertz
114 H5
To 600
dB(A)
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Total
Figure 3-4, Typical Noise Characteristics of Chain Saws
196
-------
40
100
1000
Frequency in Hertz
10000
97
dB(A)
••MM
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IE
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C
22, 78
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59
n
i
at 6 ft at 50 ft
at 500 ft
Figure 3-5* Measured Noise Characteristics of Muffled
and Un muff led Model Airplane Engines
197
-------
firing frequency is in evidence in both cases. Indeed, the noise level at the firing
frequency is higher for the muffled engine than for the unmuffled engine. However,
since the "A" scale (and the human ear) discriminates against this 200-Hz signal.by
10 decibels, the levels of this frequency are not quite as audible as are similar .levels
between 1000 and 4000 Hz.
Thus, the unmuffled noise levels at 1200 and 2400 Hz are considerably
more audible when the bareiengine is operated than when the muffled engine is
operated. Even with muffling, the double peak frequency characteristic is very
much in evidence, but the character of the engine sound has changed from an "angry
mosquito" to something more like a noisy electric motor, while reducing the "A"
scale noise by 12 decibels.
3.3 Environmental Noise Characteristics
It is characteristic of small internal combustion engines that the equip-
ment being powered is operated by a single person or is unattended. The low noise
equipment, such as generators which have been well-muffled,1 operate unattended.
However, the typical generator is used for supplying power to a camper, mobile home,
or boat, and is built into a metal frame which also houses the owner and his family.
When its vibrational energy is communicated to this frame, considerable annoyance
may result even though its directly radiated acoustic levels are very low.
The operator of lawn-care equipment attends the equipment at all times.
Usage is generally during daylight hours in urban and suburban areas. A given user
will operate a lawnmower for one or two hours per week and may then run an edge
trimmer for approximately one-half hour. He may continue with a leaf blower to
pick up the clippings and then use either a garden tractor or tiller in his garden.
During such a hypothetical day, the operator may be exposed to four or five hours
of noise in the high 80 to low 90 dB(A) range, depending upon the manufacturer's
dedication to noise control and to the user's maintenance of the equipment.
198
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Some other lawn-care equipment transports the operator, as in the
popular Hiding mowers or garden tractors. Here the operator is directly behind
or directly above the engine. The muffler and intake ports are generally somewhat
closer'to the operator's ear than the 6 feet characteristic of the push-type equipment.
Also/ equipment which can carry the operator generally requires a larger engine than
would be required otherwise. These two factors combine to create considerably
higher sound pressures at the operator's ear. The A-weighted noise level for this
situation generally' ranges from the low to mid-90's, presenting the operator with a
risk of permanent hearing damage when long periods of operation are endured each
day, or when shorter time periods of operation are endured by a person who is especially
sensitive to hearing damage.
A third type of engine characterized by high speed and minimal muffling
is the chain saw. Operator ear levels for this device may be as high as 115 decibels,
78
with quieter machines operating near 102 to 103 dB(A). ' Such levels present a
definite risk of permanent hearing damage, and use of ear protective devices should be
recommended as a prudent precaution in the operating instructions and the labeling
of such equipment.
The noise of model airplane engines and other small devices is usually
not of a sufficient level to impose hearing damage risk on the user, during the short
exposure times of close proximity to the engine.
A well-built generator will seldom exceed 70 to 72 A-weighted decibels
at a distance of 50 feet when installed in a motor home or other such vehicle. It will
not generally cause speech interference; however, when the generator is used during
early evening and beyond, there may be considerable interference with sleep and
relaxation to persons nearby. As the market for these devices expands, they will
become a greater nuisance. Consequently, current production units are being
improved as rapidly as technology and cost permit.
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The norHparticipant noise environment generated by lawn-care equip-
ment has at least some 'effect on a large portion of the population in the United States.
This extensive effect is the result of the large numbers of engines being used in this
application in heavily-populated areas. The equipment generates A-weighted noise
levels in the low 70's at:50 feet and produces some speech interference. Where any
kind of solid barrier exists between the source and receiver, a decrease of 5 to 15
decibels can be expected. Thus, a solid wooden fence or the house itself will gen-
erally reduce the speech interference to acceptable levels. In many cases, the lawn-
care equipment will not become a cause of complaint by the non-participant, as long
as its use is restricted when people are sleeping and in early evenings when people
are relaxing on their patios. In other cases, where a wire fence or no fence at all
exists, complaints might well be forthcoming. •
) The non-participant environment generated by chain saws is fully capable
of causing speech interference at distances of several hundred feet. Non-participants
within 25 feet of the chain saw will be exposed to potentially damaging levels, as is
the operator. The chain saw is not frequently used in areas of heavy population and
is therefore not of frequent concern in the non-participant environment. When it is
used in populated areas, considerable reaction may be experienced from those exposed
to the noise. It is probable that a reduction of the noise levels for the operator to
the levels of lawn-care equipment would minimize problems in the non-participant
environment. However, it must be recognized that a great deal of study would be
required to accomplish this noise reduction within the cost, weight, and power con-
siderations imposed upon chain saws by their preferred use.
In all cases of the non-participant environments mentioned, the persons
affected will be in their homes or at other locations where they have gone for leisure
time activities. Apartment dwellers are not exempt since the lawns around their
apartments are mowed by larger, noisier equipment. Children attend schools where
lawns are mowed, and even most hospital rooms are within earshot of a lawnmower.
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Generators and chain saws both have a small effect on the general community
since they are used outside populated areas. Chain saws affect the operator and helper
at levels between 90 pnd 110 dB(A) with the operator receiving the highest levels; When
this equipment is used in populated areas, all persons within 500 feet will generally
be annoyed. However, duration is short and occurrence is infrequent so that their total
impact is small. It is estimated that fewer than 5 million people per year will be
adversely affected by these devices.
Generators affect their half-million owners plus another 1.5 million family
members., In addition,; each generator may annoy two other families, bringing the total
number of persons affected to 12 million, roughly 5 percent of the population.
The non-participant environment for model airplanes can range from 78 dB(A)
for nearby planes to 40 or 50 dB(A) at distance. Audibility is present at distances of
many hundreds of feet. When short flights are made during daylight hours, annoyance
is small., When flying is continuous or is conducted when people are relaxing outdoors,
annoyance.becomes great. . ;•
3.4 Industry Efforts Towards Noise Reduction
Historically, noise reduction has not been of primary consideration to the
i
manufacturers of small internal combustion engines although unmuffled equipment has
not been produced for many years because of buyer resistance to an excessively noisy
product. Public tolerance, combined with some noise control, has produced a com-
promise situation between the consumers and the manufacturers.
i \ ' . '
Generally, noise reduction achieved by the engine manufacturers has
resulted in engines which make somewhat less noise than the equipment they are
designed to power. However, equipment manufacturers are not completely convinced
of this conclusion, and tend to attribute the noise of the entire unit to the engine.
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This situation is particularly characteristic of the small equipment manufacturer who
purchases the engine from an outside source, having no involvement with engine
design. In this category are large numbers of lawn-care equipment units which are
constructed of pressed sheet metal in production shops around the country.
Many of the manufacturers of internal combustion engine powered equip-
ment feel that they are being placed in the difficult position of being required to meet
several divergent nuisance lawS. These laws have been promulgated by various
individual cities and towns, where noise restrictions are related to local economic
and social conditions. This situation is typified by the experience of the manufacturers
of lawnmowers. The recently enacted ordinance for the City of Chicago lists a
descending scale for allowed noise for lawnmowers over the next few years which most
manufacturers interviewed agree is realistic, and they are working toward compliance
within the allotted time. However, in the recent ordinance enacted by the City
of Minneapolis, the equipment is riot allowed to exceed certain ambient levels at the
property line by more than 6 decibels. Lawn care equipment is specifically
exempted from these requirements, but is restricted to operation between the hours of
7:30 a.m. to 9:00 p.m. on weekdays, and 9:00 a.m. to 9,:30 p.m. on Saturdays,
Sundays, and holidays. If the lawn-care equipment can comply with the specified
ambient requirements, then it may be used during any hours.
Other cities around the country have ordinances with noise levels as
12
low as 40 dB(A) at the property line. Although there does not appear to be a strong
effort to comply with or enforce this latter ordinance, no manufacturer can look with
impunity upon such a law, and he might even decide not to market in that area. As
other localities pass noise ordinances, such inequities could proliferate, making the
manufacturers' task much more difficult.
, The extent of noise reduction within the industries supplying small internal
combustion engines has been directly related to its effect on sales and the existence of
noise ordinances. With the exception of the small generator industry, public pressure
has not been sufficient to produce significant noise reduction efforts in most of these
devices.
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As a result, noise abatement1 programs have not been consistent. For
instance, one manufacturer has demonstrated that a small generator, using a 3 horse-
power engine with a vertical shaft housed within a complete enclosure, may be quieted
to 70 decibels at a position 6 feet from the engine. If this same treatment were applied
to a lawnmower, it would achieve an improvement of approximately 20 dB over most
current production lawnmowers, and would make the engine quite inaudible in the
presence of a rotating blade. However, no serious plans are being made for production
of such a mower because of the high cost of the noise reduction treatment.
Another manufacturer is presently producing a lawnmower operating at a
noise level of 50 dB(A) at 50 feet. This is some 13 decibels below the average machine
and is accomplished through the use of a 2-cycle engine with a large muffler, and
cast frames where pressed steel was previously used. Only 10 percent of the engines
manufactured in the United Stares are of the 2-cycle type, so that a changeover to
that type of engine from the present majority of 4-cycle types would be a very long
and expensive task. High fuel cost could also create resistance in the marketplace.
Some manufacturers were questioned as to the feasibility of producing
3
2-cylinder engines for use in lawn-care equipment and other such devices. This
change from the singles-cylinder engine has the advantage of allowing the exhaust
pulse from one cylinder to partially cancel the pulse from the other cylinder. While
many manufacturers admitted the feasibility of this concept, estimates of cost for such
engines ran from 30 percent to 50 percent higher than the single-cylinder engines for
a given horsepower rating. Such a penalty would make the "quiet" engines non-
competitive with the lower-priced models of current design.
Chain saw manufacturers recognize the existence of a serious noise
problem with their equipment. The high power-to-weight ratio necessary in a device
that must be hand-carried and be capable of quickly cutting trees and large brush
requires a structure not capable of containing its own noise. Further, the noise
produced" by the chain itself is of the order of 100 dB(A) at the operatpr position
and reduction of the engine noise below this level would not reduce total output to
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an acceptable level. In addition, where experimental prototypes have been built
using electric motors to achieve very low engine noise, the more apparent mechanical
noise of the chain gives the impression of a device "ready to fly apart, " causing
operators to resist using it. Some experimental work is being done to reduce the.
noise of the chain, but cost limitations rapidly become prohibitive when exotic , ,
materials are used to damp the response .of the blade to the chain.
Considerable engineering work has been expended to make the mufflers
more efficient within weight and size limitations, and some success has rewarded
these efforts. Sound levels have been reduced to as low as 103 dB(A) by some special
mechanical devices with power losses of no more than 10 to 12 percent.
Noise control within the industry served by small internal combustion
engines will be affected by various laws and ordinances as enacted by the government ,
bodies concerned. However, there will always be difficulty in encouraging noise
abatement until public education advances to the point where the charisma of noise
1 , . ' : "'>•.• •'.-.)••••
is gone. The motorcyclist who removes his mufflers to obhain more power may well
degrade his performance and still feel he has gained power and status. He has his
counterpart in the backyard garden. This man may remove the muffler from his tiller
in order to dig his garden faster (he thinks). He may not remove his lawnmower muffler,
but as it becomes old and less efficient, he may rationalize that the lessened back
pressure will tend to compensate for losses of power through aging of other parts of
the engine.
Whatever the basis for associating loud noise with productivity, an
educational program is required to reduce public acceptance of noise. When each
person is convinced that his contribution to noise reduction is meaningful, he may
go to the manufacturer of the quietest machine, even if the cost is higher, and may "
take pride in his accomplishment. When this happens, as it has in the-small generator
field, manufacturers will probably respond decisively toward reduced noise levels.
Interviews have shown that most manufacturers can respond, but, at the present time
have found little market for quiet products when the public is asked to pay a premium
for the quiet product.
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3.5
Noise Reduction Potential
The combined effort by the public in demanding quieter products powered
by internal combustion engines and successful response to this demand by the manu-
facturers, should provide a substantial decrease in annoyance from this equipment.
This reduction in annoyance of intruding noise from'lawnmowers, chain saws, et cetera,
will be the principal benefit of a broad noise reduction program for devices powered
by internal combustion engines. The estimated potential noise reduction that might be
expected in the future for these devices is summarized in Table 3-1. The noise
reduction values are relative to current noise levels and are specified in terms of
potential reductions that can be achieved by the 1975, 1980 and 1985 time periods.
Full accomplishment of these noise reductions would largely eliminate
annoyance problems in residential areas associated with use of lawn care equipment.
However, the noise reduction potential for chain saws using existing technology is
not sufficient to eliminate their annoyance problems or hearing damage risk for
operators. Further noise' reduction' research is called for with these unique devices.
Table 3-1
Estimated Noise Reduction Potential for Devices '
Powered by Internal Combustion Engines •,
•>
Source
Lawn Gare Equipment
Chain Saws
Generator Sets
Noise Reduction, d6 *
1975
10
2 '
5
1980
13
2
•7
1985
15
5
17
*Noise reduction relative to typical current noise levels in dB(A) at 50 feet.
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4.0 ENVIRONMENTAL IMPACT FOR TRANSPORTATION VEHICLES
AND SMALL INTERNAL COMBUSTION ENGINES
The preceding chapters have illustrated the nature of the noise character-
istics as well as an estimate of current and future noise reduction potential for each
major element of the transportation system and for small non-industrial internal com-
bustion engine powered devices. With, this background, one would like to have an
overall view of the impact of these noise sources on the observer in a community and
on the operator or passenger. As with any complex situation, several viewpoints are
desirable in order to obtain such an overall perspective.
First, a simplified overview of the relative contribution of each of the
source categories is provided by comparing their estimated daily outputs of acoustic
energy. Next, the sources are compared to estimate their relative contributions to
the outdoor residual noise level in average urban residential areas. Third, the sources
are reviewed with respect to their individual single event intrusive characteristics,
and their potential impact in terms of community reaction. Finally, the operator/
passenger noise environment is reviewed with respect to the potential hazard for
hearing damage and speech interference. Each of these comparisons is examined in
terms of today's situation and in terms of one possible estimate of the potential change
in the future. This example of a possible estimate of future noise helps to provide
some insight into potential changes in the relative impact of the various source
categories that could be effected with current or advanced technology.
A detailed discussion of the methods and sources of data used in carrying
out this impact analysis is presented in Appendix B. Key assumptions utilized are
summarized as follows.
J
• The impact analysis is based on current figures for the number
and use pattern of the noise sources as determined from nationwide
statistical data. These data, coupled with the definition of
characteristics of the noise sources, provided the basis for evalu-
ating noise impact for 1970 in statistically-average communities.
206
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To project- changes in the noise impact to the year 2000, a conservative
model was chosen for growth of the transportation system and growth in
numbers of internal combustion devices. Major assumptions for the
model included (a) conservative population growth,of,1.15 percent per
year from 1970 to 1985 and 1.05 percent thereafter, and (b) conserva-
tive estimates for numbers of noise sources with growth rates approaching
estimated urban population growth rates by the year 2000.
The potential change in noise levels for transportation vehicles and
internal combustion engine devices has been estimated for three
possible options for future noise reduction:
Option 1 — No change in source noise levels after 1970. This
represents a base-line condition wherein changes in
noise impact would be due only to changes in number
or use-patterns of the noise sources.
\ 7 ' '
Option 2 — Estimated noise reduction that would be achieved by
;. extrapolating current industry trends by the year 1985,
with no further reductions thereafter. This option
assumes no new noise control regulations by local,
state or Federal agencies, or any change in consumer
demand for quieter vehicles.
Option 3 — Example of projected noise reduction achieved by
implementation of an incremental regulatory program
to achieve a specified amount of noise reduction by
the years 1975, 1980, and 1985.. The criteria used
for defining these estimates for potential noise reduction
r under this option example are as follows:
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— By 1975, what noise reduction could be achieved by
reducing levels to those for a typical quieter model
now on the market.
— By 1980, what noise reduction could be achieved
that industry has already demonstrated can be
accomplished.
- By 1985, what is a practical limit for the potential
noise reduction that could be achieved utilizing, if
necessary, advanced technology.
The estimates of potential noise reduction utilized for Option 3
are summarized in Table 4-1 for the major transportation categories
and in Table 4-2 for Internal Combustion Engine Devices.
Due to the very different use-patterns for transportation vehicles in
contrast to non-industrial stationary internal combustion engine devices, it is desirable
*
to evaluate their impact separately. Transportation vehicles are considered first.
4.1 Total Noise Energy Output per Day for Transportation Systems
A small, but no longer insignificant, byproduct of the growth in transporta-
tion is the conversion of a tiny fraction of the mechanical energy expended by the
industry into sound — normally an unwanted sound or noise. For example, to propel
87 million automobiles and 19 million trucks and buses in the United States, an energy
equivalent to approximately 7800 million kilowatt-hours is consumed every 24 hours
— approximately one-third of the total energy consumption in the United States from
all sources of power. Approximately one-millionth of this portion for transportation
is converted into noise. The amount of noise energy per day for each element of the
transportation system is a function of its noise level, number of units, and number of
hours per day operation. Thus, a source category which has high noise levels, but
only a few units in operation^ can produce the same total noise energy per day as a
source category which has a lower noise level but a very large number of units in
208
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Table 4-1
Example of Potential Noise Reduction for Externally
Radiated Noise for Transportation System Categories
Source
HIGHWAY VEHICLE1
Medium and Heavy Duty Trucks
Utility and Maintenance Vehicles
Light Trucks and Pickups
Highway Buses
City and School Buses
Passenger Cars (Standard)
Sports, Compact, and Import Cars
Motorcycles (Highway)
AIRCRAFT
Commercial —with Turbofan
Engines^
3
General Aviation — Propeller
Heavy Transport He li copters '-
3
Medium Turbine- Powered Helicopters
Light Piston- Powered Helicopters^
RAILWAY1
Locomotives and Trains
Existing Rapid Transit and Trolley Cars
RECREATIONAL VEHICLES1
Snowmobiles
Minicycles and Off-Road Motorcycles
Outboard Motorboats
Inboard Motorboats
Effective Date
1975
3
3
2
3
2
2
6
2
4
0
0
5
10
0
5
10
2
2
5
1980
8
8
5
8
5
4
8
7
7
5
5
12
15
5
10
12
7
4
6
1985
10
10
8
10
8
5
9
10
10
10
10
17
20
8
15
14
10
6
7
1
Relative reduction in average noise levels in dB(A) at 50 feet.
2
Relative reduction in EPNdB at FAR-36 Measurement Position for Takeoff.
3
Relative reduction in EPNdB at 1000 feet from aircraft during takeoff.
209
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Table 4-2
Estimated Noise Reduction Potential for Devices
Powered by Internal Combustion Engines*
Source
Lawn Care Equipment
Chain Saws
Generator Sets
Effective Dates
1975
10
2
5
1980
13
2
7
1985
15
5
17
* Noise reduction relative to typical current noise levels in dB(A) at 50 feet
operation. Although this energy comparison does not relate directly to impact on
people, it does identify and give some perspective to the major noise sources.
Table 4-3 summarizes the estimates of the A-weighted noise energy
generated throughout the nation during a 24-hour day, by each category of the
transportation system as it exists today. The top ten transportation categories, as
ranked by their noise energy, constitute 96 percent of the total, and of these, heavy
trucks and 4-engined aircraft alone produce over 50 percent of the noise energy.
The approximate A-weighted noise energy expended per day has also been
estimated for the year 2000 for most of the surface transportation categories except
aircraft for each of the three options defined above. The results are summarized in
Table 4-4. The estimated value for 1970, specified earlier, is listed in the first
column for reference. The second column, based on Option 1 (no noise reduction),
shows the increase in noise energy per day due solely to the estimated increase in
number and usage of sources. The third and fourth columns show the estimated trend
in noise energy by the year 2000 for Option 2 (current industry trends) or Option 3
(possible noise regulation).
With the Option 3 noise reduction program, the noise energy by the
year 2000 for all categories is always less than 1970 values. The reduction for
Option 2 relative to Option 1 by the year 2000 reflects the current effort by the
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Table 4-3
Estimated Noise Energy for Transportation System Categories in 1970
Major Category
Noise Energy
(Kilowatt-Hours/Day)
Aircraft
Highway
Vehicles
Recreational
Vehicles
Rail Vehicles
• Commercial — 4-Engine Turbofan
• Commercial — 2- and 3-Engine Turbofan
• General Aviation Aircraft
Helicopters
• Medium and Heavy Duty Trucks
• Sports, Compact, and Import Cars
• Passenger Cars (Standard)
• Light Trucks and Pickups
• Motorcycles (Highway)
City and School Buses
Highway Buses
• Minicycles and Off-Road Motorcycles
Snowmobiles
Outboard Motorboats
Inboard Motorboats
• Locomotives
Freight Trains
High Speed Intercity Trains
Existing Rapid Transit
Passenger Trains
Trolley Cars (old)
Trolley Cars (new)
3,800
730
125
25
5,000
1,000
800
500
250
20
12
800
120
100
40
1,200
25
8
6.3
0.63
0.50
0.08
Total ~ 15,000
Top ten categories which each generate at least 125 kilowatt-hours per day.
211
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Table 4-4
Example of Estimated Future Change in Noise Energy for Major Surface
Transportation System Categories with Three Options for Noise Reduction
Source
HIGHWAY VEHICLES
Medium and Heavy Duty Trucks
Sports, Compact, and Import Cars
Passenger Cars (standard)
Light Trucks and Pickups
Motorcycles (Highway)
City and School Buses
Highway Buses
RECREATION VEHICLES
Minicycles & Off-Road Motorcycles
Snowmobiles
Outboard Motorboats
, Inboard Motorboats
RAIL VEHICLES
Locomotives
Existing Rapid Transit
Noise Energy in Kilowatt-Hours/Day
1970
5,000
1,000
800
500
250
20
12
800
120
100
40
1,200
6
- — : —
1
10,000
2,500
1,200
1,000
800
20
12
2,500
400
160
63
1,200
10
NA — Not available.
*Option 1 — No noise reduction.
2 — Estimated Industry trend in noise reduction.
3 — Example of possible Incremental program of Noise
2000
Option*
2
4,000
1,600
800
400
320
8
5
NA
NA
NA
NA
1,200
6.3
Regulation.
/
3
800
250
400
160
80
3
1.2
250
16
40
12
200
0.5
212
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various industries to achieve a quieter product, while the additional reduction indicated
for Option 3 shows the significant additional benefit that could be obtained, through
noise regulation. ..-.•••
These values of noise energy provide a rough indication of change in the
relative magnitude of potential noise impact from transportation vehicles. By the
year 2000 the noise energy values in Table 4-4 indicate a 100 percent increase from
those in 1970 if no further action were taken to reduce noise (Option 1). Assuming
V.
current industry trends are continued (Option 2), there is little significant change in
estimated noise energy indicated by the year 2000. Thus, the estimated noise reduction
just offsets the increase in numbers of vehicles. However, by implementation of a
positive regulatory program (Option 3 example), the aggregate noise energy per day
for these sources in the year 2000 might be approximately 78 percent less than the
current,amount.
4.2 Contribution of Transportation System Components to the Residual
Background Noise Level in an Average Community
As discussed in Reference 2, the residual noise level in a community is
the slowly changing nonidentifiable background noise which is "always there" when-
ever one listens carefully outside the home. This residual noise level is originated
by all forms of traffic moving throughout the community, and the large number and
variety of stationary sources in a community, such as dispersed industrial plants or
multiple air conditioning systems. The method for predicting this residual noise level
is discussed in Appendix B.
Table 4-5 summarizes the estimated daytime residual noise levels for-
1970 for each significant type of highway vehicle that operates in an average urban
community. It is apparent that automobiles and light trucks are the principal sources"
which control the contribution to the residual noise level from transportation sources.
The average residual level was also predicted with the same technique
for the years 1950 and 1960. The estimated values for the typical urban community are:
» .
• For 1950 - Daytime Residual Level (L9Q) « 45 dB(A) '. .
• For 1960- Daytime Residual Level (L9Q) «46 dB(A)
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Table 4-5
Predicted Contributions to Daytime Residual Noise Levels
By Highway Vehicles for a Typical Urban Community in 1970
Source
Passenger Cars (Standard)
Sports, Compact, and Import Cars
Light Trucks and Pickups
Medium and Heavy Duty Trucks
Motorcycles (Highway)
City Buses
Approximate
Source Density
Units/Square Mile
~ 50
- 20
- 20
- 1.5
~ 1
- 0.8
Residual Noise Level
dB(A)
43
41
42
33
18
15
ITotal (All Vehicles) 47 dB(A)
These estimates indicate an increase over 10 years of approximately one dB
in the residual level O-Q/\)- This conclusion is consistent with the available measure-
ments which are summarized in Reference 2. Although these estimated values for the
residual level are certainly no more accurate than ±3dB, they agree very well with
the available data and clearly indicate the prime sources of the residual noise in a
2
typical urban community.
Although the average residual level 0-9f.) in an urban community may not
have changed significantly over the past two decades, the residual noise level in any
given neighborhood may have changed. Such change is expected in neighborhoods
where the land use has changed or where new service arterials (highway or freeway)
have been developed. Thus, the development of rural land into suburban communities
has increased the residual level, as has the construction of a freeway through an
existing fully developed community.
The same model for estimating residual noise levels for 1970 has been
applied to forecast trends for 1985 and 2000 as a function of the noise reduction options
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for highway vehicles only. The result of this projection, including the estimated
residual levels for 1950 and I960, is shown in Figure 4-1. The trend for Option 1
is clearly an upper bound, and indicates an additional growth of about 2.5 dB in
the residual level in an average community by the year 2000 due solely to growth
in the number and density of the noise sources. The lowest line (for the Option 3
example) represents the cumulative effect of achieving the 3-step noise reduction
values summarized in Table 4-1. It estimates a net reduction in average residual
noise level of 5 dB relative to today by the year 2000, whereas little change is
forecast by the year 2000 for the projection of current industry trends (Option 2).
%
In summary, therefore, if no further action were taken to reduce noise
levels of highway vehicles, the residual noise level in an average urban residential
community would be expected to increase an additional 2 to 3 dB by the year 2000
over today's levels. On the other hand, a positive program of noise reduction for
highway vehicles could prevent such an increase and achieve a desirable and reason-
able reduction in average .residuaI noise levels of about 5 dB over the next 30 years,
not including any additional noise reductions to be achieved after 1985.
4.3 Relative Annoyance Potential of Intruding Single Event Noise
As discussed in Reference 2, the reaction of a community to excessive
noise is the summation of annoyance from successive intruding single event noises such
as aircraft flyovers or many cars driving by. It is desirable, therefore, to rank trans-
portation noise sources according to their noise levels at a fixed distance, or, as
illustrated in Figure 4-2, define the distance from the source within which the single
event noise is greater than a specific value.
Two measures of the noise level are useful for this comparison; the maxi-
mum noise level which occurs when the vehicle passes by, and the single event noise
exposure level (SENEL)* which integrates the A-weighted noise throughout the entire
passby. This latter measure accounts for both noise level and duration, both of which
have been found to be factors in annoyance. An SEN EL of 72 dB has been chosen as
See Appendix B for definition.
215
-------
50
<,
CO
48
- 46
0)
d>
'5
1 44
O
o
O 42
Option 1
(No Noise Reduction)
Option 2 _
(Current Trend)
Full
Effects of
Limits
Option 3 Example
(Effect of Regulatory Limits Set in
1975, 1980 and 1985)
Dates of
Introduction
of Limits
Option 3
m
40
1950
1960
1970
1980
1990
2000
Figure 4-1, Estimated Long-Term Trend in Outdoor Residual Noise
Levels in a Typical Residential Urban Community
216
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Single
Event A
Noise T
Level
Noise Level
Generally
Acceptable
S89.
i Noise Level
' Potentially
Annoying
Figure 4-2. Decay of Noise Level with Distance from Single Source
Defines Relative Bounds of Annoyance Zone
217
-------
the reference value for comparing the distances required between a receiver and each
of the various sources if the sources are to be judged equally annoying. This SENEL
value is approximately that experienced at a distance of 50 feet from a residential
street when a standard passenger car passes. In subjective tests with motor vehicles '
of all types, this SENEL value has been found to be a dividing line betv/een "quiet"
and 'acceptable", and is approximately 10 dB below the dividing line between
2 3
'acceptable "and "noisy". ' In these tests, the effective duration of the vehicle
noise was approximately one second, so that the maximum noise level during the pass-
by was numerically equal to the SENEL. Thus the maximum noise level found 'accept-
able" ranged between 72 and 82 dB(A), which brackets the sound level of one's own
voice as measured at the ear. This "self voice level" has been suggested as a possible
annoyance reference level.
Table 4-6 summarizes typical values for maximum noise levels and SENEL
values at a representative distance for transportation sources. The table also lists the
distance within which the SENEL exceeds a fixed level of 72 dB. Examination of the
various categories in Table 4-6 clearly shows that aircraft are obviously the outstanding
source of annoying sounds. However, heavy trucks, highway buses, trains and rapid
transit vehicles, which normally operate along restricted traffic routes, will also be a
distinct source of intrusion — potentially affecting more people. This noise intrusion of
single events is more severe in communities where the residual noise level is inherently
low. For example, in a rural or "quiet" suburban community located well away from
major highways, the residual noise level is 10 to 15 dB lower than in urban areas, and
the passby of a noisy sportscar at night may momentarily increase the noise level by as
much as 40 dB. Similarly, during the nighttime near a major highway, noise intrusion
from single trucks is readily apparent due to the lower density of automobile traffic.
Recreational vehicles operating on land are in a class by themselves. .
Their high noise levels, wide usage in both residential and recreational areas, and the
rapid increase in their number have all contributed to the current concern regarding
noise pollution from these devices. The growth pattern is particularly significant, as
indicated in Figure 4-3, which also illustrates the growth pattern of other consumer
devices operated by internal combustion engines.
218
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Table 4-6
Comparison of Major Surface Transportation System Categories According to Typical
Maximum Noise Levels, Single Event Noise Exposure Levels (SENEL),
and the Distance Within Which the SENEL is Greater than 72 dB
Category
AIRCRAFT
Commercial — 4-Engine
Turbofan
Commercial — 2rEngine
Turbofan
Helicopters
General Aviation Aircraft
HIGHWAY VEHICLES
Medium and Heavy Duty
Trucks
Motorcycles (Highway)
Utility and Maintenance
Vehicles
Highway Buses
Sports Cars (etc.)
City and School Buses
Light Trucks and Pickups
Passenger Cars (Standard)
RAIL VEHICLES
Freight and Passenger Trains
Existing Rapid Transit
Trolley Cars (Old)
Trolley Cars (New)
RECREATIONAL VEHICLES
Off-Road Motorcycles
Snowmobiles
Inboard Motorboats
Outboard Motorboats
Typical Single Event Levels
Distance
Feet
1000
1000
1000
1000
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
A-Weighted
Noise Levels1
dB re:
20 |jN/m2
103
96
77
83
84 (88)
82 (88)
82 (88)
82 (86)
75 (86)
73 (85)
72 (86)
69 (84)
94
86
80
68
85
85
8.0
80
SENEL
dB re:
20 MN/m2
and 1 sec
111
104
87
96
87
85
85
83
78
78
75
72
114
96
83
71
90
90
85
85
Distance2 '
for SEN EL
Less Than
72 dB
Feet
>8000
>8000
>2000
>2000
700
540
540
540
170
120
100
50
>2000
480
260
40
750
750
400
400
Values inside parentheses are typical for maximum acceleration. All other
values are normal cruising speeds. Variations of 5 dB can be expected.
without shielding loss.
219
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1970
o I960
o>
1950
0
Legend:
Gas Powered Lawnmowers
Motorboats
Motorcycles
Chain Saws
Snowmobiles
10
Number of Units
(in millions)
20
Figure 4-3. Approximate Growth of a Few Types of Noisy Recreational
Vehicles and Outdoor Home Equipment. There were Negligibly
Few Gas Powered Lawnmowers, Chain Saws
and Snowmobiles in 1950
220
-------
The noise intrusion of water craft is generally regarded to be fairly low,
particularly since power boats are legally required to be at least 100 feet from shore
when operating at high speed, thus minimizing their impact in local communities.
Looking ahead, the potential change in annoyance or intrusiveness of
single events from surface transportation vehicles can be roughly evaluated by applying
the potential noise reductions listed earlier in Table 4-1. This noise reduction also
can be translated into a reduction of the spatial extent of potentially annoying single
event levels by applying the following approximate corrections to the fourth column of
Table 4-6.
Noise Reduction Correction Factor for SENEL
(From Table 4-1) Distance (Table 4-6)
OdB 1
2 0.7
4 0.5
6 ,0.4
8 0.3
10 0.2
Applying the full potential noise reduction limits suggested in Table 4-1
for 1985, a substantial decrease in the annoyance would be achieved for most of the
transportation categories. For example, with the exception of motorcycles and main-
tenance trucks, the vehicles commonly operating on urban streets would .tend to have
SENEL values less than 72 dB at 50 feet — a typical distance between a street and a
residence.
4.4 Overall Assessment of Noise Impact by the Transportation System on
Non-Participants
As suggested above, the cumulative effect of the repeated occurrence of
intruding noises will place a different emphasis on individual transportation system
categories than is obtained by considering only a single event. The land area within
a Communify Noise Equivalent Level (CNEL) of 65, as defined in Reference 2, is
utilized to obtain a minimum estimate of the integrated noise impact for major urban
221
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highway systems and airport operations — the most important elements of the transporta-
tion system with respect to noise impacted areas. The general method for estimating
noise impact contours around airports has been briefly described in Section 2.1. A
summary of the method for estimating noise impact contours near highways is presented
in Appendix B.
The noise impacted land within a Noise Exposure Forecast (NEF) 30 con-
tour for airport operations throughout the nation in 1970 was 1450 square miles. This
2
NEF value is essentially equivalent to a CNEL of 65. Therefore, for comparison, a
CNEL of 65 was chosen as the outer boundary of noise impacted land near major urban
highways. Calculations of the area enclosed between an effective "right of way"
boundary and the CNEL 65 boundary for freeways, major arterials and collector streets
gave a total impacted area of 540 square miles. This area was associated with free-
ways only, since the distance to the CNEL 65 boundary for the other types of roads
was less than their effective right of way distance. Thus, the estimated noise
impacted land within a CNEL 65 boundary for the two major transportation systems as
of 1970 was approximately:
Highways ~ 545 square miles
Airports ~1450 square miles
Total ~1995 square miles
It should be emphasized that both of these estimates include land area
which has compatible land use, as well as land area which does not. If it is assumed
that the land use is similar to the average urban use, then the population density in
1970 would be approximately 5 thousand people per square mile. Thus, approximately
10 million people could be living in the noise impacted areas defined by this criterion.
However, the expected reaction of a residential urban community to a noise intrusion
2
which produced a CNEL of 65 would be "widespread complaints." Therefore, this
choice of a criterion for the contour boundary is conservative and the total impact for
both commercial airports and freeways is certainly greater.
222
-------
Furthermore, the criterion value for widespread complaints is a function
of the residual noise level in the community. Consequently, a more accurate figure
of noise impact would require assessing the number of people actually living within
the CNEL 65 boundary in urban residential areas, plus the number of people within
the CNEL 60 boundaries in normal suburban areas and the number within the
CNEL 55 boundary in quiet suburban and rural areas. These lower CNEL boundary
values account for the lower values expected for the residual noise levels in the
quieter areas — thus allowing for an equal amount of relative noise intrusion for each
type of residential community, as discussed in Reference 2. Accounting for the
factors, it is conservative to estimate that at least 10 to 20 million individuals are .
impacted by these two types of noise intrusion.
The noise impacted land near rapid transit lines was not included in this
analysis as there are only 386 miles of electric railway lines compared to about 9200
miles of urban freeways. This fact, combined with the effect of intermittent operation
along rapid transit lines compared to the steady noise levels along freeways, indicates
noise impacted land for the former will be much less.
Because helicopter flight route patterns are essentially random at present,
it is practically impossible to define their noise impact in terms of land area or popu-
lation. A sustained public reaction has not materialized, despite the intrusive nature
«
of the sound, probably because of the irregularity of this usage pattern. However,
widespread complaints have arisen due to air taxi services in New York and police
operations in Los Angeles.
The airport noise impact due to general aviation aircraft operations is
quite small when compared to the impact of commercial jet aircraft operations. This
is due primarily to the lower noise levels for general aviation aircraft and to the fact
that most of the airports are located in outlying sparsely populated areas, or the air-
ports are sufficiently large that NEF 30 contours do not enclose significant residential
areas. However, at some general aviation airports that have a high rate of operations
for executive jets, a significant amount of residential land may be impacted by their
noise. The amount of land area involved is not known.
223
-------
To indicate past and future trends, the total impacted land area near free-
ways and airports has been estimated from 1955 to the year 2000. The resulting values,
given in Table 4-7 represent the incompatible land area lying within a Community
Noise Equivalent Level (CNEL) of 65. Future projections of noise impacted land
have considered the effect of implementing the noise reduction options discussed at
the beginning of this chapter. Thus, estimates of noise impacted land areas are given
for 1985 and the year 2000 for both Option 2 (values in parentheses) and Option 3
examples. A marked reduction in impact is achieved by the latter. For Option 3,
the estimated noise impacted land near airports is reduced by 88 percent from the 1970
value of 1995 square miles to 240 square miles. Based on a CNEL 65 boundary, noise
impacted land near freeways is reduced to zero by the year 2000 on the assumption of
a net noise reduction by vehicles and freeway noise barriers of about 5 dB beyond
today's values.
These changes in land area, based on very conservative criteria for the
noise impact boundary, correspond to an increase from a minimum of about 10 million
people impacted today to about 17 million by the year 2000 assuming no further regu-
latory action (Option 2). Alternately, the estimated number of people impacted (based
on this criterion) could be reduced by the year 2000 to no more than 1.2 million with
a positive regulatory program to achieve further noise reduction for aircraft, highway
vehicles and freeways. It is particularly important to note that the effect of imposing
the noise limits on aircraft .by FAR-36 is already showing at least a "holding action"
; . ' - - . ' '
on noise impact around airports. However, without any similar policy for highway
vehicles at the national level, the potential growth in noise impact near freeways is
severe.
These results must be viewed with extreme caution. First, they are based
on a widespread complaint boundary which may or may not be deemed publicly
acceptable. Second, they do not count the additional impacted area in communities
with lower residual noise levels. Third, they do not account for the effect of lowering
the future residual noise levels. For example, the 5 dB reduction of average residual
224
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Table 4-7
Summary of Estimated Noise Impacted Land (Within CNEL65 Contour)
Near Airports and Freeways from 1955 to the Year 2000 with Future
Estimates Based on Option 2 (Values in Parentheses)
and Option 3 Examples
1955
1960
1965
1970
1985
2000
Impacted Land Area —Square Miles
Near Airports
~ 20
200
760
1450
780 (870)*
240 (1210)
Near Freeways
8
75
285
545
400 (1470)*
0 (2050)
Total
28
275
1045
1995
1180 (2340)*
240 (3260)
*Number in parentheses is the estimated impact area if no further regulatory
action is taken (Option 2). It assumes FAR Part 36 remains in force for
aircraft, no new limits established for highway vehicle noise, and no change
in existing freeway design concepts to increase noise reduction. Numbers
outside of parentheses assume FAR-36 minus 10 dB for aircraft and additional
combined noise reduction for freeways and highway vehicles of 3 dB by 1985
and 5 dB by the year 2000.
225
-------
noise level estimated for Option 3 (see Figure 4-1) would require a 5 dB reduction in
the level of intruding noises just to maintain the status quo. In this instance, a CNEL
60 in the year 2000 would be equivalent in terms of predicted community reaction to
a CNEL 65 today. On the other hand, the interpretation of the results does not account
for the long term 30 year evolution of land use patterns which undoubtedly will occur.
For example, one of the principal reasons why railroads are not generally considered
a major community noise problem today, is that, for the most part, the land use around
railroads has slowly evolved to compatible usage over the past 30 to 60 years. The
extent to which this factor will offset the previous factors is unknown.
Estimates have been made of the relative cost-effectiveness of alternate
methods for reducing noise impacted land. For airports, reduction of noise at the
source (i.e., quieter engines) has been shown to be clearly more cost-effective than
reducing impact by land acquisition. Continued progress to reduce jet aircraft noise
should remain a first priority for Federal action on noise pollution. For freeways,
improvement of design to increase noise reduction with barriers is more cost-effective
by about 2 to 1 over land acquisition. Vehicle noise reduction is probably least cost-
effective for reducing freeway noise impact only, but it gives other benefits for the
total urban population. Thus, a balanced approach for reducing noise impact for the
highway transportation system should emphasize both vehicle noise reduction and
improved freeway design.
4.5 Impact on Participant or Passengers in Transportation Systems
The two significant effects of noise for participants or passengers in trans-
portation systems are (a) potential hearing damage from excessive noise exposure, and
(b) interference with speech communication for passengers.
Potential Hearing Damage
The potential hazard with respect to hearing damage for all categories of
the transportation system is summarized in Figure 4-4 in terms of an equivalent 8-hour
exposure level. This equivalent level is determined from the actual passenger noise
226
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Average
Maximum
Highway Vehicles (Typical Hours Use Per Day)
Motorcycles (Highway) (1)
Medium and Heavy Duty Trucks (4)
Highway Buses (4)
Utility and Maintenance Vehicles (1)
Light trucks and Pickups (1.5)
City and School Buses (2)
Passenger Cars-All Types (1)
Aircraft
Light Piston-Powered Helicopters (2)
Commercial - Propeller (1.4)
General Aviation — Propeller (1)
Commercial —2- and 3-Engine Turbofan (1.4)
Heavy Transport Helicopters (0.5)
Medium Turbine-Powered Helicopters (0.5)
Commercial —4-Engine Turbofan (1.4)
Commercial — Widebody (1 .4)
General Aviation — Executive Jet (0.5)
Rail Vehicles
3ay)
g
IE
153
anfl.4) 171
I
X
T
Iso
[so
77 |85
^5 J85
3 ]s
|80
75
.
I 84
75 87
75 87
Iso
i
»
95
95
8
90
Existing Rapid Transit (1.5)
Trolley Cars (1.5)
Passenger Trains (6)
High Speed Intercity (2)
Recreational Vehicles (Typical)
Snowmobiles (2)
Minicycles and Off-Road Motorcycles (2)
Inboard and Outboard Motorboats (2)
I I
50
60
65
Occupational
Safety and
Health Act
Criteria
70
80
90
Equivalent 8-Hour Exposure Level, dB(A)
Figure 4-4. Potential Hearing Damage Contributions from Transportation System Categories
in Terms of Equivalent 8-Hour Exposure Levels, for Passengers or Operators
227
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exposure using the same rule for trading off time of exposure and level that is utilized
in the Occupational Safety and Health Act. The estimated equivalent 8-hour exposure
levels of five of the transportation categories exceed the Occupational Safety and
Health Act criteria for an equivalent 8-hour day. In each case, even though the
number of days of exposure per year is much less than in a working year, noise protec-
tion for the operator's ear is highly desirable. In addition, many of the other sources,
including all those exceeding an equivalent 8-hour exposure level of 80 dB(A) are
potentially hazardous to some indivuals, particularly in combination with their expo-
sure to other noise environments. A proper evaluation of hearing damage risk for the
individual must account for this cumulative effect of his entire daily exposure to all
potentially harmful noises. Consequently, efforts should be made to reduce this noise
to minimize its potential hazard for hearing damage.
The effect of implementing the potential noise reduction outlined in Table
4-1 for transportation vehicles would be a substantial reduction of this risk of hearing
damage.
Speech interference criteria specify maximum desirable noise levels at the
listener's ear as a function of talker-listener separation for effective normal speech
communication. Table 4-8 summarizes typical talker-listener separation distances in
various transportation systems and corresponding maximum desired noise levels to mini-
mize speech interference at these distances.
Comparing the last two columns, average internal levels for the principal
passenger-carrying transportation categories generally fall within the desired limits to
avoid speech interference. V/STOL rotary-wing aircraft are a notable exception for
which internal noise levels are generally much higher than desired for effective speech
communication.
It should be noted that a lower bound can exist for internal sound levels
inside multiple passenger vehicles based on speech privacy requirements. While setting
minimum levels is not necessarily desirable for short-haul rapid transit vehicles or buses
228
-------
used daily by commuters, long-haul passenger vehicles such as aircraft with close seat
spacing are potential candidates for minimum levels based on speech privacy.
Table 4-8
Typical Passenger Separation Distances and Speech Interference Criteria
Compared to Average Internal Noise Levels for
Major Transportation Categories
Passenger Cars
Buses
Passenger Trains
Rapid Transit Cars
Aircraft (Fixed Wing)
V/STOL Aircraft
Talker- Listener
Separation
Feet
1.6 To 2. 8
1 to 1.7
1 to 1.7
1 to 1.7
1.1 to 1.7
1.1 to 1.7
Speech
Interference
Limits*
dB(A)
73 to 79
79 to 85
79 to 85
79 to 85
79 to 84 '
79 to 84
Average
Internal Noise
Levels
dB(A)
78
82
68 to 70
82
82 to 83
90 to 93
* Maximum noise levels to allow speech communication with expected voice level
at specified talker-listener separation distances.
A comparison of the average interior levels listed in Table 4-8 with speech
privacy criteria shows that aircraft and rapid transit vehicles tend to meet this "minimum"
level requirement for a typical seat pitch distance. However, internal levels for auto-
mobiles, buses and passenger trains generally fall below speech privacy criterion levels
for typical seat-to-seat distances. Reduction of minimum levels required for speech
privacy can be achieved only be increasing the seat spacing or increasing the barrier
attenuation of sound between seats.
In summary, the impact of internal noise levels on current commercial
passenger vehicles appears to be minimal, with the exception of V/STOL propeller or
rotary-wing aircraft. For the latter, internal levels tend to be excessive according to
both speech interference and potential hearing damage criteria. Noise levels for
229
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operators of heavy trucks, motorcycles and most gas engine-powered recreational
vehicles are excessive and should be reduced to avoid potential hearing damage risks.
4.6 Environmental Impact for Internal Combustion Engine Devices
As indicated earlier in Figure 4-3, various labor-saving devices powered
by internal combustion engines are a rapidly growing source of intrusive noise in many
communities.
The principal characteristics of internal combustion engines as sources of
potential noise impact are summarized in Table 4-9 using the same parameters presented
earlier for transportation vehicles. In general, these devices are not significant
contributors to average residual noise levels in urban areas. However, the relative
annoyance of most of the garden care equipment tends to be high. This is due to the
long duration of noise for these sources. This leads to a Single Everit Noise Exposure
Level much greater than the approximate annoyance threshold of 727dB at a distance
of 50 feet, a typical neighbor-to-neighbor distance. Clearly, further noise reduction
for these devices is desirable. Similarly, a distinct local increase in the residual level
in rural or wilderness areas may be experienced at distances up to one mile from such
devices as chainsaws. As a result, they constitute a persistent source of annoyance for
persons seeking the solitude of wilderness areas. In addition, use of chain saws can
result in equivalent 8-hour exposure levels of 83 to 90 dB(A) for the operator, indicating
the desirability of hearing protection for operators.
Potential Change in Noise Impact of Internal Combustion Engine Devices
The future growth in numbers of these devices is difficult to forecast
accurately due to the lack of detailed data on their current usage. Such devices often
have a short life span and, since they are seldom registered in any systematic way, the
accuracy of future growth projections is questionable. The past growth of some of
these devices has been spectacular, as shown in Figure 4-3. However, once the device
has completed its basic market penetrati9n, its growth rate should be expected to slow
down to that-of the general economy. Therefore, one can at least expect a general
upward trend in their utilization as convenient and normally effective labor-saving
230
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Table 4-9
Summary of Noise Impact Characteristics of Internal Combustion Engines
Source
Lawn Mowers
Garden Tractors
Chain Saws
Snow Blowers
Lawn Edgers
Model Aircraft
Leaf Blowers
Generators
Tillers
A-Weighted(1)
Noise Energy
KiJowatt-Hrs
Day
63
63
40
40
16
12
3.2
0.8
0,4
Typical A-Weighted
Noise Level
at 50 Feet
dB(A)
74
78
82
84
78
.78. -
76
71
70
Typical
SENEL(4)at,
50 Feet
and 1 sec
111
N/A
118
120
111
108
106
—
106
• " (2)
8-Hr Exposure
Level
dB(A)
Average
74
N/A
85
61
67
70(3>
67
-.
72
Maximum
82
N/A
95
75
75
79(3)
75
-
80
Typical
Exposure
Time
Hours
1.5
N/A
1
1
1/2
1/4
1/4
•
1
(1) Based, on estimates of the total number of units in operation per day.
(2) Equivalent level for evaluation of relative hearing damage risk.
(3) During engine trimming operation.
(4) See Appendix B for definition of SEN EL.
CO
-------
devices which will always be in demand. This clearly represents an upward trend in
their noise intrusion potential.
The combined effort by the public in demanding quieter products powered
byinternal combustion engines and successful response to this demand by the manu-
facturers, should provide a substantial decrease in annoyance from this equipment.
This reduction in annoyance of intruding noise from lawn mowers, chain saws, et
cetera, will be the principal benefit of a broad noise reduction, program for devices
powered by internal combustion engines. The estimated potential noise reduction that
might be expected in the future for some of these devices has been summarized earlier
in Table 4-2. The noise reduction values were relative to current noise levels and
were specified in terms of potential reductions that could be achieved by the 1975,,
1980 and 1985 time periods (i.e., Option 3).
Full accomplishment of these noise reductions would largely eliminate
annoyance problems in residential areas associated with use of lawn care equipment.
However, the noise reduction potential for chain saws using .existing technology is not
sufficient to eliminate their annoyance problems or hearing damage risk for operators.
Further noise reduction research is called for with these unique devices. .
232
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5.0 CONCLUSIONS AND RECOMMENDATIONS
The data and discussions presented in this report have attempted to sum-
marize many aspects of a very complex environmental problem. The manufacturing and
transportation industries involved are a major segment of our national economy. Further,
the transportation industry provides the essential service which enables the remainder of
our economy to function. Unfortunately, noise is a byproduct of these industries. Thus,
the majority of the sources discussed in this report contribute to noise'.pollution.
Highway vehicles are responsible for the outdoor residual noise level in our
communities, as'well as for freeway noise. Aircraft are responsible for the noise in the
vicinity of airports. Recreation vehicles are responsible for disturbing noise in the
remote wilderness areas, and lawn care equipment is responsible for excessive noise in
the neighborhood. In addition, some of the sources in each of :these general categories
represent a potential hazard of hearing damage and most of the sources are often respon-
sible for single-event noise intrusion in residential neighborhoods. Consequently^ there
are a variety of noise problems to be examined and solved within acceptable economic,
technical and social constraints. '.,... . -
It will be a very difficult task to solve all of the major noise problems in
the environment within these constraints. Such a task requires development of national
noise goals, cause-and-effect noise system models, and economical and technical
feasibility analyses which are beyond the scope of this report. However, the data pre-
sented in this report forms a necessary point of departure and suggests several useful
directions for accomplishing the much needed task of controlling our noise environment
for the benefit of our entire population.
This chapter presents the initial conclusions from this work, including the
total impact on people of the noise sources discussed herein, industry's need for public
guidance if it is to successfully implement noise reduction, and an identification of
possible priorities for Federal action. It also contains a brief summary of major recom-
mendations for the development of noise measurement standards, noise reduction
demonstration projects, and research programs.
233
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5.1 Noise Impact on People
The noise of each of the source categories in this report has been evaluated
in Chapter 4, with reference to its potential impact. This evaluation, together with the
1 o
analysis of the effect of noise in companion reports, ' provides a basis for assessing the
impact of the noise of the source categories on the population of this country. This
assessment is made for (1) continuous outdoor noise sources which interfere with speech,
(2) other noises resulting in community reaction and annoyance, and (3) noise which may
be potentially hazardous to hearing.
Continuous Outdoor Noise which Interferes with Speech
The noise environment is primarily a product of man and his machine. It
consists of an all-pervasive and non-specific residual noise, to which are added both
constant and intermittent intrusive noises. The residual noise level in urban residential
communities is generally the integrated result of the noise from traffic on streets and.
highways, principally automobiles and light trucks in the daytime, and including
heavier trucks at night. The daytime outdoor residual noise levels vary widely with
the type of community and can be grouped into the following approximate ranges:
• wilderness and rural 16-35 dB(A)
• suburban residential 36 - 45 dB(A)
• • urban residential 46 - 55 dB(A)
• very noisy urban residential g., _,. IR/A\
and downtown city
Residual noise levels in suburban and rural areas do not appear to interfere
with speech communication at distances compatible with normal use of patios and back-
yards and often provides beneficial masking for speech .privacy. However, some inter-
ference with outdoor speech is found in urban residential communities, and considerable
continuous interference is found in the very noisy urban and downtown city areas. Thus,
f-he use of outdoor space for conversation is effectively denied to an estimated 5 to
10 million people who reside in very noisy urban areas.
234
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The backyards, patios and balconies facing an urban freeway are similarly
rendered useless on a continuous basis, except when traffic is very light in the early
morning hours. Although windows are kept closed in many dwelling units adjacent to
freeways to keep out the noise, the level inside the dwelling may still be too high for
relaxed-conversation. An estimated 2.5 to 5 million people living near freeways are
impacted significantly by this intrusive noise source. Probably another 7 to 14 million
people are impacted to a lesser degree by the noise from traffic on the 96 thousand
miles of major arterial roads in urban communities.
Thus, the combination of continuous daytime noise pollution caused by
traffic on city streets, major arterials and freeways impairs the utility of the patios,
porches and yards outside the dwelling units of approximately 7 to 14 percent of the
total population. The analysis of Chapter 4 suggests that this situation will grow worse
by the year 2000, unless the noise from automobiles and trucks is reduced. However,
it could be improved by about 5 dB if noise reductions of 5 and 10 dB for automobiles
and trucks, respectively, were accomplished by the 1985 time period. Such a reduc-
tion in the residual noise level should not destroy speech privacy in suburban areas and
would improve the situation in the higher noise level urban areas. However, it would
need to be supplemented by better land use planning and design of freeways and
arterials to solve current and future noise problems.
Other Noises Resulting in Community Reaction and Annoyance
Adverse community reaction may be expected when the energy equivalent
2
level of an intruding noise exceeds the residual noise level. The degree of reaction
depends primarily on the amount of the excess, and secondarily on additional factors
such as season, personal attitude, and characteristics of the noise. For example, wide-
spread complaints may generally be expected when the energy equivalent level exceeds
the residual level by approximately 17 dB, and vigorous community action when the
excess is approximately 33 dB. For these two values, the approximate percentage of
the affected residents who are "very much annoyed" was found in one survey to be 37
and 87 percent, respectively. The impact of several forms of noise pollution, including
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intermittent noise from multiple single events such as aircraft overflights, infrequent
diesel trucks on the highway, and the use of lawn care equipment, is often most
effectively evaluated in terms of community reaction.
: The most outstanding national problem which can be defined in these terms
is the impact of aircraft noise. It is conservatively estimated that the number of .people
living in areas where aircraft noise exceeds the level required to generate widespread
complaints is 7.5 million. This estimate assumes that all of the people affected live in
residential urban communities. A more realistic estimate, including the people affected
by aircraft noise who live in quiet and normal suburban communities, is 15 million.
Most of the people impacted experience noise levels which interfere with speech/ TV.
enjoyment, and indoor and outdoor speech communication every time an aircraft passes,,
and are often awakened or disturbed during sleep.
This has been a most difficult problem to splye because i;t, jgrew to enormous
proportions in only a few.years, with no technically or economically feasible means
available for its solution. Partial solutions of the noise psoblems of fixed-wing aircraft
are now available. These solutions have resulted from Federal action to regulate noise.
and the incorporation of new noise reduction technology, which meets or. exceeds the
Federal standards^ into.new aircraft. However, an additional 10 dB of noise reduction^
over that achieved to date must be obtained through future technological research and
development; otherwise, the problem cannot be solved for the remainder of this century
withoutia massive alteration, in land use near airports or the development of an entire
new airport system well removed from urban areas. Realization, of this additional noise
reduction through technical advance and Federal regulation, together with effective
procedures for implementing compatible land use planning should effect a solution
through the year 2000.
-,•••' t
In addition to the people impacted by aircraft noise, there are uncounted
• .. • • j •'
millions who are annoyed by sources such as: motorcycles, minicycles and sportscars
i
operated in a noisy manner on residential streets; dunebuggies, chainsaws and snow-
> . . i i
mobiles operating in the wilderness; power lawnmowers, edge clippers and snowblowers
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operated by a neighbor on Sunday morning; and heavy trucks transporting freight at
night. The single event noise exposure levels of almost every noise source category
examined in this report can be classified as noisy when the source is operated in the
urban residential environment. The principal exception is the automobile in its normal
operation oh a residential street, although automobiles, particularly sportscars and
small imported compact cars, are judged noisy when operated with unnecessarily high
acceleration. '
The number of people who experience intermittent Interference with speech •
and are otherwise annoyed by one or more of these sources at various times, probably
include at least 75 percent of the population. However, the degree of lasting annoyance,
and its accompanying'probable community reaction, depends critically on the number'of
times the source'operates per day, the time of day that it operates, people's attitude
2
toward the source, and other factors. There is no simple way of quantifying the magni-
tude of the overall impact in these terms since, unlike the airport or other industrial
noise problems, there has been no centralized focal point for citizen expression. There-
fore, perhaps the best indicator of the true community reaction is the significant incre'ase
of political activity by citizens operating through all levels of government to attempt to
reduce the noise output of most of these sources through governmental regulation.
If the noise reductions selected in the Option 3-example of Chapter 4 were
achieved by 1985, most of these noise sources would be expected to be judged accept-
able when operated properly in the appropriate land use areas. However, considerable
technical development is required to achieve this result with production hardware, and
local operational and noise regulations will be required to ensure proper operation and
restriction to appropriate land use areas.
Noise Which May be Potentially Hazardous with Respect to Hearing Damage
There is a long history of occupational noise environments which have
resulted in hearing impairment of various degrees for some of the working population.
For the most part, workers are now protected from such hazard through Federal enforce-
ment of the provisions in the Occupational Health and Safety Act.
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However, there are also many occasions where people may be exposed to
potentially hazardous noise in a non-occupational, environment. The more significant
of these potential hazardous noise exposures for the sources in this report are summarized
in Table 5-1. The equivalent 8-hour exposure level for these sources, on any day of
use, was estimated in Chapter 4 to exceed 80 dB(A). Although the average person who
is infrequently exposed to such noises will not necessarily suffer permanent hearing
damage, frequent exposure to any one or several of these noises, or infrequent exposure
in combination with industrial noise, will increase the risk of incurring permanent
hearing impairment.
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Table 5-1
Approximate Number of People (Operators and Passengers) in
Non-Occupational Situations Exposed to Potentially
Hazardous Noise with Respect to Hearing Damage from
Various Significant Sources
Source
Snowmobiles
Chain Saws
Motorcycles
Motorboats (over 45 HP)
Light Utility Helicopters
General Aviation Aircraft
Commercial Propeller Aircraft
Internal Combustion Lawnmowers
and other Noisy Lawn Care
Equipment
Trucks (Personal Use)
Highway Buses
Subways
Noise Level in dB(A)
"Average**
108
100
95
95
94
90
88
87
85
82
80
Maximum
112
110
110
105
100
103
100
95
100
90
93
Approximate Number
of People
In Millions
1.60
2.50
3.00
8.80
0.05
.0.30
5.00
23.00
5.00
2.00
2.15
**
Although average use of any one of these devices by itself may not produce
permanent hearing impairment, exposure to this noise in combination, or
together with occupational noise will increase the risk of incurring permanent
hearing impairment.
k
Average refers to the average noise level for devices of various manufacture
and model type.
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Summary
These data show that approximately 22 to 44 million people have lost part
of the utility of their dwellings and yards due to noise pollution from traffic and aircraft,
and an even larger number of people are frequently subjected to intermittent speech
interferences and annoyance from most of the sources considered in this report. Further-
more, some of these people, and others, are exposed to potentially hazardous noise,
principally when operating or riding in noisy devices. Although the number exposed
to potentially hazardous noise cannot be accurately assessed, since the people enumer-
ated in Table 5-1 are not additive, a total of 30 million people might be a reasonable
estimate.
Thus, noise pollution from these sources appears to impact at least
50 million people, or 25 percent of the population. Roughly one-half of this total
impact is a potential health hazard, and the remaining one-half is anlinfringement on
the ability to converse in the home environment. When the number of people who have
occasional interference with speech as a result of intruding single event noise sources
is included, the total number of people impacted probably rises to the order of 75 per-
cent of the population. These percentages clearly show the need for action to reduce
the number of devices which have potentially hazardous noise and are used by the
public, and to reduce the outdoor noises which interfere with the quality of life.
5.2 Interaction Between Public and Industry
Much of the strength of the nation's economy, and the accompanying high
standard of living, resulted from technical innovation and its utilization by industry in
the development of new and better machines which fulfill people's needs. By-and-
large, the performance criteria for these machines are defined in terms of the useful
work which they will accomplish and the value of this work with respect to its cost.
The success of any new product is determined in the market place, primarily in terms
of the potential economic value of the product to the customer relative to its total
cost, including both initial and operating costs.
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In the case of acoustical devices such as musical instruments, hi-fi sets
and speech communication equipment, sound characteristics are a primary performance
criterion. However, for the other devices, noise is generally an unwanted byproduct
which is not associated with the primary performance criteria. Only when a need for
less noise is articulated, through customer preference or public action, does noise
become one of the primary performance criteria. The information feedback process
from the public to industry generally takes many years and often presents a conflicting
set of needs. .For example, the purchasers of devices such as motorcycles, sportscars,
trucks and power lawnmowers often consider noise as a positive indicator of high per-
formance. For the same reasons, the owners of many types of devices purposely operate
them in their noisiest mode or modify them by removing their mufflers for "added power."
In such cases, where the consumer and public interests diverge, industry responds to the
consumer until the offended public articulates its requirements.
One of the best examples of the possible long term noise accommodation
among industry, public and the market place is the standard American passenger car.
In its sixty-year history, it has evolved from a noisy, sputtering, crude, low-powered
vehicle to a relatively quiet, efficient, high-powered vehicle. Mufflers were installed
before World War I to prevent scaring horses, and thus win a wider acceptance in the
market place. In the 1920's, cities and towns set regulations requiring that all cars be
muffled, primarily to ensure that owners retained the mufflers supplied with the vehicle
in good working order. Without further action in the public sector, industry has made
continuous progress toward quieting the automobile interior to gain wider acceptability
in the market place, and in so doing has also attained reasonably acceptable exterior
noise levels for individual automobiles.
Thus, although the market place provides industry with sufficient infor-
mation to act in the national interest in the primary performance and cost aspects of its
products, it does not necessarily provide such information about secondary performance
factors such as noise'. Consequently, unless the public articulates its requirements for
noise, industry has little basis for establishing noise criteria and developing products
which meet these criteria.
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During the last few years, various governmental bodies have begun to express
the public concern by developing and implementing noise regulations for various sources.
With the exception of aircraft noise, where the Federal Government has begun to act/
many of the remaining sources are being subjected to a series of separated, uncoordinated
and often conflicting regulations. Actions by the public, as well as the data presented
in this report, give clear evidence of the need for noise reduction. However, if industry
is to make an effective response in controlling the noise of its products, it must have
clear and consistent guidance. Only the Federal Government can fulfill this role.
5.3 Federal Action to Reduce Source Noise
Most of the sources discussed in this report have additional noise reduction
potential which can be attained with application of today's technology. In many cases,
these potential improvements will probably be sufficient to control noise pollution in the
public interest. However, in some cases, including aircraft engines,-tires and chain-
saws, present technology is clearly insufficient to provide adequate noise control, and
research is necessary. In either case, the eventual reduction of noise pollution in the
nation requires establishment of a balanced set of noise goals which will enable priorities
to be set for systematic exploitation of existing technology and development of new
technology.
Together with these goals, source noise standards and the implementation of
regulations must be promulgated to give industry a definite set of performance criteria
for all of its products which are capable of causing noise pollution. Such standards
should have time scales for achievement which are consistent with industrial design,
prototype test and production cycles to encourage the most economical and effective
incorporation of noise performance criteria into the total design of the product.
Regulations should cover at least all the sources which were shown in this
report to be responsible for the significant noise pollution. High priority should be
given to the sources which may constitute a potential hazard for hearing. This includes
most of the recreational vehicles, internal combustion powered lawn care equipment
and some transportation vehicles, as presented in Table 5-1. In addition, high priority
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should be given to all types of aircraft and large highway vehicles which are associated
with the airport and freeway noise problems, and to the other elements of city traffic,
so that the people living in major cities will eventually be able to enjoy relaxed con-
versation outdoors. Finally, high priority should be given to the lawn care equipment
and recreational vehicles which cause unnecessary intrusion, intermittent interference
with speech, and annoyance. Without an effective noise regulatory program, today's
noise pollution problems will grow in size and impact an ever-increasing number of
people.
5.4 Recommendations for Noise Reduction
Specific recommendations for programs to reduce the overall noise pollution
of transportation systems and internal combustion engine devices are summarized in the
following paragraphs. These recommendations are provided in four general groups in
approximate descending order of priority within each group. The four types of programs
and their basic objectives are:
• Demonstration Programs — Provide a clearly visible (or really audible)
demonstration of the application of existing technology to noise reduc-
tion for a particular category. Economic practicality shall be con-
sidered but shall not be a firm constraint.
• Research Programs — Carry out applied or basic research to develop new
technology required to define the ultimate noise reduction potential
available beyond existing technology or achieve economically practical
methods for utilizing existing technology, where adequate.
• Measurement Standards Programs — Develop, in conjunction with industry
and professional organizations, effective procedures for noise certifica-
tion of all categories of the transportation system not currently covered
by Federal noise standards.
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• Noise Certification Programs — Develop national standards for maximum
noise levels of major transportation vehicles (similar conceptually to
FAR Part 36) and internal combustion engine devices so that manufac-
turers can plan product development for noise control on a uniform
basis. Control on usage relative to community noise abatement should
be retained by local state, county and city governments.
Several criteria have been used to establish the approximate priority for the recommended
programs. These criteria include:
• Action to reduce potential hearing damage risk to passengers or non-
commercial operators of transportation vehicles or internal combustion
engine devices.
• Action to reduce the noise impacted land area near airports and major
urban highways.
• Action to reduce the annoyance from noise of increasing numbers of
vehicles or ICE devices which generate higher noise levels.
Demonstration Programs
• Commercial Aircraft — Continue Federal commitments to the full range
of aircraft noise reduction programs. Commercial jet aircraft are and
will continue to be for the foreseeable future the major source of noise
pollution in urban communities. Reduction of this noise impact will
require vigorous pursuit by the Federal government, in conjunction with
aircraft engine and airframe manufacturers of the .currently planned
demonstration programs. These include:
The "Quiet Engine" Program (NASA Lewis/General Electric)
Development of flightworthy nacelle retrofit packages (FAA/Boeing)
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Prototype 150-passenger STOL aircraft to meet 95 EPNdB at
500 feet (NASA Program anticipated)
Small engine noise reduction program (WPAFB/AiResearch)
Establish a program to demonstrate maximum noise reduction potential
within the present state of the art for helicopters intended for law
enforcement and other general governmental functions.
General Aviation Aircraft — A major program should be formed at the
Federal level to demonstrate the optimum state of the art in reducing
propeller and engine noise for general aviation aircraft. The projected
growth of the general aviation fleet over the next 20 years is sufficient
to indicate that the growth innumberand operation of urban general
aviation airports will provide another source of significant noise impact
for urban populations unless counteracting action is taken to minimize
any increase in noise pollution corresponding to the growth in the gen-
eral aviation fleet.
Demonstration of very significant noise reductions for executive jet
aircraft is now being made by some manufacturers. Further demonstra-
tion and implementation of this noise reduction should be fostered by
strict enforcement of FAR Part 36 for all new or modified aircraft
requiring a new flight certification.
Highway Vehicles — Noise levels of new passenger cars are generally
being limited by existing or proposed limits imposed by state law. No
specific Federally-funded demonstration program is considered necessary
at this time for such vehicles. However, tire noise presents a major
obstacle to further substantial reduction of automobile noise and
requires a separate high priority effort.
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Noise levels for new trucks are also being partially limited by state laws.
However, a demonstration program is recommended to foster industry
competition to achieve substantial additional reduction in truck noise
levels. Excluding tire noise, truck noise can be reduced substantially
within the present state of the art. The principal objective of this
demonstration program would be to define this state-of-the-art limit
with due consideration given to economic practicality. The results of
the program would provide a baseline for establishing research goals
to improve the state of the art.
Noise levels for trucks generally increase with age. A demonstration
program is recommended to define an optimum concept for truck over-
haul which combines practical noise reduction concepts with optimum
performance objectives to extend the economic life of the truck while
minimizing :ts noise signature.
Sufficient demonstrations have been made of potential reduction in
tire noise to indicate that an extensive research program is required
to advance the state of the art.
A program to demonstrate practical noise reduction retrofit packages
for existing utility and maintenance trucks (such as garbage trucks)
would provide a basis for achieving compliance with desired reduction
in annoyance from these vehicles.
Recreation Vehicles — The motorcycle is the primary source of noise
pollution from recreation vehicles. A program to demonstrate "quiet
motorcycles" for both highway and off-highway use is recommended.
This could take the form of an industry competition to achieve the maxi-
mum practical noise reduction within the present state of the art.
An educational program for the potential user should be part of this
effort to motivate the motorcyclist to employ a quiet muffler for all
recreational uses.
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Stringent reductions in noise from snowmobiles are imposed by state
laws now in existence or proposed. It is felt that compliance with
these regulations will effectively demonstrate noise reduction potential
(within the current state of the art) for these vehicles.
A related program would provide a demonstration of an acceptable
compromise between noise reduction and performance for high-
powered pleasure boats used for ski-towing.
• Rapid Transit Vehicles — Substantial improvements have been made in
reducing noise for several different rapid transit systems. However,
there is a real need to bring together into one program, a demonstra-
tion of the best noise reduction features of all these systems — in other
words, demonstrate the best noise reduction available with a rapid
tranit system designed with noise reduction as a principal constraint.
• Internal Combustion Engine Devices — A demonstration program is
recommended to achieve substantially lower noise levels for lawn
mowers and chain saws. This might take the form of an industry
competition and would have the objective of defining practical limits
for noise reduction within the current state of the art, thus leading to
research goals for improving this state of the art.
Research Programs
• Commercial Aircraft — Increased research on:
Fan/compressor noise
- Core engine noise
Supersonic jet engine noise reduction
Advanced technology quiet aircraft
V/STOL propulsion systems.
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General Aviation Aircraft
Basic research on propeller noise should be pursued by propeller
manufacturers .
Pursue improved concepts in engine muffler designs for reciprocat-
ing and turboshaft propeller aircraft.
Develop optimum lightweight methods for cabin noise treatment
of general aviation aircraft.
Develop "quiet" turbofan engines specifically designed for mission
requirements of executive jet aircraft.
Highway Vehicles
Conduct a broad ranging research program on tire noise reduction.
Objectives should include, but not be limited to, overcoming the
K
current economic and safety constraints of quiet recap tires for the
trucking industry.
Advanced technology research for quieting of truck noise with
emphasis on overall system design tradeoff problems involving intake
noise reduction versus engine block cooling concepts, engine casing
enclosure techniques versus engine compartment cooling require-
ments, exhaust noise reduction versus exhaust pressure effects on
engine performance.
Basic and applied research on noise reduction potential for new
types of truck engines such as turboshaft drive, unique engine cycles
(i.e., Wankel engine), or turbocharged two or four cycle diesel
engines instead of roots-type blowers for diesels.
Basic and applied research on quieting of transit buses. Research
objectives to emphasize reduction in wayside noise of engine
intake experienced by bystanders as bus departs; and elimination
of brake squeal.
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Applied research program to establish improved methods for
evaluating noise levels generated by highway vehicle traffic.
Study should include models for evaluating residual noise levels
as well as noise impact areas near freeways as a function of free-
way noise reduction design features.
Recreation Vehicles
- Wide ranging research program directed toward development of
lightweight muffler designs adaptable to motorcycles, minicycles,
snowmobiles, etc. Program should include full exploration of
advanced materials and acoustics technology to achieve optimum
performance with design constraints for these vehicles.
t . ' - '
- Applied research program to overcome systems problems in achiev-
• . ' *\ ... - »
ing additional noise reduction for gasoline-powered recreational
vehicles. Approaches should reflect new technology or utilization
of new techniques to reduce engine intake and engine casing noise
on the assumption that the engine muffler program will be suffi-
ciently successful so as to make these sources dominant.
Rail Transit Vehicles and Ships
Conduct analysis of future noise impact from high speed above
ground, ground surface and below ground rapid transit systems
that may be developed over the next 15 to 25 years in major urban
areas. Srudy to include evaluation of probable transportation
demands and the noise impact generated by alternate methods for
meeting this demand. :
Conduct similar study for potential noise impact for high speed
water transportation systems such as surface effect machines or
hydrofoils that may be included in significant numbers in future
urban transportation systems.
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Depending on results of above programs, conduct advanced
research on noise reduction techniques applicable to urban rapid
transit systems for which a significant growth in noise impact is
predicted.
• Devices Powered by Small Internal Combustion Engines
Adopt noise reduction research results or objectives for recreation
vehicles to requirements for low-cost engine design constraints of
lawn care and yard maintenance equipment. Particular attention
to be paid to reducing noise of chain saws and lawn mowers with
the use of advanced technology.
Measurement Standards and Noise Certification Limits
• Commercial Aviation
Continue utilization and periodic updating of FAR Part 36 for
noise certification of commercial aircraft.
Establish comparable standards for STOL and VTOL aircraft.
• General Aviation Aircraft
Continue development of noise certification limits and measure-
ment' techniques for all categories of general aviation aircraft.
• Highway Vehicles
Update existing industry measurement standards for highway
vehicles (such as the SAE method) to reflect more realistic
operating conditions for the vehicle and measurement procedures
more readily adaptable to local agency enforcement.
Develop standard techniques for noise measurement of individual
components on trucks and cars to provide a uniform basis for noise
control at the manufacturers level. Particular emphasis should be
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placed on engine intake air and cooling components as well as
tires. Specification of limits for these components should be the
responsibility of manufacturers who must meet total system noise
limits imposed by local or Federal government agencies.
Develop a noise measurement procedure and outline potential
noise certification limits for vehicles at highway speeds (50 mph
or greater) which fairly accounts for the influence of tire noise.
Recreation Vehicles
Develop national standards for noise measurement techniques and
minimum noise levels for all classes of recreation vehicles with
emphasis on motorcycles.
Devices Powered by Small Internal Combustion Engines
Standardize, at the national level, measurement techniques
and noise certification limits for newly manufactured internal
combustion engine devices such as lawn mowers and chain saws.
Establish minimum standards for noise certification of portable
generators to be used for mobile homes.
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REFERENCES - 1.0 Introduction
"Report to the President and Congress on Noise, " U.S. Environmental
Protection Agency, December 31, 1971.
252
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REFERENCES-2.0 Transportation Systems
1. Statistical Abstract of the United States 1970,, Bureau of Census, U.S.
Department of Commerce.
2. 1969 Statistical Supplement to the Survey of Current Business, Bureau of
Census, U.S. Department of Commerce.
3. Auto Facts, Automobile Manufacturers Association, Inc., Detroit, Michigan,
WT
4. Air Transport 1971, Air Transport Association of America, Washington, D.C.
5. 70-71 Transit Fact Book, American Transit Association, Washington, D.C.
6. Leisure Time Product Noise, Sub Council Report, National Industrial
Pollution Control Council, May 1971.
253
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REFERENCES-2.1 Commercial Aircraft
1. Statistical Abstract of the United States 1970, Bureau of Census, U.S.
Department of Commerce.
2. Air Transport 1971, The Air Transport Association of America, Washington,
D.C., 20036.
3. "Aviation Week and Space Technology Forecast and Inventory Issue,"
Aviation Week and Space Technology, March 8, 1971.
4. McPike, Ad., "Community Noise Levels of the DC-10 Aircraft, " Paper
5957, McDonnell-Douglas, Long Beach, California, 1971.
5. Sperry, W.C., "Information Brief on Estimated Maximum Noise Levels of the
Existing Turbojet/Fan Airplanes," Report on Agenda Item 1, International
Civil Aviation Organization Committee on Aircraft Noise (CAN), Montreal,
Canada, September 28-October 2, 1970.
6. Sperry, W.C., "Information Brief on Boeing 747 Noise Characteristics,"
Federal Aviation Administration Briefing, September 23, 1970.
7. Bishop, D.E. and Horonjeff, R.D., "Procedures for Developing Noise
Exposure Forecast Areas for Aircraft Flight Operations," FAA DS-67-10,
Federal Aviation Administration, August 1967.
8. Bishop, D.Eoand Horonjeff, R. D., "Noise Exposure Forecast Contours for
Aircraft Noise Tradeoff Studies at Three Major Airports, "FAA-NO-07-7,
Federal Aviation Administration, July 1970.
9. Bray, D.E., "Noise Environments in Public Transportation," 81st Meeting of
The Acoustical Society of America, Washington, D.C., April 21, 1971.
10. Gebhardt, G.T., "Acoustical Design Features of Boeing Model 727,"
Journal of Aircraft, Vol. 2, No. 4, July/August 1965.
11. McPike, A.L., "Cabin Sound Pressure Levels During Cruise of the DC-8 with
JT3C-6 Engines," Report No. SM-23966, McDonnell-Douglas, Long Beach,
California, May 1960.
12. Anon., "Cruise Flight Cabin Acoustics, DC-9, Nos 23 and 28," TM-DC9-4132,
McDonnell-Douglas, Long Beach, California, July 1966.
254
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13. Aircraft Noise Staff, The Boeing Company, private conference, August 1971.
14. Wyle Laboratories, (unpublished data),
15. Higgins, T., Federal Aviation Administration, private telecon, August 1971.
16. Hubbard, H.H., "The Nature of the Sonic-Boom Problem," 70th Meeting of
The Acoustical Society of America, St. Louis, Missouri, November 3-6,
1965.
17. Kryter, K.D., "Sonic Booms from Supersonic Transports," Science, Vol. 163,
January 24, 1969.
18. Wyle Laboratories Research Staff, "Community Noise," WR 71-17, Office of
Noise Abatement and Control, Environmental Protection Agency, Washington,
D.C., November 1971.
19. Supporting Papers, DOT TST-10-5 and NASA SP-266, Joint DOT-NASA
Civil Aviation Research and Development Policy Study, Department of
Transportation and National Aeronautics and Space Administration, March
1971.
20. Bishop, D.E. and Simpson, M.A., "Noise Exposure Forecast Contours for
1967, 1970 and 1975 Operation at Selected Airports," FAA-NO-70-8,
Federal Aviation Administration, September 1970.
21. Wyle Laboratories Research Staff, "Palmdale Intercontinental Airport Study of
Noise Attenuation Requirements for Palmdale Schools and Residences Based on
Noise Contours Projected for the 1985 Time Period," Department of Airports,
Los Angeles, California, July 1971.
22. "NASA Acoustically Treated Nacelle Program," NASA SP-220, National
Aeronautics and Space Administration, September 1971.
23. Kramer, J.J., "The NASA Quiet Engine," NASA TM-X-67884, National
Aeronautics and Space Administration, September 1971.
24. Pendley, R.E., "The Integration of Quiet Engines with Subsonic Transport
Aircraft," NASA CR-72548, National Aeronautics and Space Administration,
August 1969.
25. Copeland, W.L., et al, "Noise Measurement Evaluations of Various Takeoff-
Climbout Profiles of a Four-Engine Turbojet Transport Airplane," NASA TN
D-3715, National Aeronautics and Space Administration, July 1966.
255
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26. Quigley, H.C., et al, "Flight Simulation Investigations of Method for
Implementing Noise Abatement Landing Approaches," NASA TN D-5781,
National Aeronautics and Space Administration, May 1970.
27. Zalovcik, J.A., "NASA Research on Noise - Abatement Approach for
Multiengine Jet Transport Aircraft," NASA TN D-4044, National Aero-
nautics and Space Administration, April 1967.
28. Russell, R.E. and Kester, J.D., "Aircraft Noise. Its Source and Reduction,"
Society of Automotive Engineers — Department of Transportation Conference
on Aircraft and the Environment, Washington, D.C., February 8-10, 1971.
29. FAA Request for Proposal WASS-1 (DC), Federal Aviation Administration,
1970.
30. Acoustics Research Staff, National Aeronautics and Space Administration,
Lewis Research Center, private conference, August 1971.
31. Yaffee, M.L., "NASA Building STOL Transports," Aviation Week and Space
Technology, September 13, 1971.
32. Aerodynamics Staff, National Aeronautics and Space Administration, Ames
Research Center, private conference, August 1971.
33. Aviation Forecasts, Fiscal Years 1970- 1981, Federal Aviation Administration,
January 1970.
34. Report of the Department of Transportation Air Traffic Control Advisory
Committee, Department of Transportation, Washington, D.C., December 1969.
256
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REFERENCES -2.2 V/STOL Aviation
1. Jagger, D.H. and Kemp, E.D.G., "The Potential and Development of V/STOL
Intercity Airlines, " Aircraft Engineering, Vol. 42, No. 1, 1970.
t
2. "Study of Aircraft in Short Haul Transportation Systems, " NASA CR-986,
National Aeronautics and Space Administration, 1968.
3. Metygar, F.B. and Foley, W.M., "STOL Aircraft Noise Certification - A
Rational Approach, " SAE No. 700325, Society of Automotive Engineers,
April 1970.
4. Snodgrass, I.C., "Position Paper on STOL Noise Rules, " Aerospace Industries
Association of America, Washington, D.D., May 27, 1970.
5. Directory of Helicopter Operations in the United States and Canada 1971,
Aerospace Industries Association of America, Washington, D.C.
6. "Aviation Week and Space Technology Forecast and Inventory Issue, "
Aviation Week and Space Technology, March 8, 1971.
7. Statistical Abstract of the United States 1970, Bureau of Census, U.S.
Department of Commerce"
8. Air Transport 1971, The Air Transport Association of America, Washington,
D.C., 20036.
9. Supporting Papers, DOT TST-10-5 and NASA SP-266, Joint DOT-NASA Civil
Aviation Research and Development Policy Study, Department of Transportation
and National Aeronautics and Space Administration, March 1971.
10. Bell Helicopter Company, private correspondence, August 1971.
11. Hamilton Standard Division, United Aircraft Corporation, private correspond-
ence, August 1971.
12. Sikorsky Aircraft, United Aircraft Corporation, private correspondence,
August 1971.
13. The Boeing Company, Vertol Division, private correspondence, August 1971.
257
-------
14. Wyle Laboratories (unpublished data).
15. Yaffee, M.L., "NASA Building STOL Transports, " Aviation Week and
Space Technology/ September 137 1971.
16. Rosen, G., "Prop-Fan —A High Thrust, Low Noise Propulsor, " Society of
Automotive Engineers, National Air Transportation Meeting, Atlanta, Georgia,
May 10-13, 1971.
17. Jane's All The World's Aircraft 1970.
18. Holmes, D.R. and Cox, C.R., "Noise Reduction Possibilities fora Light
Helicopter, " Society of Automotive Engineers National Aeronautic and Space
Engineering and Manufacturing Meeting, Los Angeles, California, October
6-10, 1969.
19. "Flying Neighborly- Hot to Operate the Light Helicopter More Quietly, "
Bell Helicopter Company, Fort Worth, Texas.
20. Duke, F.H. and Hooper, W.E., "The Boeing Model 347 Advanced Technology
Helicopter Program, " 27th Annual National V/STOL Forum of the American
Helicopter Society, Washington, D.C., May 1971.
258
-------
REFERENCES -2.3 General Aviation Aircraft
1. Statistical Abstract of the United States 1970, Bureau of Census, U.S.
Department of Commerce.
2. Air Transport 1971, The Air Transport Association of America, Washington,
D.C., 20036.
3. General Aviation Manufacturers Association of America, Washington, D.C.,
private telecon, August 1971.
4, Adcock, B.D. and Ollerhead, J.B., "Effective Perceived Noise Level
Evaluated for STOL and Other Aircraft Sounds, " FAA-NO-70-5, Federal
Aviation Administration, May 1970.
5. Tobias, J.V., "Noise in Light Twin-Engine Aircraft, " Sound and Vibration,
September 1969.
6. Tobias, J.V., "Cockpit Noise Intensity; Fifteen Single Engine Light Aircraft, "
FAA AM 68-21, Federal Aviation Administration, September 1968.
7. Tobias, J.V., "Cockpit Noise Intensity: Eleven Twin-Engine Light Aircraft, "
FAA AM-68-25, Federal Aviation Administration, October 1968.
8. The Magnitude and Economic Impact of General Aviation, 1968-1980, Speas
Association, Aero Publications, Manhasset, New York, 1970.
9. Piper Aircraft Corporation, private conference, August 1971.
10. Acoustics Group, AiResearch Manufacturing Company, Phoenix, Arizona,
(unpublished data).
11. Woolsey, J.P., "Operators, FAA Seek Jet Noise Solution, " Aviation Week
and Space Technology, September 20, 1971.
259
-------
REFERENCES -2.4 Highway Vehicles
1. Auto Facts, Automobile Manufacturers Association, Inc., Detroit, Michigan,
T97T
2. "Automotive News, 1971 Almanac, " Automotive News, Detroit, Michigan,
April 1971.
3. American Trucking Trends 1970-71, Departments of Research and Transport
Economics and Public Relations, American Trucking Association, Inc.
Washington, D.C. ,
4. '70 - '71 Transit Fact Book, American Transit Association, Washington, D.C.
5. Cities of Los Angeles, Long Beach, Torrance and Culver City, California,
private telecons, September 1971.
6. "Statistical Study of Traffic Noise, " National Research Council of Canada,
Division of Physics, Ottawa, Canada, 1970.
7. "Passenger Car Noise Survey, " California Highway Patrol, January 1970.
8. Little, R.A., California Highway Patrol, private correspondence, August 1971.
9. Wyle Laboratories "Traffic Noise" (unpublished data).
10. Galloway, W.J., Clark, W.E. and Kerrick, U.S.., "Highway Noise: Measure-
ment Simulation and Mixed Reactions, " NCHRP Report 78, Highway Research
Board, National Academy of Sciences, 1969.
11. RoK. Hillquist, Standards Engineer, General Motors Company, private
conference, September 1971. <
12. Leasure, W.A., Jr., "Truck Tire Noise — Results of a Field Measurement
Program, " Purdue Noise Control Conference, Purdue University, July 14-16,
1971.
13. Tetlow, D., "Truck Tire Noise, " Sound and Vibration, August 1971.
T4. Leasure, W.A., Jr., et al, "Interim Progress Report of Research Activity —
Truck Tire Noise Investigation," NBS Report 10567, National Bureau of
Standards, April 1971.
260
-------
15. Wyle Laboratories, "Truck Noise" (unpublished data).
16. Austen, A.E. and Priede, T., "Origins of Diesel Engine Noise," Proceedings
of the Symposium on Engine Noise and Noise Suppression, London, England,
October 24, 1958, The Institute of Mechanical Engineers, London, England.
17. Dickerson, D.O. (Editor), "Transportation Noise Pollution: Control and
Abatement, " NASA Contract NGT 47-003-028, National Aeronautics and
Space Administration, Langley Research Center and Old Dominion University,
Summer 1970.
18. Soroka, W.W. and Chien, C.F., "Automotive Piston-Engine Noise and Its
Reduction -A Literature Survey, " SAE No. 690452, Society of Automotive
Engineers Midyear Meeting, Chicago, Illinois, May 19-23, 1969.
19. Wyle Laboratories,"Truck Tire Noise" (unpublished data).
20. "The Noise Trucks Make, " American Trucking Association, Inc., Washington,
D.C., September 1964.
21. "Noise Measurements of Motorcycles and Trucks, " Automobile Manufacturers
Association, Inc., Detroit, Michigan, June 1971.
22. Engineering Representatives, Stempco Manufacturing Co., Longview, Texas,
private conference, August 1971.
23. "Sources of Truck Noise, "Acoustic Technical Bulletin No. 20, Donaldson
Company, Inc., Minneapolis, Minnesota, December 1970.
24. Engineering Representatives, Ford Motor Company, private correspondence,
August 1971.
25. "Submission of Caterpillar Tractor Company, " Regional Hearing on Transportation
Noise (Highway), Chicago, Illinois, Office of Noise Abatement and Control,
Environmental Protection Agency, Washington, D.C., July 28, 1971.
26. Millard, R.S. (Editor), "A Review of Road Traffic Noise - The Working Group
on Research into Road Traffic Noise, " LR 357, Road Research Laboratory,
Crowthorne, England, 1970.
27. "Donaldson SBM Intake Silencer— Product Technical Information, " Donaldson
Company, Inc., Minneapolis, Minnesota, July 1968.
261
-------
28. Hempel, R.E., "Does Turbocharging Increase Diesel Engine Noise? —
Observations on the Generation, Emission and Reduction of Diesel Engine
Noise," SAE No. 680406, Society of Automotive Engineers Midyear Meeting,
Detroit, Michigan, May 1968.
29. Priede, T., "Noise of Internal Combustion Engines," Paper C-2, National
Physical Laboratory Symposium No. 12, England, July 1961.
30. Mr. S.A. Lippmann, Uniroyal Tire Company, Detroit, Michigan, private
conference, July 1971.
31. Engineering Representatives, Firestone Tire and Rubber Company, Akron,
Ohio, private conference, July 1971.
32. Hayden, R. E., "Roadside Noise for the Interaction of a Rolling Tire with the
Road Surface," Purdue Noise Control Conference, Purdue University/
July 14-16, 1971.
33. "Submission of General Motors Company," Regional Hearing on Transportation
Noise (Highway), Chicago, Illinois, Office of Noise Abatement and Control,
Environmental Protection Agency, Washington, D.C., July 28, 1971.
34. Engineering Representatives, Automotive Division,. Anv.in Industries, Columbus,
Indiana, private conference, July 1971.
35. Groening, J.A., "Characteristics and Control of .Gar/ Truckand Bus Noise,"
Purdue Noise Control Conference, Purdue University, July 14-16, 1971.
36. Wiener, P.M., "Experimental Study of the Airborne Noise Generated by
Passenger Automobiles," Noise Control, July/August 1960.
37. Wyle Laboratories, "Bus Noise" (unpublished data). .
38. Wyle Laboratories, "Garbage Compactor Noise" (unpublished data).
39. "Study on the Effect of Noise Pollution, Section on Community Noise,"
Office of Noise Abatement and Control, Environmental Protection Agency,
Washington, D.C., October 1971.
40. Wyle Laboratories, "Automobile Interior Noise Levels" (unpublished data).
41. Solomon, L. and Underwood, J., "Interior Automotive Noise Measurements
Under Various Operating Conditions," Journal of the Acoustical Society of
America, Vol. 49, 1971.
262
-------
42. "More on Interior Car Noise," Noise Measurement, The General Radio
Company, Spring 1971.
f : .-. • ' - .
43. Ford, R.D., Hughes, G.M. and Sanders, D.J., "The Measurement of Noise
Inside Cars," Applied Acoustics, January 1970.
i>-. - : :'. •: •
44. Engineering Representatives, International Harvester Company, private
conference, August 1971.
45. Rowley, D., Manager, Acoustics Section, Donaldson Company, Inc.,
Minneapolis, Minnesota, private telecon, August 1971.
46. "Comments on Traffic Noise Reduction," Society of Automotive Engineers
Vehicle Sound Level Committee, Detroit, Michigan, September 1971.
47. Priede, T., Stockman, E.S., Bonora, A., and Zimmerman, K., "Cut
Engine Noise Three Ways," Journal of the Society of Automotive Engineers,
April 1970.
'•..'»'"• • '
48. Magazine Advertisement, Time, Ford Motor Company, July 1971.
49. "Sound Levels for Passenger Cars and Light Trucks," SAE J986a, Society
of Automotive Engineers Standard.
50. Starkman, E.S., Vice President, Environmental Activities Staff, General
Motors Company, private correspondence, August 1971.
51. "A Proposal to Reduce Exhaust Emissions and Decrease Vehicle Noise from
Existing Diesel Coaches," Truck and Coach Division, General Motors Company,
December 1970.
52. Kelly, K., "CMC Offers Quiet Refuse Truck," Automotive News,
February 22, 1971.
263
-------
REFERENCES-2.5 Rails/stems
1. Yearbook of Railroad Facts, Association of American Railroads, Chicago,
Illinois, 1971.
2. Statistical Abstract of the United States 1970, Bureau of Census,
U.S. Department of Commerce
3. 70-71 Transit Fact Book, American Transit Association, Washington, D.C.
4. Embleton, T.F.W. and Thiessen, G.J., "Train Noises and Use of Adjacent
Land," Sound: Its Uses and Control, January/February 1962.
5. Locomotive Engineering Department, General Electric Company, private
conference, July 1971.
6. "The Visibility and Audibility of Trains Approaching Rail-Highway Grade
Crossings," FRA-RP-71-1, Federal Railroad Administration, May 197.1,
7. Bray, D.C., "Noise Environments in Public Transportation," Joint American
Society of Civil Engineers - American Society of Mechanical Engineers
Transportation Engineering Meeting, Seattle, Washington, 1971.
8. Advanced Technology Industrial Systems Group, Rohr Corporation, private
conference, August 1971.
9. St. Louis Car Division, General Steel Industries, private conference, July
1971.
10. Kendall, H.C., "Noise Studies in Retarder Yards," Railway Systems
Control/Vol. 2, No. 7, 1971.
11. Blazier, W.E.,\Jr., et al, "Chicago Urban Noise Study, Phase I, Noise in
the Urban Environment," The City of Chicago Department of Environmental
Control, Chicago, Illinois, November 1970.
i
12, "Acoustic Studies," Technical Report No. 8, San Francisco Bay Area Rapid
Transit District, San Francisco, California, June 1968.
13. Wilson, G.P., "Noise and Vibration Characteristics of High Speed Transit
Vehicles," Office of Noise Abatement ancj Control, Environmental Protection
Agency, Washington, D.C., June 1971.
264
-------
14. Wilson, G.P., "Environmental Features and Advantages of Rapid Transit
Systems," Institute of Electrical and Electronic Engineers, Region Six
Conference, May 1971.
15. Kirschner, F., "Control of Railroad Wheel Screech Noise," 6th International
Congress on Acoustics, Tokyo, Japan, August 1968.
16. Research Center, B.F. Goodrich Company, private correspondence, August
1971.
17. Bender, E.K. and Hirtle, P.W., "The Acoustic Treatment of Stations to
Alleviate Wheel-Squeal Noise," Prepared under Contract No. CS-009 for
Massachusetts Bay Transit Authority, Boston, Massachusetts, October 1970.
18. Kramer, M. and Richards, A., "Data on New York City Subway Noise,"
(unpublished data), Sunset Park Family Health Center, Brooklyn, New York.
19. Botsford, J.H., "Damage Risk," Transportation Noises -r A symposium on
Acceptability Criteria, Chalupnik, J.D. (Editor), University of Washington
Press, 1970.
1 i •
20. Northwood, T. D., "Rail Vehicle Noise>" Research Paper No. 155, Division
of Building Research, National Research Council, Ottawa, Canada, 1963.
21. Harris, C.M. and Aitken, B.H., "Noise in Subway, Cars, " Sound'and
Vibration, February 1971.
22. Davis, E.W., et'al, "Comparison of Noise and Vibration Levels in Rapid
Transit Vehicle Systems," National Capitol Transportation Agency, <
Washington, D.C., April 1964.
23. Bender, E.K., et al, "Noise Generated by Subways above Ground and in
Stations," Report No. OST-ONA-70-1, Office of Noise Abatement and
Control, Environmental Protection Agency, Washington, D.C., January 1970.
24. "Aerial Structure and Rail Support Methods,". Technical .Report No. 11,
San Francisco Bay Area Rapid Transit District, San Francisco, California, 1968.
25. Bonvallet, G.L., "Levels and Spectra of Transportation Vehicle Noise,"
Journal of the Acoustical Society of America, Vol. 22, No. 2, 1950.
26. Wyle Laboratories (unpublished data).
265
-------
27. Engineering Department, Chicago Transit Authority, private conference,
July 1971.
28. Franken, P. A,, et al, "Chicago Urban Noise Study, Phases III and IV,
Noise Control Technology, Federal Aid for Noise Abatement, Noise Control
Program Recommendations," The City of Chicago Department of Environ-
mental Control, Chicago, Illinois, November 1970.
29. New York Transit Authority, private correspondence, August 1971.
30. Land Systems, Sikorsky Aircraft, private correspondence, August 1971.
266
-------
REFERENCES-2.6 Ships
1. Statistical Abstract of the United States 1970, Bureau of Census, U.S.
Department of Commerce.
2. Air Transport 1971, The Air Transport Association of America, Washington, D.C.,
20036.
3. Shearer, K., "Ambient Sound Measurements on Two Passenger Ships and an Oil
Tanker, " Report No. 424, British Shipbuilding Research Association, 1962.
4. Conn, R., "Noise Levels in the Engine Room and Bridge of Merchant Ships,"
Report No. NS 121, British Ship Research Association, 1966.
5. Hagan, R., and Babcock, G., 'Shipboard Vibration Problems and Their
Solutions," Noise Control, p. 36, July 1959.
6. Hagan, A.f and Hammer, N., "Shipboard Noise and Vibration From a
Habitobility Viewpoint, " Marine Technology, January 1969.
267
-------
REFERENCES -2.7 Recreation Vehicles,
1. Leisure Time FVoduct Noise/ Sub Council Report, National •Industrial
Pollution Control Council, May 1971.
2. Dickerson, D.O. (Editor), "Transportation Noise Pollution: Control and
Abatement," NASA Contract NGT 47-003-028, National Aeronautics
and Space Administration, Langley Research Center, and Old Dominion •
University, Summer, 1970.
3. "A Continuing Market Analysis of Snowmobile. Owners," Snow Goer, .1971
» i ''•",•••
4. Boating 1970, Marketing Department of the Boating Industry Association,
The Outboard Boating Club of America and the National Association of
Engine and Boat'Manufacturers. . -7>
5. "Snowmobile Noise, Its Sources, Hazards and Control, " APS-477,
Division of Physics, National Research Council of Canada, Ottawa,
Canada, 1970.
i . • _•')>•
6. Harrison, R., Acoustical Engineer, U. S. Department of Agriculture,
Testimony before the California Assembly Committee on Natural Resources
and Conservation, April 2, 1971. .:
• • t • . • , •.'•-•••..'• ,£r1 .
7. "Resolution to Control Noise," Motorcycle Industry Council, Inc.,
April 14, 1971.
8. Campbell, R.A., "A Survey of Passby Noise from Boqts," Sound and
Vibration, September 1969. .
9. Lincoln, R.H., Manager, (Environmental Engineering, Outboard Marine
Corporation, Milwaukee, Wisconsin. Private correspondence, , i
August 23, 1971.
; j, '
10. Tests conducted by the State of California, Resources Agency, Department
of Parks and Recreation, Millerton Lake State Recreation Area,
W.J. Reinhardt,, Area Manager, May 19, 1971 and July 21, 1971.
. • , . ' ' ' -i -.•'.',
11. Campbell, R. A., "Noise Levels During an Outboard Boat Race, "
Veterans Administration Hospital, Miami, Florida, July 1970.
12. Engineering Representatives, Birmingham Small Arms Corporation.
Private correspondence, August 27, 1971.
268
-------
13. Wyle Laboratories, "Motorcycle Operator Noise Levels" (unpublished data).
14. Rowley, D., Manager, Acoustics Section, Donaldson Company, Inc.,
Minneapolis, Minnesota. Private telecon, August 1971.
15. "Power of Silence", Cycle News, Western Edition, Vol. VIII, No. 43,
November 10, 1970.
16. Engineering Representatives, Harley Davidson Motor Co., Inc.,
Milwaukee, Wisconsin. Private conference, September 1971.
17. Hoene, J.V., "Snowmobiles and Noise", International Snowmobile •
Industry Association, Minneapolis, Minnesota, 1970.
t . ! ' •
18. Nesbitt, John F., Engineering Director, International Snowmobile '
Industry Association. Private correspondence, August 1971.
19. Foster, L.W., "Snowmobile — Safety and Ecology", Twin Cities Section,
Society of Automotive Engineers, Minneapolis, Minnesota', May 1971.
i .
20. Wyle Laboratories "Dune Buggy Noise Levels" (unpublished data).
21. Harrison, R., "Study of Sound Propagation and Annoyance under Forest-
Conditions", Project Record ED and T 1768, U.S. Department of Agri-
culture, Forest Service, Equipment Development Center, December 1970.
22. Wyle Laboratories, "Motorcycle Noise" (unpublished data).
23. Campbell, R.A., "A Survey of Noise Levels On Board Pleasure Boats",
Veterans Administration Hospital, Miami, Florida, 1970.
24. "Mufflers and Exhaust Systems (for Snowmobiles)", Bulletin 1200-154,
Donaldson Company, Inc., Minneapolis,-Minnesota, March 1971. (
25. Conover, W.C., "Noise Control of Outboard Motors", Noise Control,
March 1955.. ~~
26. Kueny, D.F., and Boerma, M.J., "A Modern Outboard Design — Johnson
and Evinrude 50 HP", SAE No. 710580, Society of Automotive Engineers
Midyear Meeting, Montreal, Canada, June 1971 .
269
-------
27. Engineering Representatives, Outboard Marine Corporation, Milwaukee,
Wisconsin. Private conference, August 1971.
28. Handle/, W.C., "Noise Level Studies of Motorcycles in Bermuda",
Noise Control, July 1959.
29. "Motorcycle Noise Test Procedure Evaluation", California Highway
Patrol, January 1971.
30. Little, R.A., California Highway Patrol. Private conference, September 1971
31. Snowmobile Safety, Laws Section Conservation 55 — Mufflers, State of
Minnesota, 1969-1970.
32. Hiraboyashi, T., "Noise and the Snowmobile, " Purdue Noise Control
Conference, Purdue University, July 1971. ..
270
-------
/'REFERENCES — 3.0 Devices Powered by Small Internal Combustion Engines
1. Industrial Report/ Bureau of Census, U.S. Department of Commerce, 1963-1969.
2. Moody's Industrials, 1971.
3. Kohler Company, Kohler, Wisconsin and Onan Corporation, Minneapolis,
Minnesota. Private conference, August 1971.
4. Wyle Laboratories (unpublished data).
5. Nelson Muffler Corporation, Stoughton, Wisconsin, private conference, ;
August 1971.
6. McCulloch Corporation, Los Angeles, California, private conference, July 1971
7. Leisure Time Product Noise, Sub Council Report, National Industrial Pollution
Control Council, May 1971.
8. Outboard Marine Company, Milwaukee, Wisconsin, private conference,
August 1971.
9. Rettinger, M., "Noise Level Reductions of Barriers. l! Noise Control,
September 1957.
10. Noise Ordinance, City of Chicago, Illinois.
11. • Noise Ordinance, City of Minneapolis, Minnesota.
12. Noise Ordinance, City of Costa Mesa, California.
271
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REFERENCES —4.0 Environmental Impact for Transportation Vehicles and
Small Internal Combustion Engines
1, Statistical Abstract of the United States 1970, Bureau of Census, U.S.
Department of Commerce. '. -{
i
2. "Study on the Effect of Noise Pollution, Section on Community Noise,"
Office of Noise Abatement and Control, Environmental Protection Agency,.
Washington, D.C., October 1971.
3. Mills, C.H.G. and Robinson, D.W., "The Subjective Rating of Motor
Vehicle Noise," The Engineer, pp. 1070-1075, June 30, 1961.
4. vonGierke, Dr. H.E., Director, Biodynamics Bionics Division, 6,570th
Aerospace Medical Research Laboratory, Wright Patterson Air Force Base,
private conference, September 1971.
5. George, R.W., et al, "Jet Aircraft, A Growing Pollution Source,"
Journal of the Air Pollution Control Association, pp. 847-855,
November 19, 1969.
6. Miller, J.D., "The Effects of Noise on the Quality of Human Life,"
Central Institute for the Deaf, 1971.
7. Webster, J., "SIL—Past, Present, and Future," Sound and Vibration,
pp. 22-26, August 1969.
272
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REFERENCES- 5.0 Conclusions and Recommendations
1. Miller, J., "Effects of Noise on People, " Central Institute for the Deaf,
to be published by the Environmental Protection Agency, Washington, D.C.,
November 1971.
2. Wyle Laboratories Research Staff, "Community Noise, " WR 71-17, Office of
Noise Abatement and Control, Environmental Protection Agency, Washington,
D.C., November 1971.
273
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NOISE FROM TRANSPORTATION
SYSTEMS, RECREATION VEHICLES
AND DEVICES POWERED BY SMALL
INTERNAL COMBUSTION ENGINES
APPENDICES
A MEASUREMENT STANDARDS
B METHODOLOGY FOR IMPACT ANALYSIS
C NOISE GENERATOR CHARACTERISTICS
-------
APPENDIX A
MEASUREMENT STANDARDS
In this appendix, several typical measurement standards relevant to the
categories of Transportation Systems and Devices Powered by Internal Combustion
Engines are summarized. The purpose of this discussion is to provide insight into the
procedures used to obtain the standard levels contained in the body of this report.
However, it is not all-inclusive since an analysis of every standard applicable to
these categories is beyond the scope of this appendix.
The purpose of a noise measurement standard is to establish a practical
formal procedure for determining the noise output of a device under realistic and
repeatable operating conditions.
In some instances, measurement standards may be created by civil agen-
cies whereby they are set forth as a basis for verifying that the noise output of a
device falls within specified legal limits. The FAR-36 specification for certification
of jet aircraft contains such a measurement standard. The new-vehicle noise
measurement procedure utilized by the California Highway Patrol is another example.
Voluntary measurement standards may also be created by manufacturers'
associations, professional societies, or other member bodies of the American National
Standards Institute. In these instances, the purpose of the standard is to establish a
common measurement basis which may be utilized by manufacturers and users through-
out the nation. It also serves as a guide to groups with a peripheral involvement in
the product, such as subcontractors and distributors, as to the basis for measurement
on the completed system. This type of standard is typified by the SAE standards for
measuremenfs on commercial vehicles, automobiles, and other types of internal com-
bustion engine powered equipment. These voluntary measurement standards may
frequently be incorporated into government regulations and ordinances which specify
maximum noise levels for various devices. An example is SAE Standard J192 for
snowmobiles, which is utilized by a number of states as the basis for legislation of
A-1
-------
maximum snowmobile noise. Although a number of the voluntary measurement
standards have gained fairly wide acceptance in industry and government, they
generally have not been developed for regulatory use. Therefore, the quantities
measured and the operating procedures utilized may not be appropriate for regu-
lation of noise at the source.
For example, the State of California adopted SAE Standard J986a as
a noise test compliance method for automobiles. This approach has been criticized
because it penalizes certain vehicles by rating them in a maximum noise-producing
mode which, in a large percentage of cases, does not typify normal operation. As
a result, luxury American automobiles with 400 to 500 cubic inch displacement
engines have difficulty passing the full-throttle acceleration noise test, w.hereas
small imports and sports cars have little difficulty. Yet in use, the luxury vehicle
is generally considered acceptably quiet, whereas the smaller car often is not so
judged. This inequity results from the fact that the luxury automobile normally
operates at only a fraction of its potential power, whereas the small low~powered
vehicle normally operates near maximum power. This situation exemplifies the
case of a standard, designed to serve as a common measurement basis, being
incorrectly applied to noise regulation.
The principal noise source categories analyzed in this report are
summarized in Table A-l, with a listing of the major measurement standards which
apply to these categories. As can be observed, a number of these categories are
not covered by any specific measurement or regulatory standards.
Following Table A-l are brief descriptions of the test methods incor-
porated in the standards and the recommended noise levels produced under these
operating conditions. In addition, because of its significance as the first noise
standard promulgated by the Federal Government, the FAR Part 36 Noise Standard
for Aircraft Type Certification is presented in its entirety at the conclusion of this
appendix. This certification standard demonstrates the detail and complexity
required in some standards, and appropriate sections of it may serve as a model
for future standards.
A-2
-------
Table A-1
Summary of Major Noise Measurement Standards
Category
General Aviation
Aircraft
i '.
V/STOL:
Business Jets
Subsonic Commercial
Aircraft
Trains
Passenger Cars and
Light Trucks
GVW < 6000 pounds
Trucks and Buses
GVW > 6000 pounds
Motorcylces
Snowmobiles
Pleasure Boats
Other Devices
Powered by I/C
Engines, Lawn
Mowers, etc.
Applicable Noise Measurement Standard — Observer
None
X
X
X
X
FAR1
Part
36
X
X
ISO2
R362
X
X
X
CHP3
Article
10
X
X
X
SAE4
J331
Proposed
X
SAE
J366
X
SAE
J986a
X
SAE
J192
X
SAE
J952b
X
'Federal Aviation Regulation.
"International Organization for Standardization,
California Highway Patrol.
Society of Automotive Engineers.
A-3
-------
Title:
Originator:
Noise Source:
Purpose:
Measurement
Location:
Procedure:
Maximum
Noise Ljmits:
FAR 36 - NOISE STANDARDS: AIRCRAFT TYPE CERTIFICATION
Issued Novembers, 1969, last revision November 24, 1969 .:
Federal Aviation Agency
Subsonic Transport and Turbojet Powered Aircraft
FAR-36 is an FAA procedure for flight certification of all subsonic
transport and turbojet aircraft. It establishes maximum allowable
noise levels for new aircraft and a standardized procedure for
their measurement.
Landing — 1 nautical mile from threshold, directly under the
aircraft path,
Takeoff- 3.5 nautical miles from brake release, directly
under the aircraft path, and
Sideline - at the location of maximum noise along a line parallel
to and at a distance of 0.35 nautical miles from the runway center-
line, for,aircraft which have four or more engines; and 0.25 nau-
tical miles from the runway centerline, for aircraft which have
three or fewer engines.
Appropriate measurement instrumentation is set up at the specified
locations. A series of takeoffs and landings are made by the air-
craft to be certified, in accordance with prescribed engine power
and flight profiles. This procedure is performed with the aircraft
operating at maximum gross takeoff weight. Noise data taken
during this procedure is subsequently analyzed for compliance
with the specified limits.
The noise limits of this regulation are set forth in terms of Effective
Perceived Noise Levels and gross takeoff weight. For landing and
sideline, these levels range from 102 EPNdB to 108 EPNdB. For
takeoff, the levels range from 93 EPNdB to 108 EPNdB.
A-4
-------
Other
Requirements:
Additional specifications are set forth relating to the measurement;
instrumentation, weather conditions, flight profiles, test aircraft
operating conditions, and the appropriate technique for calcula-
ting EPNdB.
Title:
;' M
e '
Originator:
Noise Source:
Purpose:
Measurement
Location:
Procedure:
ISO RECOMMENDATION R362 - MEASUREMENT OF NOISE
EMITTED BY VEHICLES - First Edition, February, 1964.
International Organization for Standardization
Motor Vehicles
Establishes a procedure for measurement of the maximum exterior
noise level for motor vehicles, consistent with normal driving
conditions, and is capable of giving easily repeatable results.
Should consist of an extensive flat open space of some 50 meters
radius, of which the central 20 meters would consist of concrete
or asphalt paving =
j^
Locate microphone 7.5 meters from the centerline of the vehicle
path. Approach microphone in low gear range (generally second
gear) at 50 kph, or 3/4 maximum rated engine rpm, or 3/4 maxi-
mum engine speed permitted by governor, whichever is lowest.
At a point 10 meters ahead of microphone, accelerate fully and
hold at full throttle until the vehicle is 10 meters beyond the
microphone.
Recommended
Maximum Level:
No recommendations made.
-------
Title:
Originator:
Noise Source:
Purpose:
Measurement
Location:
Procedure:
SAE J192 - EXTERIOR SOUND LEVEL FOR SNOWMOBILES
Approved September 1970.
Society of Automotive Engineers
New Snowmobiles
Provides a procedure for measurement of maximum exterior sound
level for snowmobiles.
Test site to be flat open space, free of large reflecting objects
within 100 feet of either the vehicle or the microphone.
Locate microphone 50 feet from the centerline of the vehicle path,
Vehicle operated on grass (3-inch height). Accelerate fully from
standing start such that maximum rated engine rpm is achieved
25 feet ahead of the microphone. Hold this maximum rpm until
50 feet beyond microphone.
Recommended
Maximum Level: 82 +2 dB(A) at 50 feet.
Title!
Originator:
Noise Source:
Purpose:,
Measurement
Location:
Procedure:
SAE J331 - PROPOSED - SOUND LEVELS FOR MOTORCYCLES
Draft No. 5, April 30, 1971
Society of Automotive Engineers
Motorcycles
Establishes a procedure for determining maximum sound levels
for all classes of motorcycles.
Test site shall be a flat open space, free of large reflecting objects
within 100 feet of either the vehicle or the microphone.
Locate microphone 50 feet from the centerline of the vehicle
path. Motorcycle usually operated in low gear. Approach
A-6
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Recommended
Maximum Level:
microphone at 2/3 maximum rated engine rpm. At a point of at
least 25 feet ahead of microphone, accelerate fully to achieve
maximum rate engine rpm at a point between 15 and 25 feet past
the microphone.
Recommended dB(A)* for motorcycles manufactured after
January 1, 1972:
1972
Engine Displacement
170 cc and less
171 cc - 300 cc
More than 300 cc
*With an additional allowance of +2 dB
1973
1974
86
90
92
83
87
89
80
84
86
Title:
Originator:
Noise Source:
Purpose:
Measurement
Location:
Procedure:
Recommended
Maximum Level:
SAE J366- EXTERIOR SOUND LEVEL FOR HEAVY TRUCKS
AND BUSES - Approved July 1969.
Society of Automotive Engineers
Trucks and Buses over 6000 pounds G'VW
Establishes the method for measuring the maximum exterior
sound level for highway motor trucks, truck tractors and buses.
Test site shall be flat open space, free of large reflecting
objects within 100 feet of either the vehicle or the microphone.
Locate microphone 50 feet from the centerline of the vehicle path.
Approach microphone in a gear ratio selected such that at a point
50 feet ahead of the microphone, the vehicle is at no higher than
2/3 the maximum rated or governed engine speed. Accelerate fully
such that maximum rated engine rpm is achieved between 10 and
100 feet beyond microphone and without exceeding 35 mph at
end point.
88 +2dB(A)at50feet.
A-7
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Title:
Originator:
Noise Source:
Purpose:
Measurement
Location:
Procedure:
SAEJ952b-SOUND LEVELS FOR ENGINE POWERED EQUIPMENT
Approved May 1966, Last Revised January 1969.
Society of Automotive Engineers
Engine Powered Equipment
Establishes procedure for measuring maximum sound levels for
engine powered equipment.
Test site shall consist of a flat open area, free of large reflecting
objects within 100 feet of either the-microphone or the test specimen.
Locate microphone 50 feet from the test specimen. Operate equip-
ment at the combination of load and-speed which produces maximum
sound level without violating the manufacturer's operating
specification.
Recommended
Maximum Levels:
Type of Equipment
Maximum Sound Level
dB(A) at 50 feet*
(A-Weighting Network)
1. Construction and industrial machinery
2. Engine powered equipment of 5 hp or less intended
for use in residential areas at frequent intervals
3. Engine powered equipment exceeding 5 hp but not
greater than 20 hp intended for use in residential
areas at frequent intervals
4. Engine powered commercial equipment of 20 hp
or less intended for infrequent use in a residential
area
70
78
88
5. Farm and light industrial tractors
*An additional 2 dB allowance over the sound level limits is recommended to
provide for variations in test site, vehicle operation, temperature gradients,
wind velocity gradients, test equipment, and inherent differences in nominally
identical vehicles.
A-8
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Title:
Originator:
Noise Source:
Purpose:
Measurement
Location:
Procedure:
Recommended
Maximum
Levels:
SAE J986a - SOUND LEVEL FOR PASSENGER CARS AND LIGHT
TRUCKS - Approved July, 1967; Last Revised January, 1969
Society of Automotive Engineers
Passenger Cars and Light Trucks (of 6000 GVW or less)
Provides a method for determining the maximum sound level for
passenger cars and light trucks.
Test area to be flat open space, free of large reflecting objects,
within 100 feet of either the vehicle or the microphone.
Locate microphone 50 feet from the centerline of the vehicle path.
Approach microphone at 30 mph in a low gear range. At a point
25 feet ahead of microphone, accelerate at wide open throttle such
that maximum rated rpm is achieved 25 feet beyond microphone.
86 + 2 dB(A) at 50 feet.
A-9
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Title:
Originator:
Noise Source:
Purpose:
Measurement
Location:
Operating
Conditions:
CALIFORNIA ADMINISTRATIVE CODE, TITLE B, CHAPTER 2,
SUBCHAPTER 4, ARTICLE 10, VEHICLE NOISE MEASUREMENT
February 15, 1968.
Department of California Highway Patrol
All new motor vehicles offered for sale in the State of California.
Three categories of motor vehicles are defined: (1) trucks and buses
with gross weight greater than 6000 pounds; (2) trucks, buses, and
passenger cars with gross weight under 6000 pounds; and (3) motor-
cycles.
Establishes procedures for implementation of Section 27160 of the
California Vehicle Code which is concerned with limits on noise
output of new motor vehicles offered for sale in the State of California,
Open area, free of reflecting surfaces within a 100-foot radius of the
microphone and within 100 feet of the center!ine of the path of the
vehicle from the point where the throttle is opened to the point where
the throttle is closed.
Vehicles are operated along a path 50 feet distant from, and at
right angles to, the measurement microphone.
Category 1 (Truck and Buses >6000 pounds GVW): Operate vehicle
under conditions of grade, load, acceleration, deacceleration and
gear selection to achieve maximum noise at a speed of up to 35 mph.
Category 2 (Light Truck, Passenger Cars; GVW <6000 pounds):
Operate vehicle in a low gear range. Approach microphone at
30 mph, accelerate fully at a point 25 feet ahead of microphone
and continue to 100 feet beyond microphone or a point at which
maximum rated engine rpm is reached.
Category 3 (Motorcycles): Motorcycle driven in second gear at
constant speed corresponding to 60 percent of maximum rated engine
rpm. Accelerate full at a point 25 feet ahead of microphone.
A-10
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Noise Limits: New Vehicles offered for sale in California: *
Manufactured Prior to
January 1, 1973
:per California Vehicle Code
Manufactured After
January 1, 1973
Category 1
Category 2
Category 3
88 dB(A)
86
88
86 dB(A)
84
86
Title:
Originator:
Noise Source:
Purpose:
Measurement
Location:
Operating
Conditions:
CALIFORNIA ADMINISTRATIVE CODE, TITLE 13, CHAPTER 2,
SUBCHAPTER 4, ARTICLE 10, VEHICLE NOISE MEASUREMENTS,
February 15, 1968.
Department of California Highway Patrol
Motor vehicles and combinations of vehicles subject to registration
when operated on California highways.
Establishes procedures for implementation of Section 23130 of the
California Vehicle Code which is concerned with limits on noise
output of motor vehicles operated on all California highways.
Open area, free of large reflecting surfaces within a 100-foot
radius of the microphone and within a 100-foot radius of the point
on the centerline of the path of the vehicle nearest the microphone.
Sound level readings are recorded on vehicles which are in lanes
of travel whose centerlines are at or beyond 50 feet from the
microphone position.
A-ll
-------
Noise Limits*: Speed Limit of Speed Limit of
35 mph or less more than 35 mph
1. Motorcycles and motor vehicles
of 6000 GVW or more
(a) Before 1 January 1973 88 dB(A) 90 dB(A)
(b) After 1 January 1973 86 90
2. All other motor vehicles 82 86
* per California Vehicle Code
(with an additional allowance of +2 dB)
A-12.
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PART 36—NOISE STANDARDS:
AIRCRAFT TYPE CERTIFICATION
Subparl A—General
Bee. -
S6.1 Applicability.
36.2 Special retroactive requiremente.
S6JS Compatibility with airworthiness
requirements.
36.5 Limitation a! part.
Subpart B—Noise Measurement and Evaluation
3C.JO1 Nofee measurement.
36.1O3 Noise evaluation.
Subporf C—Noise Limits
86.201 Noise llmltB.
Subparl D (Reserved!
Subparl E IReservedl
Subpart f IReservedl
Svbpart G—Operating Information and Airplane
Flight Manual
36 J501 Procedures and other Information.
S6J581 Airplane Flight Manual.
Appendix A—Aircraft noise measurement
under $ 36.1O1 .
Appendix B—Aircraft noise evaluation under
i 36.103
Appendix C—Noise levels for subsonic trans-
port category and turbojet pow-
ered airplanes under S 36.201
AUTHORITY : The provisions of this Part 36
Issued under sees. 313(a), 601. 603, and 611
of the Federal Aviation Act of 1958; 49 U.S.C.
1354, 1421, 1423, and 1431 and sec. 6(c) of
the Department of Transportation Act; 49
U.S.C. 1655 (C),
Subpart A—General
g 36.1 Applicability.
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A—AIBCBAFT NOISE MEASOSEMENT
UNDER ! 36.101
Section A38.1 Mrtse certification test and
measurement conditions—(a) General. This
section prescribes the conditions under
wblcn noise type certlflcatlon tests must be
conducted and tbe measureuieui. procedures
that must be used to measure the noise
made by the aircraft for which the test Is
conducted.
(b) General teat conditions. (1) Tests to
show compliance with established noise type
certlflcatlon levels must consist of a series
of takeoffs and landings during which meas-
urements must be taken at the measuring
points defined In Appendix C of this part.
Tbe sideline noise measurements must also
be made at symmetrical locations on each
Bide of the runway. On each test takeoff.
simultaneous measurements must be made
at tbe sideline measuring points on both
sides of the runway and also at the takeoff
flyover measuring point. If the height of the
ground at each measuring point differs from
that of the nearest point on the runway by
more than 20 feet, corrections must be made
as defined In 5 A38.3(d) of this appendix.
(2) Locations for measuring noise from
an aircraft In flight must be surrounded by
relatively flat terrain having no excessive
Bound absorption characteristics such as
might be caused by thick, matted, or tall
grass, shrubs, or wooded areas. No obstruc-
tions which significantly Influence the sound
field from the aircraft may exist within a
conical space above the measurement posi-
tion, the cone being denned by an axis nor-
mal to the ground and by a half-angle 75*
from tbls axis.
(3) The tests must be carried out under
the following weather conditions':
(1) No rain or other precipitation.
(Jl) Relative humidity not higher than
90 percent or lower than 30 percent.
(Ill) Ambient temperature not above
88* F. and not below 41* P. at 10 meters
above ground.
(IT) Airport reported wind not above 10
knots and crosswlnd component not above
6 knots at 10 meters above ground.
(v) No temperature Inversion or anoma-
lous wind conditions that would significantly
affect tbo noise level of the aircraft when
the noise Is recorded at the measuring points
defined In Appendix C of this part.
(c) Aircraft testing procedures. (1) The
aircraft testing procedures and noise meas-
urements must be conducted and processed
In en approved manner to yield the noise
evaluation measure designated as Effective
Perceived Noise Level. EPNL, In units of
EPNdB, as described In Appendix B of this
part.
(2) The aircraft height and lateral posl-
. tlon relative to the extended centerllne of
the runway must be determined by a method
Independent of normal flight Instrumenta-
tion such as radar tracking, theodolite trl-
angulatlon, or photographic scaling tech-
niques to be approved by the FAA.
(3) Tbe aircraft position along the flight
path must be related to the noise recorded
at tbe noise measurement locations by means
of synchronizing signals. The position of the
aircraft must be recorded relative to the
runway from a point at least 4 nautical
miles from threshold to touchdown during
the approach and at least 6 nautical miles
from the start of roll during the takeoff.
(4) The takeoff test may be conducted at
a weight different from the maximum take-
off weight at which noise certification 1ft re-
quested If the necessary EPNL correction does
not exceed 2 EPNdB. The approach test
may be conducted at a weight different from
tbe maximum landing weight at which noise
certlflcatlon Is requested provided the neces-
sary EPNL correction does not exceed 1
EPNdB. Approved data may be used to deter-
mine the variation of EPNL with weight for
both takeoff and approach tcr.t conditions.
(5) The takeoff test must meet the con-
ditions of 5 C36.7 of Appendix C of this part.
(6) The approach test must be conducted
with the aircraft stabilized and following a
ft* :*-0.5* approach angle and must meet the
conditions of § C36.9.
(d) Measurements. (1) Position and per-
formance data required to make the cor-
rections referred to In !A36.3(c) of this
appendix must be automatically recorded at
an approved sampling rate. Measuring equip-
ment must be approved by the PAA.
(2) Position and performance data must
be corrected, by the methods outlined In
5 A36.3(d) of this appendix to standard pres-
sure at sea level, an ambient temperature of
77' P.. a relative humidity of 70 percent, and
Eero wind.
(3) Acoustic data must be corrected by the
methods of 5A36.3(d) of this appendix to
standard pressure at sea level, an ambient
temperature of 77" P., and a relative humid-
ity of 70 percent. Acoustic data corrections
must also be made for a mlnlmuu distance
.D_f_370_feet between the aircraft's approach
path and the approach measuring point, a
takeoff path vertically above the flyover
measuring point and for differences of more
than 20 feet In elevation of measuring loca-
tions relative to the elevation of the nearest
point of the runway.
(4) The airport tower or another facility
must be approved for use as the location at
which measurements of atmospheric param-
eters are representative of those condi-
tions existing over the geographical area In
which aircraft noise measurements are made.
However, the surfac wind velocity and tem-
perature must be measured near the micro-
phone at the approach, sideline, and take-
off measurement locations, and the tests are
not acceptable unless the conditions con-
form to § A36.1(b) (3) of this appendix.
(5) Enough sideline measurement sta-
tions must be used during tests so that the
maximum sideline noise Is clearly denned
with respect to location and level.
Section A3S.2 Measurement of aircraft
noise received on the ground—(a) General.
(1) These measurements provide the data
for determining one-third octave band noise
produced by aircraft during testing proce-
dures, at specific observation stations, as a
function of time.
(2) Methods for determination of the dis-
tance form the observation stations to the
aircraft Include theodolite trlangulatlon
techniques, scaling aircraft dimensions on
photographs made as the aircraft flies
directly over the measurement points, radar
altimeters, and radar tracking systems. The
method used must be approved.
(3) Sound pressure level data for noise
type certlflcatlon purposes must be obtained
with approved acoustical equipment and
measurement practices.
(b) Measurement system. (1) The acousti-
cal measurement system must consist of
approved equipment equivalent to the
following:
(1) A microphone system with frequency
response compatible with measurement and
analysis system accuracy as stated In para-
graph (c) of this section.
(11) Tripods or similar microphone mount-
Ings that minimize Interference with the
sound being measured.
(ill) Recording and reproducing equip-
ment characteristics, frequency response, and
dynamic range compatible with the response
and accuracy requirements of paragraph (c)
of this section.
(Iv) Acoustic calibrators using sine wave
or broadband noise of known sound pressure
level. If broadband noise Is used, the signal
must be described In terms of Its average
end maximum rms value for a nonoverlood
signal level.
(v) Analysis equipment with the response
and accuracy requirements of paragraph (d)
of this section.
(c) Sensing, recording, and reproducing
equipment. (1) The sound produced by the
aircraft shall be recorded in such a way that
the complete Information, time history In-
cluded, Is retained. A magnetic tape recorder
Is acceptable.
(2) The characteristics of the system must
comply with the recommendations given In
International Electrotechnlcal Commission
(IEC) Publication No. 179 with regard to the
sections concerning microphone and ampli-
fier characteristics. The text and specifica-
tions of IEC Publication No. 179 entitled:
"Precision Sound Level Meters" are Incorpo-
rated by reference Into this part and are
made a part hereof as provided In 6 U.S.C.
552(a)(l) and 1 CFR Part ,20. This pub-
lication was published In 1965 by the Bureau
Central de la Commission Electrotechnlque
Internationale located at 1. rue de Varembe,
Geneva. Switzerland, and copies may be pur-
chased at that place. Copies of this publica-
tion are available for examination at the
DOT Library. Federal Office Building 10A
Branch and at the Office of Noise Abatement
both located at Headquarters, Federal Avia-
tion Administration, 800 Independence Ave-
nue, Washington, D.C. Moreover, copies of
this publication are available for examina-
tion at the Regional Offices of the FAA.
Furthermore, a historic, official file will be
maintained by the Office, of Noise Abatement
and will contain any changes made to this
publication.
(3) The response of the complete system
to a sensibly plane progressive sinusoidal
wave of constant amplitude must He within
the tolerance limits specified In IEC Publica-
tion No. 179, over the frequency range 45 to
11.200 Hz.
(4) If limitations of the dynamic range
of the equipment make It necessary, high
frequency preemphasls must he added to
the recording channel with the converse de-
emphasis on playback. The preemphasls
must be applied such that the Instantaneous
recorded sound pressure level of the noise
signal between 800 and 11,200 Hz does not
vary more than 20 dB between the maximum
and minimum one-third octave bands.
(5) The equipment must be acoustically
calibrated using facilities for acoustic free-
field calibration and electronically calibrated
as stated in paragraph (d) of this section.
(6) A windscreen must be employed with
the microphone during ail measurements of
aircraft noise when the wind speed Is In
excess of 8 knots. Corrections for any In-
sertion loss produced by the windscreen, as
a function of frequency, must be applied to
the measured data and the corrections ap-
plied must be reported.
(d) Analysis equipment. (1) A frequency
analysis of the acoustical signal shall be per-
formed using one-third octave niters comply-
ing with the recommendations given In In-
ternational Electrotechnlcal Commission
(IEC) Publication No. 225. The text and spec-
ifications of IEC publication No. 225 en-
titled "Octave. Half-Octnve and Third-Oc-
tave Band Filters Intended for the Analysis
of Sounds and Vibrations" arc Incorporated
by reference Into this part and are made a
part hereof as provided In 5 U.S.C. 552 (a) (1)
and 1 CFR Part 20. This publication was
published In 1968 by the Bureau Central de
la Commission Electrotechnlque Interna-
tionale located at 1. rue de Varembe. Geneva,
Switzerland, and copies may be purchased
at that place.-Copies of this publication are
available for examination at the Office of
Noise Abatement and at the DOT Library,
Federal Office Building 10A Branch both lo-
cated at Headquarters. Federal Aviation Ad-
ministration, • 800 Independence Avenue,.
Washington. D.C. Moreover, copies of .this
publication are available for examination ait
the Regional OSlces of the FAA. Furthermore
- 12 -
A-14
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a historic, official file will be maintained by
ttoe Office of Noise Abatement and will con-
tain any changes made to this publication.
(2) A act of 24 consecutive one-third oc-
tave filters must be used. The first filter of
tbo £6i rnuct be centered c.t a gcsmetrtc n;can
frequency of 50 Hz and the last of 10 kHz.
(3) The analyzer Indicating device must
be analog, digital, or a combination of both.
The preferred sequence of signal processing
Is:
(I) Squaring the one-third octave filter
outputs;
(II) Averaging or Integrating; and
(Ul) Linear to logarithmic conversion.
The Indicating device must have a minimum
crest factor capacity of 3 and shall measure.
within a tolerance of ±l.OdB, the true root-
mean-square (rms) level of the signal In
each of the 24 one-third octave bands. If
other than a true rms device la utilized, It
must be calibrated for nonslnusoldal signals
and time varying levels. The calibration must
provide means for converting the output
levels to true rms values.
(4) The dynamic response of the analyzer
. to input signals of both full-scale and 20
dB less than full-scale amplitude, shall con-
. form to the following two requirement*:
(1) When a sinusoidal pulse of 0.6-second
duration at the geometrical mean frequency
of each one-third octave band Is applied to
tbe Input, the maximum output value shall
read 4 dB±l dB less than the value obtained
for a steady state sinusoidal signal of the
same frequency and amplitude.
(ii) The maximum output value shall ex-
ceed the final steady state value by 0.5 ±0.6
dB when a steady state sinusoidal signal at
. tbe geometrical mean frequency of each one-
third octave band is suddenly applied to tbe
. analyzer input and held constant.
(6) A single value of the rms level must
be provided every 0.5 ±0.01 second for each
of tbe 24 one-third octave bands. The levels
from all of the 24 one-third octave bands
must be obtained within a 50-m!!!!second
period. No more than 6 milliseconds of data
from any 0.5-second period may be excluded
from the measurement.
(6) The amplitude resolution of the
analyzer must be at least 0.25 dB.
(7) Each output level from the analyzer
must be accurate within ±1.0 dB with re-
spect to the input signal, after all systematic
errors have been eliminated. Tbe total sys-
tematic errors for each of the output levels
must not exceed ±3 dB. For contiguous filter
systems, the systematic correction between
adjacent one-third octave channels may not
exceed 4 dB.
(8) The dynamic range capability of the
analyzer for display of a single aircraft noise
event must be at least 55 dB in terms of the
difference between full-scale output level
and the maximum noise level of the analyzer
equipment.
(0) The complete electronic system must
be subjected to a frequency and amplitude
electrical calibration by the use of sinusoidal
or broadband signals at frequencies covering
the range of 45 to 11.200 Hz, and of known
amplitudes covering the range of signal levels
furnished by the microphone. If broadband
signals are used, they must be described In
terms of their average and maximum rms
values for a nonoverload signal level.
(e) Noise measurement procedures. (1)
Tbe microphones must be oriented so that
the maximum sound received arrives as
nearly as reasonable in the direction for
which the microphones are calibrated. The
microphones must be placed so that their
sensing elements are approximately 4 feet
above ground.
(2) Immediately prior to and after each
test, a recorded acoustic calibration of the
system must be made in the field with an
acoustic calibrator for tho two purposes of
chocking system sensitivity and providing
an acoustic reference level for the analysis
of the sound level data.
(3) For the purpose of minimizing equip-
ment or operator error, field calibrations
must be supplemented with the use of an
insert voltage device to place a known signal
at the input of the microphone, just prior
to and after recording aircraft noise data,
(4) The ambient notso. Including both
acoustical background and electrical noise
of the measurement system, must be re-
corded and determined in the test area with
the system gain set at levels which will be
used for aircraft noise measurements. .
Section A36.3 Reporting and correcting
measured data—(a) General. Data represent-
ing physical measurements or corrections to
measured data must be recorded in perma-
nent form and appended to the record except
that corrections to measurements for normal
equipment response deviations need not be
reported. All other corrections must be ap-
proved. Estimates must be made of the indi-
vidual errors Inherent In each of the opera-
tions employed In obtaining tbe final data.
(b) Data reporting. (1) Measured and
corrected sound pressure levels must be pre-
' sented in one-third octave band levels
obtained with equipment conforming to
the standards described in 9 A36.2 of this
appendix.
(2) The type of equipment used for meas-
urement and analysts of all acoustic aircraft
performance and meteorological data must be
reported.
(3) The following atmospheric environ-,
mental data, measured at hourly Intervals or
less during the test period at the observation
points prescribed in 5 A36.1 (d) (4) of this
appendix, must be reported:
(1) Air temperature in degrees Fahrenheit
and relative humidity in percent.
(11) Maximum, minimum, and average
wind in knots and their direction.
(Hi) Atmospheric pressure In Inches of
Mercury.
(4) Comments on looal topography, ground
cover, and events that might Interfere with
sound recordings must be reported.
(6) The following aircraft information
must be reported:
(1) Type, model, and serial numbers (If
any) of aircraft and engines.
(II) Dross dimensions of aircraft and lo-
cation of engines.
(Ill) Aircraft gross weight for each test
run.
(Iv) Aircraft configuration such as flap
and landing gear positions.
(v) Airspeed in knots.
(vi) Engine performance in pounds of net
thrust, engine pressure ratios, jet exit tem-
peratures, and fan or compressor shaft
rev./mln. as recorded by cockpit Instruments
and manufacturer's data.
(vll) Aircraft height in feet determined
by a method Independent of cockpit instru-
mentation such as radar tracking theodolite
trlangulatlon, or approved photographic
techniques.
(6) Aircraft speed and position and engine
performance parameters must be recorded
at an approved sampling rate sufficient to cor-
rect to the noise type certification reference
conditions prescribed in 8 A36.3(c) of this
appendix. Lateral position relative to the
extended centerllne of the runway, configu-
ration, and gross weight must be reported.
(c) Noise type certification reference con-
ditions—(1) Meteorological conditions. Air-
craft position and performance data and the
noise measurements must be corrected to
the following noise type certification refer-
ence atmospheric conditions:
(o) Sea level pressure of 2116 psf (78 cm
mercury),
(b) Ambient temperature of 77* P.
(ISA + 10'C.).
(c) Relative humidity of 70 percent,
(d) Zero wind.
(2) Aircraft conditions. Tho reference con-
dition for takeoff lo the maximum weight e'-
cept as provided In 5 36.1681 (b).
The reference conditions for approach are:
(a) Design landing weight, except as pro-
vided in i 30.1681 (1>),
(b) Approach angle of 3 *,
(c) Aircraft height of 370 feet above noise
measuring station. •
(d) Data corrections. (1) The noise data
must be corrected to the noise type certifi-
cation reference conditions as stated In
§ A3B.3(c) of this appendix. Tho measured
atmospheric conditions must be those ob-
tained in accordance with SA36.1(d)(4) of
this appendix. Atmospheric attenuation of
sound requirements are given in 5 A36.6 of
this appendix.
(2) The measured flight path must be
corrected by an amount equal to the dif-
ference between the applicant's predicted
flight paths for the test conditions and for
the noise type certification reference con-
ditions. Necessary corrections relating to air-
craft flight path or performance may be de-
rlvnd from approved data other than cer-
tification test data. The flight path correction •
procedure for approach noise must be made
with reference to a fixed aircraft height of
370 feet and a glide angle of 3'. The effective
perceived noise level correction must be less
than 2 EPNdB to allow for:
(a) The aircraft not passing vertically
above the measuring point.
(b) The difference between 370 feet and
the actual minimum distance of the air-
craft's ILS antenna from the approach meas-
uring points.
(c) The difference between the actual ap-
proach angle and 8'.
Detailed correction requirements are given
in 5 A36.6 of this appendix.
(3) If aircraft sound pressure levels do
not exceed the background sound pressure
levels by at least 10 dB in any one-third
ootavo band, approved corrections for tbe
contribution of background sound pressure
levels to observed sound pressure levels must
be applied.
(e) Validity of results. (1) The test re-
sults must produce three average EPNL val-
ues and their 90 percent confidence limits,
each being the arithmetic average of the cor-
rected acoustical measurements for all valid
test runs at the takeoff, approach, and side-
line measuring points, respectively. If more
than one acoustic measurement system Is
used at any single measurement location
(such as for the symmetrical sideline meas-
uring points), the resulting data for each test
run must be averaged as a single measure-
ment.
(2) The minimum sample size acceptable
for each of the three certification measuring
points is six. The samples must be large
enough to establish statistically for each of
the three average noise type certification
levels a 90 percent confidence limit not ex-
ceeding ±1.5 EPNdB. No test result may be
omitted from the average process unless
otherwise specified by tbe FAA.
(3) The average EPNL values and their
90 percent confidence limits obtained by the
foregoing process must be those by which
the noise performance of the aircraft Is
assessed against the noise type certification
criteria, and must be reported.
Section A36.4 Symbols and units—(a)
General. The symbols used In Appendixes
A and B of this part have the following
meanings.
- 13 -
A-15
-------
Symbol Unit
Meaning
ant
cuo dB
d BOC
D dB
EPNL EPNdB..
f(0orfl_..t H«
F(i.k) dB
h dB
H %....
th the presence of dis-
crete free of f^ownd Pressure Lerel.
The chnnpc in level between
Adjacent one-third octave
band sound pressure levels
at the i-th hand for the k-th
instant of time.
Change in Slope of Sound
Pressure tevcl.
Adjusted Slope of Sound Pres-
sure Ijfpfl. The change in
level between adjacent
adjusted one-third octave
band sound pressure levels
at the i-tli band for the k-tb
instant of time.
Average Slope of Sound Pres~
ture Jsvcl.
Sound Pressure Level. The
sound pressure level at any
instant of time that occurs
inaspcciRed frequency range.
Noy DitcnrUinulty Coordinate.
The SPL value of the inter-
section point of the straight
Hnos representing the varia-
tion of SPL with lop n.
Noy Intercept. The intercepts
on the S PL-axis of the
straight lines representing
the variation of SPL with
log n.
Sound Pressure T*eoel. The
sound pressure level at the
k-th instant of time that
occurs In the i-th one-third
octave bund.
Adjusted Sound Pressure Level.
The-flrst approximation to
background level in the i-th
one-third octave band for
the k-th Instant of time.
Background Sound Pressure
Lerel. The final approxima-
tion to background level in
the i-th one-third octave
band for the k-th instant
of time.
Maximum Sound Pressure
Level. The sound pressure
level that occurs in the 1-th
one-third octave band of
the spectrum for PNLTM.
Corrected Maximum Sound
Pressure Lerel. The sound
pressure level that occurs in
the i-th one-third octave
band of the spectrum for
PNLTM corrected for
atmospheric sound absorp-
tion.
Elapted Time. The length of
time measured from a
reference r,ero.
Time Limit. The beginning
and end of the significant
noise time history defined
by h.
Time Increment. The equal
increments of time for which
PNL(k) and PNLT(lc) are
calculated.
Nonnalifir.ft Time Conftant.
The length of time used as
. a reference in the Integration
method for computing
duration corrections.
Temperature. The ambient
atmospheric temperature.
Tat Atmospheric Absorption.
The atmospheric attenua-
tion of sound that occurs In
the 1-th one-third octave
band for the measured at-
mospheric temperature and
relative humidity.
Symbol
Unit
Meaning
. dB/fpet
. dB/IOOO
feet.
O
. degrees--.
. decrees...
. dcprecs,
. degrees.
. decrees..
. degrees...
Reference Atmospheric Absorp-
tion. The atmospheric at-
tenuation of sound tlmt oc-
curs in the t-th one-third
octnvc band for the reference
atmospheric temperature
and relative humidity,
. First Constant Climb Antfe.
. Sfcrmd Constant Climb Angle. '
Thriift CuflHict Anglct. The
anplcs defining the points
on the takeoff flight path at "
which thrust reduction is
started and ended respeo- ',
tivcly.
.. Approach Angle.
. To.kroff Koisc Anglf. The angle
between the flight path and
noise path for takeoff opera- •
tion. It is Identical for both ;
measured and corrected
flight paths.
X degrees Approach Noise Angle. The
ancle between the flight
path and the noise path for
approach operation. It is
idenlical for !>oth measured •
find corrected flight paths.
Al EPNdB.. P^LT Correct ion. The correc-
tion to ho added to the
EPNL calculated from
measured data to account .<
for noise level changes due 1
to differences in atmospheric
absorption and noise ]«ith
length between reference '
nnQ test conditions. I
A2 EPNdB.. A'oiw Path Duration Cwrcc- •
(inn. The correction to be 1
added to the EPNL calcu- j
la ted from measured data to [
account for noise level i
changes due to the noise i
duration because of differ- j
cnces in flyover altitude !
between reference and test
condition. j
Weight Correction. The onrrec- *
tion to he added to the i
EPNL calculated from !
measured data to account ]
for noise level clianpcs du* j
to differences between maxi-
mum and test aircraft
weights.
EPNdB. . Approach Anale Corredinn.
The correction to be added
to the EPNL calculated
from measured daUv to
account for noise level
changes due to differences
between 3* and the test.
A3 EPNdB..
AAB.
AjS_.__
Ay....
approach angle.
. feet. Takeoff Profile Changes. The
. degrees. changes in the basic param-
. degrees. eters defining the takeoff
. degrees. profile due to differences
; degrees. between reference and test
conditions.
FLIGHT PROFILE IDENTIFICATION POSITIONS
Position Description
A Start of takeoff roll.
B Liftoff.
C Start of first constant climb.
D Start of thrust reduction.
E Start of second constant climb.
EC Start of second constant climb on
corrected flight path. • '
P; End of noise certification takeoff
flight path.
Fc End of second constant climb on
corrected night path.
O_, Start of noise certification ap-
proach flight path.
Or Start of noise certification ap-1
-Nt_x proach on reference flight path.
H-> Posltl6n on approach path dl-j
"'' rectly above noise measuring!
station. i
I Start of level off. :
Ir Start of level off on reference ap-;
proach flight path. :
J Touchdown.
K-_ Takeoff noise measuring station.
L. S:dcl!no nclGC nicsGurlng ctatlca
(not on flight track).
- 14 -
A-16
-------
rUGHT PROFILE IDENTIFICATION
POSITIONS— Continued
Petition Description
M__... __ End of noise type certification
takeoff flight track.
N __ - _____ Approach noise measuring station.
O. _______ Threshold of approach end of
FMOOT PBOPILE DISTANCES—Continued
S
p ________ Start of noise type certification
approach flight track.
Q ______ .. Position on measured takeoff
night path corresponding to
PNLTM at station K.
Qc _______ Position on corrected takeoff
flight path corresponding to
PNLTM at statlon.K.
B_ _______ Position on measured takeoff
flight path nearest to station K.
He _______ Position on corrected takeoff
flight path nearest to station K.
Position on measured approach
flight path corresponding to
PNLTM at station N.
Sr _______ Position on reference approach
flight path corresponding to
PNLTM at station N.
T ________ Position on measured approach
flight path nearest to station N.
Tr _______ Position on reference approach
flight path nearest to station N.
X ________ Position on measured takeoff
flight path corresponding to
PNLTM at station L.
FUOHT PROFILE DISTANCES
. Distance Unit
Meaning
AB feet
AK fcet
AM feet
KQ.
KR..
•ERo.
LX..
NH.
N8..
NBr.
NT..
NTr
... feet
....'feet
.... feet
.... feet..
.... feet
..... feet
.... feet
....feet
fort
feet.......
-ON feet
Length of Takeoff Roll. The
distance along the runway
between the start of takeoff
roll and lirt on.
Takeoff Measurement Distance.
The distance from the start
of roll to the takeoff noise
measurement station alone
the extended ccntcrline
of the runway.
Takeoff Flight Track Distance.
The distance from the Start
of roll to the takeoff flight
track position along the
cxtented ccntcrlino of the
runway for which the
position of the aircraft
need no longer bo recorded.
Measured Takeoff Noise Path.
The distance from station
K to tho measured aircraft
position Q.
Corrected Takeoff Noise Path.
The distance from station
K to the corrected aircraft
position Qc.
Measured'Takeoff Minimum
IHttance. The distance from
station K to point R on the
measured fllpht path.
Corrected Takeoff Minimum
Distance. The distance from
station K to point Uc on
the corrected flight path.
Measured Sideline Noise Path.
The distance from station
L to the measured aircraft
position X.
Aircraft Approach Height. The
vortical distance between
the aircraft and the ap*
proach measuring station.
Measured Approach Noise
Path. Tho distance from
station N to the measured
aircraft position S.
Reference Approach Noise
Path. Tho distance from sta-
tion N to the reference air-
craft position Sr.
Measured Approach Minimum
Distance. The distance from
station N to point T on the
measured flight path.
Reference Approach Minimum
Distance. Tho distance from
station N to point Tr on the
corrected flight path; It
equals 3(3'.) foot.
Approach Measurement Dls~
tance. Tho distance from the
runway threshold to the ap-
proach measurement station
along the extended centw-
llae of the runway.
Symbol
Unit
Meaning
OP.. feet Approach Flight Track Dis-
tance. The distance from the
runway threshold to tho ap*
proech flight tract position
along tho extended center-
lino of the runway for which
the position of the aircraft
• need uo longer be recorded.
Section A36.5 Atmospheric attenuation of
sound—(a) General. The atmospheric at-
tenuation of sound must be determined In
accordance with the curves of Figure 15
presented In SAE ARP 866 or by the simplified
procedure presented below. SAE ARP 866 is
a publication entitled: "Standard Values of
Atmospheric Absorption as a Function of
Temperature and Humidity for Use In
Evaluating Aircraft Flyover Noise" and the
recommendations presented therein are In-
corporated by reference Into this Part and
are made a part hereof as provided In 5 U.S.C.
522(a)(l) and 1 CFR Part 20. This publica-
tion was published on August 31, 1964. by
the Society of Automotive Engineers, Inc.,
located at 2 Pennsylvania Plaza, New York,
N.Y. 10001, and copies may be purchased
at that place. Copies of this publica-
tion are available for examination at the
DOT Library. Federal Office Building 10A
Branch and at the Office of Noise Abatement
both located at Headquarters, Federal Avia-
tion Administration. 800 Independence Ave-
nue, Washington, D.C. Moreover, copies of
this publication are available for examina-
tion at the Regional Offices of the FAA. Fur-
thermore, a historic, official file will be
maintained by the Office of Noise Abatement
and win contain any changes made to this
publication. '
(b) Reference conditions. For the refer-
ence atmospheric conditions of temperature
and relative humidity equal to Tl' V. and 70
percent, respectively, and for nil other con-
ditions of temperature and relative humidity
where their product Is equal to or greater
than 4,000, the sound absorption must be ex-
pressed by the following equation:
alo' = n/500 (dB/l,OOOft.)
alo' Is the atmospheric attenuation of sound
that occurs In the 1-th one-third octave
band for tho reference atmospheric condi-
tions and fl Is the geometrical mean fre-
quency for the 1-th one-third octave band.
(c) Nonre/crence conditions. (1) For all
atmospheric conditions of temperature and
relative humidity where their product Is
equal to or less than 4.000, the rel.itlonshlp
between sound absorption, frequency, tem-
perature, and humidity must be expressed
by the following equation:
500 olVfl = (2/3) ((11/2) - (HT/1,000) ]
al' Is the atmospheric attenuation of sound
that occurs In the 1-th one-third octave
band for a relative humidity of H percent
and a temperature of T' Fahrenheit.
(2) Figure Al graphically Illustrates the
simplified relationship. The second equation
represents the Inclined line which is valid
for all values of HT up to and Including
4,000. For all values of 4.000 and greater, the
horizontal line, represented by the first
equation, Is valid. The minimum, reference.
and maximum values of humidity and tem-
perature are Indicated In Figure Al.
njlii
I
— H=30%
T = 41°F
,,,,!,„,
IJI
H=70%
1 Ji
= 86°F
HUMIDITYX TEMPERATURE, HT/IOOO, % °F
FIGURE Al. SIMPLIFIED RELATIONSHIP BETWEEN ATMOSPHERIC
SOUND ATTENUATION, FREQUENCY, HUMIDITY,
AND TEMPERATURE.
Section A36.8 Detailed correction proce-
dures—(a) General. If tho noise type certifi-
cation test conditions arc not equal to the
noise certification reference conditions, ap-
propriate positive corrections must be made
to the EPNL calculated from the measured
data. Differences between reference and test
conditions which lead to positive corrections
can result from the following:
(1) Atmospheric absorption of sound un-
der test conditions greater than reference,
(2) Test flight path at higher altitude
than reference, and
(3) Test weight less than maximum.
Negative corrections arc permitted If the
atmospheric absorption of sound under test
conditions Is less than reference and also
If the test"TirgTirpal;rr-rg-a'ra~rewCT-BlttUide
than reference.
The takeoff test flight path can occur at a
higher altitude than reference If the meteor-
ological conditions permit superior aero-
dynamic performance ("cold day" effect).
Conversely, the "hot day" effect can cause
the takeoff test flight path to occur at a
lower altitude than reference. The approach
test flight path can occur at either higher
or lower altitudes than reference Irrespec-
tive of the meteorological conditions.
The correction procedures presented In the
following discussion consist of one or more
of five possible values added algebraically to
- 3.5 -
A-17
-------
the EPNL calculated as If the tests were con-
ducted completely under the noise type certi-
fication reference conditions. The flight pro-
files must be determined for both takeoff and
approach, and for both reference and test
conditions. The test procedures require noise
and flight path recordings with a synchro-
nized time signal from which the test profile
can be delineated, Including the aircraft
position for which PNLTM is observed at the
noise measuring station. For takeoff, a flight
profile corrected to reference conditions may
be derived from manufacturer's data, and for
approach, the reference profile Is known.
The noise paths from the aircraft to the
noise mcasxirtng station corresponding to
PNLTM are determined for both the test
and reference profiles. The SPL values In the
spectrum of PNLTM are then corrected for
the effects of:
(1) Change In atmospheric sound
absorption,
(2) Atmospheric sound absorption on the
change In noise path length,
(3) Inverse square law on the change In
noise path length.
The corrected values of SPL are then con-
verted to PNLT from which is subtracted
PNLTM. The difference represents the correc-
tion to be added algebraically to the EPNL
calculated from the measured data.
The minimum distances from both the
test and reference profiles to the noise meas-
uring station are calculated and xised to
determine a noise duration correction due
to the change In the altitude of aircraft fly-
over. The duration correction is added alge-
braically to the EPNL calculated from the
measured data.
Prom approved data In the form of curves
or tables giving the variation of EPNL with
takeoff weight and also for landing weight,
corrections are determined to be added to
the EPNL calculated from the measured data
to account for noise level changes due to
differences between maximum and test air-
craft weights.
Prom approved data In the form of curves
or tables giving the variation of EPNL with
approach angle, corrections are determined
to be added algebraically to the EPNL cal-
culated from measured data to account for
noise level changes due to differences be-
tween 3° and the test approach angle.
(b) Takeoff profiles. Figure A2 Illustrates
a typical takeoff profile. The aircraft begins
the takeolf roll at point A, lifts off at point
B, and Initiates the first constant climb at
point C at an angle 0. The noise abatement
thrust cutback Is started at point D and
completed o.t point E where the second con-
stant climb Is denned by the angle 8 (usu-
ally expressed In terms of the gradient In
per cent).
The end of the noise type certification
takeoff flight path Is represented by aircraft
position P whose vertical projection on the
flight track (extended centerllne of the run-
way) Is point M. The position of the aircraft
must be recorded for a distance AM of at
least 6 nautical miles.
Position K Is the takeoff noise measuring
station whose distance AK Is specified as 3.5
nautical miles. Position L Is the sideline noise
measuring station located on a line parallel
to and a specified distance from the runway
centerllne where the noise level during take-
off Is greatest.
The takeoff profile Is defined by the fol-
lowing five parameters: AD. the length of
takeoff roll; p. the first constant climb angle;
•y. the second constant climb angle; and
£ and (. the thrust cutback angles. These five
parameters are functions of the aircraft per-
formance and weight and the atmospheric
conditions of temperature, pressure, and
wind velocity and direction. If the test con-
ditions arc not equal to the reference condi-
tions, the corresponding test and reference
profile parameters will be different as shown
In Figure A3. The profile parameter changes.
Identified as AAB. AS, Aa, A*, and A<. can
be derived from the manufacturer's data
(approved by the FAA) and can be used to
define the flight profile corrected to the
reference conditions. The relationships be-
tween the measured and corrected takeoff
flight profiles can then be used to determine
.the corrections, which If positive, must be
applied to the EPNL calculated from the
measured data.
NOTE: Under reference atmospheric con-
ditions and with maximum takeoff weight,
the gradient of the second constant climb
angle, Q, Is specified to be not less than 4
percent. However, the actual gradient will
depend upon the test atmospheric condi-
tions, assuming maximum takeoff weight
and the parameters characterizing engine
performance are constant (rpm. epr, or any
other parameter used by the pilot).
Figure A4 Illustrates portions of the meas-
ured and corrected .takeoff flight paths In-
cluding the significant geometrical relation-
ships Influencing sound propagation. EP
represents the measured second constant
flight path with climb angle -y, and EcFc
represents the corrected second constant
flight path at reduced altitude and with re-
duced climb angle Q—AJJ.
Position Q represents the aircraft location
on the measured takeolf flight path for which
jfflLTTvjC 'lsj .observed jit the noise measTTfTng
station K/and Qc'ls the corresponding posi-
tion on the corrected flight path. The meas-
ured and corrected noise propagation paths
are KQ and KQc. respectively, which form
the same angle 9 with their flight paths.
Position R represents the point on the
measured takeoff flight path nearest the
noise measuring station K, and Re IB the
corresponding position on the corrected
flight path. The minimum distance to the
measured and corrected flight paths are In-
dicated by the lines KH and KRc, respec-
tively, which are normal to their flight paths.
(c) Approach, profiles. Figure A5 Illus-
trates a typical approach profile. The begin-
ning of the noise type certification approach
profile Is represented by aircraft position O
whose vertical projection on the flight track
(extended centerllne of the runway) Is point
P. The position of the aircraft must be re-
corded for a distance OP from the runway
threshold O of at least 4 nautical miles.
The aircraft approaches at an angle u,
passes vertically over the noise measuring
station N at a height of NH, begins the level
off at position I, and touches down at posi-
tion J. The distance ON IB specified as 1.0
nautical mile.
The approach profile Is defined by the ap-
proach angle i> and the height NH which are
functions of the aircraft operating conditions
controlled by the pilot. If the measured ap-
proach profile parameters are different from
the corresponding reference approach param-'
eters (3* nnd 370 feet, respectively, as shown
In Figure A6), corrections. If positive, must
be applied to the EPNL calculated from the
measured data.
Flgu.-e A7 Illustrates portions of the meas-
ured and reference approach flight paths
Including the significant geometrical rela-
tionships Influencing sound propagation.
CI represents the measured approach path
with approach angle 17, and Grlr represents
the reference approach flight path at lower
altitude and approach angle of 3'.
Position S represents the aircraft location
on the measured approach flight path for
which PNLTM Is observed at the noise meas-
uring station N, and Sr Is the corresponding
position on the reference approach night
path. The measured and corrected nolsa
propagation paths are NS and NSr. respec-
tively, which form the same angle X with
their flight paths.
Position T represents the point on the
measured approach flight path nearest 'the
.noise measuring station N. and Tr Is the
corresponding point on the reference ap-
proach flight path. The minimum distances
to the measured and reference flight paths
are Indicated by the lines NT and NTr, re-
spectively, which are normal to their flight
paths.
NOTE: The reference approach flight path
Is defined by ij = 3° and NH = 370 feet. Con-
sequently, NTr can also be defined; NTr = 369
feet to the nearest foot and Is, therefore. '
considered to be one of the reference
parameters.
(d) PNLT corrections. Whenever the am-
bient atmospheric conditions of tempera-
ture and relative humidity differ from the
reference conditions (77° P. and 70 percent.
respectively) and whenever the measured
takeoff and approach flight paths differ from
the corrected and reference flight paths re-
spectively, It may b- necessary or desirable
to apply corrections to the EPNL values cal-
culated from the measured data. If the
corrections are required, they must bo
calculated as described below.
Referring to the takeoff flight path shown
In Figure A4, the spectrum of PLNTM ob-
served at station K. for the aircraft at po-
sition Q, Is decomposed Into Its Individual
SPLI values. A set of corrected values are
then computed as follows:
-alo) KQ
+alo (KQ-KQc)
+20 log
-------
8-th 03W--:.fe£Ta o-.-las* j«.i r »>,i:_K3 K tie
r.r.os'" —'"tx! take.-" n* •/.' "• tj..^ond
term accour-.tx ' .'. "ne etieofcs or
i. ';• uv. •'.''.• .• '-cV'nga
ta ':fca «.CIEJ v.;ila. --•' •/•.*. t-. .Q^ s She
-
rcctlo~ ic;'£3 account.' .'':; tx •'"••'Cts or th?
i'5iv~J'!'» oqners !c,w *. r.l'e ,1'Bi^e in */!ie
SLOi1*/ pfcWs length.
TSie (xrrsc'-ii. vBltur; of .?°Lio are them
converted to PNX/T o^r, t uorrnctlon term
calculatedtis follows:
i i = PHI/*- - .vHi/n :
wfaKi ysTM'ezents *.ne cnrr'sitto to be added
afc.:^ ViM i'.Sj t3 tiie E.1 K"> -j.? :.!at
-------
A
L
3.5 N.M.
K M
6 N.M. MINIMUM
FIGURE A2. MEASURED TAKEOFF PROFILE.
MEASURED
FLIGHT
PATH
AAB
FIGURE A3, COMPARISON OF MEASURED AND
CORRECTED TAKEOFF PROFILES.
- 18 -
A-20
-------
>
MEASURED
FLIGHT
PATH
Q
FIGURE A4. TAKEOFF PROFILE CHARACTERISTICS
INFLUENCING SOUND PROPAGATION.
FIGURE A5. MEASURED APPROACH PROFILE.
MEASURED
APPROACH
PATH
REFERENCE
APPROACH
PATH 370'
N
FIGURE A6. COMPARISON OF MEASURED AND
CORRECTED APPROACH PROFILES.
MEASURED
APPROACH
PATH
REFERENCE
APPROACH
PATH
FIGURE A7. APPROACH PROFILE CHARACTERISTICS
INFLUENCING SOUND PROPAGATION.
-------
EPNil
TEST
MAXIMUM
AIRCRAFT TAKEOFF WEIGHT
FIGURE A8. TAKEOFF WEIGHT CORRECTION FOR
EPNL AT 3.5 NAUTICAL MILES
FROM BRAKE RELEASE.
EPML
TEST
MAXIMUM
AIRCRAFT LANDING WEIGHT
FIGURE A9. APPROACH WEIGHT CORRECTION
FOR EPNL AT 1.0 NAUTICAL MILE
FROM RUNWAY THRESHOLD.
- 20 -
A-22
-------
EPNL
A 4
ANGLE OF APPROACH, 1
FIGURE A10. APPROACH ANGLE CORRECTION FOR
EPNL AT 1.0 NAUTICAL MILE
FROM RUNWAY THRESHOLD.
APPENDIX B—AIRCRAFT NOISE EVALUATION
XJNPEB {36.103
Section B36.1 General; The procedures In
this appendix must be used to determine the
•noise evaluation quantity designated as
'effective perceived noise level, EPNL, under
i 36.103. These procedures, which use the
physical properties of noise measured as pre-
scribed by Appendix A of this part, consist
of the following:
(a) Tlie 24 one-third octave bands of
eound pressure level are converted to per-
ceived noisiness by means of a noy table. The
noy values are combined and then converted
to Instantaneous perceived noise levels,
PNL(k).
(b) A tone correction factor, C(k), Is cal-
culated for each spectrum to account for the
subjective response to the presence of the
maximum tone.
(c) The tone correction factor Is added to
the perceived noise level to obtain tone cor-
rected perceived noise levels, PNLT(k), at
each one-half second Increment of time. The
Instantaneous values of tone corrected per-
ceived noise level are noted with respect to
time and the maximum value, PNLTM, Is
determined.
PNLT(k) = PNL(k) +C(k)
(d) A duration correction factor, D, is
computed by Integration under the curve of
tone corrected perceived noise level versus
time.
(e) Effective perceived noise level, Et>NL, Is
determined by the algebraic sum of the maxi-
mum tone corrected perceived noise level and
the duration correction factor.
EPNL = PNLTM + D
Section B36.2 Perceived noise level. In-
Btantaneous perceived noise levels, PNL(k) ,
must be calculated from Instantaneous one-
third octave band sound pressure levels,
BPL(l.fe), as follows:
Step 1. Convert each one-third octave
band SPL(l.k), from BO to 10,001) Hz, to per-
ceived noisiness, n(l,k), by reference "to
Table Bl, or to the mathematical formulation
of the noy table given In 5 B36.7 of this
appendix.
Step 2. Combine the perceived noisiness
values, n(l.k), found In step 1 by the
following formula:
N(k)=n(k>+0.15
n(l, k)
where n(k) Is the largest of the 24 values of
n(l,k) and N(k) Is the total perceived
noisiness.
Step 3. Convert the total perceived noisi-
ness, N(k) , Into perceived noise level, PNL(k) ,
by the following formula:
PNL(k) =40.0+335 log N(k)
which Is plotted In Figure Bl. PNL(k) may
also bp obtained by choosing N(k) In the
1,000 Hz column of Table pi and then read-.
Ing the Corresponding value of BPL(lJc)
which, at 1,000 Hz, equals FNL(k) .
- 21 -
A-23
-------
SPL
dB
One-Third Octave Band Center Frequencies f, HZ
90 O »»*»12S94»I00150JU«»JOO
<» wo io» u» iioo »«
Table Bl.
Perceived Noisiness
(NOYs) as a Function
of Sound Pressure
L'evel.
- 22 -
A-24
-------
5100
80
.llllll
I'l i i
1
1
' ' J ' "±
I 10 100
fatal Perceived Nolilnesi, N, noys.
Figure Jl. Perceived Nolle Level os o Function of Noy».
1000
10000
Section B36.3 Correction for spectral Ir-
repularttiei. Noise having pronounced Irreg-
ularities In the spectrum (for example, dis-
crete frequency components or tones), must
be adjusted by the correction factor C(k)
calculated as follows: ' '
Step 1. Starting with the corrected sound
pressure level In the 80 Hz one-third octave
band (band number 3). calculate the
changes In Bound pressure level (or "slopes"):
In the remainder of the one-third octave
bands as follows: '
s(3JE)=no value
) =SPL(4.k) -SPL(3Jc)
=8PL(IJc) -
-1) JtJ
6(24*) =SPL(24Jc) — SPL(23,k)
Step 2. Encircle the value of the slope,
»(l,k), where the absolute value of the
change In slope Is greater than S; that Is,
where.
|As(l, k)|=|s(l, k)-st(i-l), k)|>6.
Step 3. (a) If the encircled value of the
(lope s(l,k) Is positive and algebraically
greater than the slope s[(l—l),k), encircle
8PL(1*).
'(b) If the encircled valuevrf the slope B(ljc)
Is zero or negative and the elope s[l—l
Is positive, encircle (SPL[(1-1) ,k])
(c) For all other cases, no sound pressure
level value Is to be encircled.
Step 4. Omit all SPL(l,k) encircled In Step
3 'and compute new sound pressure levels
SPL'(i,k) as follows:
(a) For nonenclrcled sound pressure levels,
let the new sound pressure levels equal the
original sound pressure levels,
SPL'(l.kJ=SPL(l,k)
(b) For encircled sound pressure levels In
bands 1-23, let the new sound pressure level
equal the arithmetic average of the preceding
and following sound pressure levels,
BPI/(l.k)=(M)[SPL[a-l),k)+8PMa+l),k]]
(c) If the sound, pressure level In the
highest frequency band (1=24) is encircled,
let the new sound pressure level In that
band equal ;
SPL'(24Jc) =SPL(23,k) +s(23,k).
Step 5. Recompute new slopes s' (l,k), In-
cluding one for an Imaginary 25-th band, as
follows: i
8'(3.k)=s'(4,k)
s'(4,k)=SPI/(4.k)-SPI/(3,k)
8' (24, k) =SPI/(24. *) _SPL'(23, k)
e'(25.k)=s>(24. k)
Step S. For 1 from 3 to 23, compute the
arithmetic average of the three adjacent
slopes as follows:
,
Step 7. Compute final adjusted one-third
octave-band sound pressure levels, SPL"
(l,k) , by beginning with band number 3 and
•proceeding to band number 24 as follows:
8PL"(3i k) =SPL(3, k)
SPL"(4,k)=SPL"(3.k)+5(3,k)
SPL"(24, k) =SPL"(23, k) +s(23. k)
Step 8. Calculate the differences, F(1JO,
between the original and the adjusted sound
pressure levels as follows: .
•' F(l.k) =SPL(l,k) — SPL"(l,k»
and note only values greater than zero.
Step B. For each of the 24 one-third octave
•bands, determine tone correction factors from
the sound pressure level differences F(l,k)
and Table B2. ' '
Step 10. Designate the largest of the tone
correction factors, determined In Step 8, as
C(k). An example of the tone correction
procedure la given In Table. B3..
Tone corrected perceived noise levels
PNI/T(k) are determined by adding the C(k)
values to corresponding PNL(k) values, that
is, ;
PNLT(k) =PNL(k) +C(k)
For any 1-th one-third octave band, at any
k-th Increment of time,' for which the tone
correction factor Is suspected to result from
something other than (or In addition to) an
actual tone (or any spectral Irregularity
other than aircraft noise), an additional
analysis may be made using a niter with a
bandwidth narrower than one-third of an
octave. If the narrow band analysis cor-
roborates that suspicion, then a revised value
for the background sound pressure level,
8PIi"(l,k), may be determined from 'the
analysis and used to compute a revised tone
correction factor, F(l,k), for that particular
one-third octave band.
- 23 -
A-25
-------
§
4
ITTII(
1 J ( I T I I.I I 1 I ' .1 I I I I ». »
SOO^f^SOOOHZ
f -£500 HZ
f ^5000 HZ
t i r i i i i i i I i i i i i t t t i i i i i t
<0
10 15
Level Difference f-f dB
20
25
i
CO
(Frequency
f, HZ
SC^f^SDO.
Sfn -•£ f? ,_<^ 5000
^& ^ i w
-
-
-
.,
2 1/3
1 2/3
4
-
2
1
_
-
1 1/3
-
-
LL_
61
1/3
2
U23-
-
FT
©
c.
dB
Step
?
„
=.
2/3
1
r-n i
- i
<
i"~ — \
i
\
1 v
Table B3. Example of Tone Correction
for a Turbofaa Sngine
-------
,,.Scctlon B36.4 Maximum tone corrected
perceived noise level. The maximum tone
corrected perceived noise level, PNLTM, IB
the maximum calculated value at the tone
corrected perceived noise level, FNLT(k), cal-
culated In accordance with th? procefU'tw of
I B36.3 ol this Appendix. Figure B2 Is an ex-
ample of a flyover nojse time history where
the maximum value Is clearly indicated.
Half-second time Intervals, At, are small
enough to obtain a satisfactory noise time
history.
If there are no pronounced Irregularities In
the spectrum, then the procedure of ! D36.8
of this Appendix would bo redundant since
PNIjT(k) would tx> identically equal to
PNL(k). For this case, PNLTM would be the
maximum value of PNL(k) and would equal
PNLM.
"5-
> TJ
8 ss
« s
13
PNLTM
t(0
Flyover Time t, ice,
figure B2. Example of Perceived Noise Level Corrected
for Tones as a Function of Aircraft Flyover
Time
t(2)
Section B36.5 Duration correction. The
duration correction factor D Is determined
by the Integration technique denned by the
expression:
P"' ant [PNLT/10] dtT^PNLTM
1(1) J
D-101og
where T Is a normalizing time constant,
PNLTM Is the maximum value of PNLT, and
t(l) and t(2) are the limits of the significant
noise time history.
Since PNLT Is calculated from measured
values of 8PL, there will, In general, be no
•obvious equation for PNLT as a function of
time. Consequently, the equation can be re- .
written with a summation sign Instead of an
Integral sign as follows:
D- 10 log Fu/T) £ AtanttPNLT(k)/10]l-PNLTM
where At Is the length of the equal incre-
ments of tune for which PNLT(k) Is calcu-
lated and d Is the time Interval to the
nearest 1.0 second during which PNLT(k) Is
within a specified value, h, of PNLTM.
Half-second time Intervals for At are small
enough to obtain a satisfactory history of the
perceived noise level. A shorter time Interval
may be selected by the applicant provided
aproved limits and constants are used..
The following values for T, At, and h, must
be used in calculating D :
T=10sec,
At=0.6 sec, and
h=10dB.
Tldng the above values, the equation for D
becomes
D-10 Ipg f-E'ant [PNLT(k)/10]"j-PNLTM-13
where the Integer d Is the duration time
defined by the points that are 10 dB less
than PNLTM.
If the 10 dB-down points fall between cal-
culated. PNLT(k) values (the usual case),
the applicable limits for the duration time
must be chosen from the PNLT(k) values
closest to PNLTM—10. For those cases with
more than one peak value of PNLT(k), the
applicable limits must be chosen to yield the
largest possible value for the duration time.
If the value of PNLT(k) at the 10 dB-
down points Is 90 PNdB or less, the value of
d may be taken as the time Interval between
the initial and the final times for which
PNLT(k) equals 90 PNdB.
Section B36.6 Effective perceived noise
level. The total subjective effect of an air-
craft flyover Is designated "effective per-
ceived noise level," EPNL, and Is equal to
the algebraic sum of the maximum value of
the tone corrected perceived noise level,
PNLTM, and the duration correction, D.
That is,
EPNL=PNLTM+ D
where PNLTM and D are calculated under
55B36.4 and B30.6 of this appendix.
The above equation can be rewritten by
substituting the equation for D from 5 B36.5
of this appendix, that is,
EPNL=101og F£ ant[PNLT(k)/10l"]-13
Section B36.7 Mathematical formulation
of nay tables. The relationship between sound
pressure level and poroeived noisiness given
In Table Bl Is illustrated in Figure B3. The
Variation of 8PL with log n for a given one-
third octave band can be expressed by either
one or two straight lines depending upon the
frequency range. Figure B3(a) Illustrates the
double line case for frequencies below «X»
Hz. and above 6,300 Hz and Figure B3(b)
Illustrates the single line case for'all other
frequencies.
The important aspects of the mathematical.
formulation are:
1. the slopes of the •straight lines, p(b)
andp(c),
2. the Intercepts of the lines on the 8PL-
axls, SPL(b), and BPL(c), and
3. the coordinates of the discontinuity,
8PL(a),andlogn(a).
The equations are as follows:
Case 1. Figure B3 (a) f <400 Hz.
f > 6300 Hz.
SPL(a) =-
p(c)SPL(b) -p(b)SPL(c)
logn(ft)=-
p(c)-p(b)
8PL(c) —SPL(b)
p(b) -p(c)
(a) SPL(b)£BPI/g8PI«(a).
8PL-SPL(b)
n=ant-
P(b)
(b) SPL >8PL(a).
SPL-SPL(c)
n=ant-
P(c)
(c) 06300 Hz.
_M(b)6PL(b)-M(c)SPL(c)
M(b) -M(c)
. _ M(b)M(c) tSPL(c) -.8PL(b) 1
log n(a, . M(c)_M(b)
(a) SPL(b) <, SPL £ SPL(a) .
n=ant M(b) [SPL— SPL(b) ]
(b) SPLgSPL(a).
n = ant M(c) |SPL— SPL(c) 1
(c) 0SPL(c).
I n=antM(c)[SPL— SPL(c)J
I (b) logn>0.
Table B4 lists the values of the Important
constants necessary to calculate sound
pressure level as a function of perceived
noisiness.
- 25 -
A-27
-------
1
J
SPL (a)
SPL(b)
SPL(c)
f -£ 400 HZ
f 5* 6300 HZ
log n(«)
Log Perceived Noisiness, log n
K)
00
v>
%
*S
"5
3
V)
SPL(c)
400 a^ f-=£6300 HZ
(b)
Log Perceived Noisiness, log n
Figure B3. Sound Pressure Level as a function of Noys.
Bond
(i)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
f
HZ
50
63
80
100
125
160
200
250
315
400
500
630
800
1000
1250
1600
2000
2500
3150
4000
5000
6300
8000
10000
M(b)
0.043478
0.040570
0.036831
II
0.035336
0.033333
II
0.032051
0.030675
-
-
-
-
-
-
-
-
-
-
_
-
-
0.042285
II
SPL
(b)
dB
64
60
56
53
51
48
46
44
42
-
-
-
-
-
-
-
-
_
-
-
-
-
37
41
SPL
(a)
dB
91.0
85.9
87.3
79.9
79.8
76.0
74.0
74.9
94.6
.
-
-
-
• -
-
-
-
_
_
-
-
44.3
50.7
M(c)
0.030103
"
11
H
"
"
11
11
"
"
"
11
"
11
"
0.029960
"
it
»
••
"
"
H
11
SPL
(<=)
dB
52
51
49
47
46
45
43
42
41
40
»
"
"
11
38
34
32
30
29
II
30
31
3<
37
Table B4. Corsfants for Mathematically Formulated NOY Values
APPENDIX C—NOISE LEVELS TOR SUBSONIC
TRANSPOHT CATEGORY AND TURBOJET
POWERED AIRPLANES UNDER { 36.201
Section C36.1 Noise measurement and
evaluation. Compliance with this appendix
must be shown with noise levels measured
and evaluated as prescribed, respectively, by
Appendix A and Appendix B of this part, or
under approved equivalent procedures.
Section C36.3 Noise measuring points.
Compliance with the noise level standards
of { C36.5 must be shown—
| (a) For takeoff, at a point 3.5 nautical
miles from the start of the takeoff roll on the
extended centerllne of the runway;
J (b) For approach, at a point l nautical
mile from the threshold on the extended
centerllne of the runway: and
(c) For the sideline, at the point, on a line
parallel to and 0.25 nautical miles from the
extended centerllne of the runway, where
the noise level after liftoff Is greatest, except
that, for airplanes powered by more than
three turbojet engines, this distance must
be 0.35 nautical miles.
Socuon
-------
(b) Tradeoff. The noise levels In paragraph
(a) may be exceeded at one or two of th«
measuring points prescribed In § CSC.3. If—
(1) The sum of the exceedances Is not
greater than 3 EPNdB;
(2) No exceedance Is greater than 2
EPNdB; and
(3) The exceedances are completely offset
by reductions at other required measuring
points.
(c) Prior applications. For applications
made before December 1, 1969. for airplanes
powered by more than three turbojet engines
with bypass ratios of two or more, the value
prescribed In paragraph (b)(l) of this sec-
tion may not exceed 5 EPNdB and the value
prescribed In paragraph.(b) (2) of this sec-
tion may not exceed 3 EPNdB.
Section C36.7 Takeoff test conditions, (a)
This section applies to all takeoffs conducted
In showing compliance with this part.
(b) Takeoff power or thrust must be xised
from the start of the takeoff to the point
at which an altitude of at least 1.000 feet
above the runway .Is reached, except that,
for airplanes powered by more than three tur-
bojet engines, this altitude must not be less
tnan 700 feel.
(c) Upon reaching the altitude specified
In paragraph {b) of this section, the power
or thrust may not be reduced below that
power or thrust that will provide level flight
with one engine Inoperative, or below that
power or thrust that will maintain a climb
gradient of at least 4 percent, whichever
power or thrust Is greater.
(d) A speed of at least V2 + 10 knots must
be attained as soon as practicable after lift-
off, and must be maintained throughout the
takeoff noise test.
(e) A constant takeoff configuration, se-
lected by the applicant, must be maintained
throughout the takeoff noise test.
Section C36.9 Approach test conditions.
(a) 'nils section applies to all approaches
conducted In showing compliance with this
part.
(b) The airplane's configuration must be
that specified by the applicant.
(c) The approaches must be conducted
with a steady glide angle of 3' ±0.5' and
must be continued to a normal touchdown
with no alrframe configuration change.
(d) A steady approach speed of not less
than 1.30 V. + 10 knots must be established
and maintained over the approach measuring
point.
(e) All engines must be operating at ap-
proximately the same power or thrust, and
must be operating at not less than the power
or thrust required for the maximum allow-
able nap setting.
[P.R. Doc. 69-13368; Filed, Nov. 17, 1969;
8:08 a.m.]
(As published in_the Federal Register
/34F.R. 18355/ on Nov. 18, 1969)
- 27 -
(Pbge 28 is blank)
A-29
-------
Title 14—AERONAUTICS AND
SPACE
Chapter I—Federal Aviafion Adminis-
tration, Department of Transportation
[Docket No. 9993; Amdt. 38-1J
PART 36—NOISE STANDARDS:
AIRCRAFT TYPE CERTIFICATION
Approach Noise Test Conditions
This amendment changes 'the type
certification approach noise test condi-
tions for subsonic transport category air-
planes and for subsonic turbojet powered
airplanes regardless of category. The pur-
pose of this amendment is to insure that
the approach noise type certification test
(1) Is conducted with the same airplane
configuration as that used during air-
worthiness type certification; and (2)
does not result in noise levels less than
those that will be generated by the
airplane in normal operation.
Parfc-36, Noise standards: Aircraft type
certification was issued by the Adminis-
trator on November 3, 1969, and will be
effective on December 1, 1969. Section
C36.9 of Appendix C of that part contains
the test conditions applicable to all
approaches conducted in showing com-
pliance with Part 36. That section
contains two provisions that require
amendment when Part 36 becomes
effective.
First, paragraph (b) of section C36.9
currently provides that the airplane's
configuration must be "that specified by
the applicant." It now appears that this
language could be regarded as permitting"
the applicant to specify configurations
that are not the same as those used in
showing compliance with the landing
requirements in the airworthiness regu-
lations. This result is not intended. While
the general requirement of compatibility
between noise and airworthiness type
certification test conditions and proce-
dures includes approach noise test condi-
tions and procedures, it is believed
advisable to remove any question that
may be caused by section C36.9(b).'
Therefore, that paragraph is amended to
specifically provide, in part, that the air-
plane's configuration during the ap-
proach noise test must be "that used in
showing compliance with the landing
requirements in the airworthiness regu-
lations constituting the type certification
basis of the airplane."
Second, paragraph (e) of section C36.9
currently provides that the aooroach
noise test must be conducted with engines
operating at not less than the "power or
thrust required for the maximum allow-
able flap setting." The intent of this
provision is to ensure that the noise gen-
erated during the approach noise type
certification test will not be less than
that later generated by the airplane in
normal operation. However, configura-
tion aspects other than flaps may affect
the noise of the airplane. In addition,
there is no need to specify a particular-
power or thrust once a specified con-
figuration is identified since section
C36.9 also specifies the glide angle and
minimum approach speed, requires that
both be "steady," and requires that tne
approach be continued to a normal
touchdown with no configuration change.
In the light of the above, it is believed
that the objective of ensuring that ap-
proaches made later in normal opera-
tion will not be noisier than the published
noise levels of the airplane can be more
effectively achieved by providing, in sec-
tion C36.9(b), that "if more than one
configuration is used in showing com-
pliance with the landing requirements in
the airworthiness regulations constitut-
ing the type certification basis of the air-
plane, the configuration that is most
critical from a noise standpoint must be
used" in showing compliance with the
approach noise requirements of Part 36.
This amendment is necessary to ensure
that the approach noise levels generated
by the airplane during type certification
will be representative of approach noise
levels generated in normal operations.
This amendment is issued in full con-
sideration of comments received with
respect to Notice 69-1, issued on Janu-
ary 3, 1969 (34 F.R. 453), including con-
sideration of economic data submitted
by affected aircraft manufacturers and
operators, and has been determined to be
economically reasonable, technologically
practicable, and appropriate to the air-
craft to which it applies.
Pursuant to section 611 of the Federal
Aviation Act of 1958 (49 U.S.C. 1431) the
Administrator has consulted with the
Secretary of Transportation concerning
the matters contained herein, prior to the
adoption of this amendment.
Like Part 36, which becomes effective
on December 1. 1969, this amendment to
that part applies to airplanes now near-
ing the completion of the type certifica-
tion process. Therefore, it is essential that
this amendment become effective on the
same date as Part 36. Therefore, I hereby
find that notice and public procedure, in
addition to that already provided by
Notice 69-1, is impracticable. In addition,
I find, for the reasons stated above, that
good cause exists for making this amend-
ment effective on less than 30 days notice
after publication thereof in the FEDERAL
REGISTER.
In consideration of the foregoing, sec-
tion C36.9 of Appendix C of Part 36 of
the Federal Aviation Regulations which
becomes effective on December 1, 1969,
is amended, effective on that date, to read
as follows:
Section C30.9 Approach, teat conditions.
(a) This section applies to all approaches
conducted In showing compliance with this
part.
(b) The airplane's configuration must be
that used In showing compliance with the
landing requirements In the airworthiness
regulations constituting the type certifica-
tion basis of the airplane. If more than one
configuration Is used In showing compliance
with the landing requirements In the air-
worthiness regulations constituting the type
certification basis of the airplane, the con-
figuration that Is most critical from a noise
standpoint must be used.
(c) The approaches must be conducted.
with a steady glide angle or 3 *±0.5* and
must be continued to a normal touchdown
with no airframe configuration change.
(d) A steady approach speed of not less
than 1.30V,-)10 knots must be established
and maintained over the approach measur-
ing point.
(e) All engines must be operating at ap-
proximately the same power or thrust.
(Sees. 313(a), 601, 603, 611, Federal Aviation
Act of 1958; 49 U.S.C. 1354, 1421, 1423, 1431;
sec. 6(c), Department of Transportation Act,
49 U.S.C. 1655(c))
Issued in Washington, D.C., on Novem-
ber 21. 1969.
J. H. SHAFFER,
Administrator.
IP.B. Doc. 69-14010; Piled, Nov. 21, 1969;
11:63 a.m.)
(As published in the Federal Register
/34F.R. 1881^7 on Nov. 25, 1969)
- 29 -
(Page 30 Is blank)
A-30
-------
Title 14—AERONAUTICS AND
SPACE
Chapter I—.Federal Aviation AdTHni*-
fralion, Department of Transportation
' [Docket No. 9337}
PART 36—NOISE STANDARDS:
AIRCRAFT TYPE CERTIFICATION
Corrections
The following corrections are hereby
made to the preamble and regulatory
material of.new Part 36—Noise Stand-
ards: Aircraft Type Certification, which
was published in the FEDERAL REGISTER
on Tuesday, November 18, 1969 (34 F.R.
18355-18379):
(1) On page 18360 of the preamble,
the word "noise" was Inadvertently
omitted from the statement, in the
right-hand column, second paragraph,
that 58 21.93(b) and 3C.KO will insure
that noise reduction technology suffi-
cient to achieve Appendix C limits must
be applied "before further aircraft
growth can occur." The quoted words
are hereby corrected to read "before
further aircraft noise growth can occur."
(2) On page 18364, paragraph (a) of
§36.2 contains a typographical error In
In 571.181 (34 P.R. 4637), the New
Bern. N.C., transition area is amended
hereby corrected to read: "5 36.201 (b)
and (cHl)."
(3) On page 18379, paragraph (e) of
8 C36.7 is not correct as it now stands,
and this paragraph is hereby corrected
to read as follows:
Section C36.7 Takeoff test conditions. • • •
(e) A constant takeoff configuration, se-
lected by the applicant, must be maintained
throughout the takeoff noise test, except
that the landing gear may be retracted.
Issued in Washington, D.C., on Novem-
ber 24,1969.
J. H. SHAFFER,
Adminstrator.
(FJEt. Doc. 09-14169: Filed, NOT. 28, 1969:
8:45 a.m.)
(As published in the Federal Register
/34 F.R. 1902J7 on Nov. 29, 1969)
- 31 -
A-31
-------
APPENDIX B
METHODOLOGY FOR IMPACT ANALYSIS
This appendix summarizes the various analytical models and supporting data used in
Chapter 4 for evaluating impact of noise from transportation vehicles and from internal
combustion engine devices. Emphasis is placed on the former category as the primary
source of noise impact in most communities today. Specifically, this appendix summa-
rizes each of the following approaches used for evaluating noise impact.
• total noise energy
• residual noise levels
• single event noise levels for major transportation noise sources as a
function of distance
• noise impacted land areas around freeways and airports
• noise impact on operators or passengers of transportation vehicles
and internal combustion engine devices.
In addition, a brief glossary of key terminology is presented at the end of
this appendix.
B. 1 Total Noise Energy
The total A-weighted noise energy produced on an average day by each
noise source category was estimated in order to provide one simple way of ranking the
potential noise impact of each category. Categories with higher noise levels which
exist in greater numbers and are used more hours per day will tend to rank highest in
terms of their noise energy. The noise energy for a given category, such as standard
passenger automobiles, was estimated by the following expression:
E = 10~3 N • T • W , kilowatt-hours/day (1)
where . . . . ,
N = total number of units
T = Average hours per day usage
W = approximate A-weighted noise power, watts
B-l
-------
and
W
10 log ^r - LA + 20 log R + 7.5 dB re 10 watts
1Q-13 A
where
L-A = typical A-weighted noise level in dB(A) at a reference distance
R (in feet).
o
The four input parameters required for this calculation (N,-T, L. and R )
^\ O
are summarized in Table B-l for all of the categories considered under transportation
vehicles and internal combustion engine devices. The values for number of units and
usage shown are based on estimated figures for 1970 compiled from available statistical
data. Where up-to-date figures were not available for 1970, linear extrapolations
were made based on available data, or where necessary, engineering estimates made of
probable values.
For ground transportation vehicles, the "typical A-weighted noise levels"
correspond to average values at a 50-foot distance for the type of vehicle under normal
operating conditions at typical speeds. For aircraft, the noise levels correspond to
values at a slant distance to the aircraft of 1000 feet and hours of usage were based on
estimates of the duration of landing and takeoff operations in the vicinity of airports.
Estimates of noise levels were based on the noise level data for all categories cited
earlier in Chapters 2 and 3.
For projections of noise energy to the year 2000, extrapolations in usage
were made on the basis of historical trends. For example, Figure B-l(a) illustrates the
past trends in passenger-miles of urban travel by various transportation vehicles. These
figures have been obtained from published data — or estimated from information on
I _ f o
vehicle-miles and average passenger loading. ' They clearly show the marked
increase in travel by the average citizen — primarily by increase in personal travel in
the passenger automobile.
This general increase in mobility is summarized in Figure B-l(b) which
shows the total urban passenger miles per urban population for all the transportation
B-2
-------
Table B-l
Parameters Used to Define Noise Energy
for Each Category in 1970
Category
AIRCRAFT (Takeoff Only)
4- Engine Turbofan
i
2- and 3-Engine Turbofan
General Aviation
Helicopters
HIGHWAY VEHICLES
i
Medium and Heavy Duty Trucks
Sports, Compact and Import
Cars -;,
Passenger Cars (Standard)
Light Trucks and Pickups
Motorcycles (Highway)
City and School Buses
Highway Buses .
RECREATIONAL VEHICLES
Minicycles, Off-Road
Motorcycles
Snowmobiles
Outboard Motorboats
Inboard AAotorboats
N 1
Number
894
1174
128,900
16
3.64M2
23 M
64 M
15.3 M
2.6M
0.38 M
.02 M
1 M
1.6M
5.2M
.65M
T
Average Use
Hours/Day
0.23
O.I3
0.0173
6
4
1
1
1.5
0.5
2
4
1
0.2
. .05
.5
LA
Noise Level
dB(A)
103
96
77
83
84
75
69
72
82
73
83
88
85
75
80
R
;o
Distance
ft
1000
1000
50
-
50
50
50
B-3
-------
Table B-l (Continued)
Category
RAIL VEHICLES
Locomotives
Freight Trains
High Speed Intercity Trains
Existing Rapid Transit Trains
Passenger Trains
Trolley Cars, (Old)
Trolley Cars (New)
INTERNAL COMBUSTION
ENGINE DEVICES
Lawn Mowers
Garden Tractors
Chain Saws
Snow Blowers
Lawn Edgers
Model Aircraft
Leaf Blowers
Generators
Tillers
N
Number
27,100
10,000
2800
21,000
185
300
1200
17M
5M
2.5 M
0.8 M
3.3 M
1 M
0.5 M
0.55M
3.5M
T
Average Use
Hours/Day
12
5
6
0.5
12
12
12
0.1
.15
.05
.1
.05
.05
.1
.1
.01
LA
Noise Level
dB(A)
94
85
85
87
83
'80 ! .
66
.
74
78
83
85
78
78
76
70
69
RO
Distance
Feet
50
50
50
Compiled from Ref. 1-4
M = millions
Estimated hours per day while operating on and near airports and noise level is greater
than 80 dB(A).
B-4
-------
1000
o
CO
I
t/i
0)
ID
D)
£
100
10
, I
_ a) Passenger Miles
Automobile
City'Bus and Trolley
Trucks (Estimated
, Personal Use)
X
Surface Rail and Subway
Motorcycles
(Estimate)
1930 1940 1950 1960
Year
1970
1980
a) c :
ro o ^
o
v. O
b) Total Urban Passenger Miles Per Year
Normalized by Urban Population
1930
I
1940
1950 1960
Year
1970
Figure B-rl. Growth in Urban Travel
(Compiled or Estimated from Data in References 1-6, 9)
B-5
-------
modes shown In Figure B-l(a). Figures B-2(a) and B-2(b) show the same information
for intercity travel. This upward trend In passenger travel per capita is due primarily
to the increase in numbers of vehicles per capita and not miles traveled per year by
each vehicle. The past trend in these two statistics is summarized for highway vehicles
in Table B-2 which shows that the mileage per vehicle has not increased markedly in
the last 20 years, while numbers of automobiles and trucks per capita has increased
substantially.
Projections of the number of vehicles to the year 2000 was therefore
made by extrapolation of the trend in number of vehicles per person from Table B-2
taking into account the decrease in rate of growth so that the rate approached the
population growth rate by the year 2000. The population growth to the year 2000
was based on the most conservative projection (Series D) made by the Bureau of the
Census in 1968, which is in general agreement with 1970 census figures.
Similar projections were made for the change in numbers of internal
combustion engine devices to the year 2000. Results of these projections for several
of the categories are shown in Figure B-3. It was assumed that the average number
of hours of usage per day of each of the categories will not change significantly.
Changes in typical noise levels to the year 2000 were made on the basis of the three
future noise reduction options, discussed in Chapter 4, which were then applied to
the base-line noise levels for 1970.
While the resulting estimates of noise energy (see Tables 4-3 and 4-4
in Chapter 4) are subject to appreciable error, they are considered sufficiently
reliable for the purpose of rank-ordering the general magnitude of noise generated
by each category.
B.2 Residual Noise Levels
The residual noise level in any area is generated by all forms of traffic
moving in and around the community, and by the large number and variety of dis-
persed stationary sources. The magnitude of the residual noise level in a given
community has been shown to vary only slowly if at all in a community with stable
B-6
-------
1000
100
I
I
I/I
0)
10
- a) Passenger Miles
Automobile
Rail
—VHi
Highway Bus
Private ..
Flying ~N .V'
Airline
-Trucks
/ x"**" /' (Estimated Personal Use)
/ •/. /
^--^/' V I I
1930 1940 1950 1960
Year
1970
1980
H
"5.
0) O
?8
a> r~,
85 "
3.
b) Total Intercity Passenger Miles Per
Year Normalized by Total Population
I
I
I
1930 1940 1950 1960
Year
1970
1980
Figure B-2. Growth in Domestic Intercity Travel
(Compiled or Estimated from Data in References 1-6, 9)
B-7
-------
Table 8-2
Trends in Highway Vehicles per 1000 persons and Mileage per Year
1
Vehicle
Passenger Cars
Light Trucks and Pickups
Medium and Heavy
Duty Trucks
City Buses
Highway Buses
Vehicles per 1000 Persons Mileage per Year
1950
'268
46
10.6
0.38
0.097
1960
341
53
12.7
0.27
0.070
1970
426
75
17.8
0.24
0.075
1950
9078
10,7762
53,833
20,910
65,41 13
1960
9474
10,5802
59; 590
'"-.'. '••»
• -t- • .
16,004
65,567s
1968
9507
11,5702
68,303
14,122
58,4233
1
Compiled from Reference 1
Average Mileage for all types of trucks which are dominated by light trucks.
Average mileage for intercity motor carriers.
B-8
-------
400
*Number of units x 10
1950
1960
1970
1980
1990
2000
Figure B-3. Growth Trends for Population and Numbers
of Several Major Noise Sources Considered
for the Noise Impact Analysis. (Compiled and
Projected from data in References I/ 2, and 6)
B-9
-------
land-use patterns. It has also been shown that this residual noise level is a key
foundation for evaluation of a community's reaction to intruding noise.
An available model for community noise has therefore been modified to
12
provide estimates of this residual noise level. As illustrated conceptual/ in
Figure B-4, the model assumes that discrete sources of noise in a community can be
replaced by a distribution of noise sources with a uniform density n throughout the
community. The model provides an estimate of the quasi-steady state residual noise
level (L90) in terms of four basic parameters:
. • The reference A-weighted noise level for each source, LA,
at a reference distance.
• The reference distance R .
o
• The excess attenuation of sound over and above that due to
spherical spreading of the sound, and
• The density n of the distributed sources in number of sources
per unit area.
The relationship between the residual noise level predicted by this model
and the reference noise level for each contributing source (assumed constant) can be
defined as follows: For the distribution of discrete sources shown on the left sede of
Figure B-4, the effective boundary of influence for one source is defined by a circle
with an area equal to the area of one of the 6-sided cells bounding each such sourcec
The radius R of this equivalent circle can be shown to be equal to l/V^n where
n is the number of sources per unit area. The noise from the local source within this
zone is considered identifiable as a local intruding noise and is not included as part
of the residual noise. The latter is made up, then, of the summation of noise from all
the other sources outside this local zone so that the residual noise level, expressed
B-10
-------
Uniformly Distributed
Sources of Community
Noise
Equivalent Continuous
Distribution of Sources
Excluding Locally Intruding
Source Density n
(Units per square mile)
B-4. Model for Residual Noise Level (Excluding Local Source)
B-11
-------
2 12
in terms of the mean square pressure P., , is:
where
P = mean square reference pressure of each source at the
reference distance R
o
, R. = distance from observer at the center of the local zone
' , *u -th
to the i source .
m = the excess attenuation loss coefficient per unit distance
s. = local shielding loss between observer and i source.
By replacing the distribution of discrete sources with a continuous distri-
bution and integrating from the outer radius R of the "local zone" out to infinity to
sum up the contribution of all but the local source to the residual noise level, one
can express this level (LOQ) in decibel form as
L90 = LA + 10 log LE/ MJ + 10 log n + 20 log -o " S " 66>5/
dB re: 20 fjN/m2 (3)
where . ,
L» = Reference A-weighted noise level of each source at the
R in feet
o
CO
s*
c /v\ — / -mR
E' (x) - / e dR
13
the exponential integral of the first kind of argument X
B-12
-------
= 0.686 a
a = attenuation loss coefficient in dB/1000 feet
n = density of sources per square mile
S = average shielding loss between observer and
surrounding noise sources, dB.
The relationship predicted by this expression for the residual noise level
relative to the reference noise level L, of a source at a distance of 50 feet is shown
in Figure B-5 for a range of values of the excess attenuation coefficient and zero
shielding loss. A typical minimum value of excess attenuation rate, due to air
14 15
absorption only, for ground transportation sources is about 1 - 2 dB per 1000 feet. '
These are approximate values for the effective attenuation.rate when applied to the
overall A-weighted noise level and are based on recently revised models for air
absorption. ' ' These are considered more accurate for predicting losses at low
19 20
frequencies than earlier prediction methods. ' The additional shielding loss
due to diffraction or reflection by buildings between the sources and the observer
21 22
has been found to be about 6 dB. ' Substantially higher values of shielding loss
(10 - 15 dB) have been reported from horizontal propagation tests of warning sirens
over communities; however, these higher values do not appear to be entirely appli-
23
cable for predicting shielding loss of traffic noise.
The source density n is estimated by the product:
n = P . r • F • T/24
Where P is the population density, r is the number of sources per person, F is the
fractional usage in the type of community being considered, and T is the number of
operating hours per 24-hour day. The primary objective in applying this model is to
illustrate the approximate contribution to the residual noise level by transportation
sources. Average values for these parameters were chosen, therefore, to represent
the source density and usage in a typical urban community. On this basis, the
average urban population density for 1970 was assumed to be 5000 persons per
B-13
-------
+10
CO
O
10
JJ
a>
u
§
oo
-10 -
-3 -20 -
a
tn
-------
square mile. While there has been a progressive decrease in the average population
density of urbanized areas over the last 20 to 30 years due to urban sprawl, the rate
of this decrease is slowing down and is being counteracted by the growth of apartment
1 24
dwellings in close-in areas. ' Thus, for purposes of projection of noise impact in
the future, it was considered reasonable to assume that the average urban population
density remained constant. The number of sources per person was assumed equal to the
total number operating in the nation, divided by the total population (see Table B-2
and Figure B-3). Only sources operating on roads and highways were considered for
estimating ambient levels. Normally, the other transportation sources do not contribute
significantly to the urban residual noise environments. Estimates of the fractional usage
in an urban community and operating time for each source were made on the basis of
available information on urban highway usage. The resulting estimates of the usage
and density of operating sources per square mile for the years 1970, 1985 and 2000
are summarized in Table B-3. Note that the projected increase in source density from
1985 to the year 2000 is slight due to the assumed trend of number of sources per capita
approaching a constant by the year 2000.
The estimated trends in the daytime residual noise level in a typical urban
residential area, based on this model, have been shown in Figure 4-1 in Chapter 4.
For 1970 conditions, the three most significant contributing sources for this residual
noise level are:
• Passenger Cars (All Types) 45 dB(A)
• Light Trucks and Pickups 42 dB(A)
• Heavy and Medium Trucks 33 dB(A)
Total 47 dB(A)
During nighttime the contribution by passenger cars and light trucks will
decrease substantially, but the contribution by heavy trucks tends to remain nearly
constant. This is illustrated by Figure B-6 which shows the hourly and daily traffic
25
flow rates on intercity highways in California. Since this intercity travel normally
involves travel on urban freeways, the contribution by trucks to the residual noise'
B-15
-------
Table B-3
Summary of Estimates of Density of Operating Highway Vehicles
in Urban Residential Areas from 1970-2000
Source
Passenger Cars
(Standard)
Sports, Compact
and Import Cars
Light Trucks and
Pick-ups
Medium and Heavy
Duty Trucks
Motorcycles
(Highway)
City Buses
Fractional
Use in
Urban1
Areas
80
80
60
10
80
100
Operating
Time
Hours
Per Day
1
1
1.5
4
1
2
Operating Source Density2
Units/Square Mile
1970
50
20
20
1.5
1
0.8
1985
62
26
23
1.8
2.3
0.7
2000
65
30
25
2.0
2.5
0.6
Use in urban residential communities.
Assuming constant population density at 5000 people/square mile.
B-16
-------
1.000
12
10 12 2
Hour of the Day
12
14,000
13,000
0)
TO
g 12,000
0)
11,000
^^
• ^
" 10,000
s
§ 9,000
O 8,000
*.
O
^ 7,000
.i? 6,000
14-
*~ 5,000
•g "'t100
-^ 3,000
2,000
1,000
0
\
\
^
A
^
^^
All
A
$
All
^-*
Veh
—
X
Veh
Tru
-^
Tru
cle:
//
//
//
//
//
//
/
i
cle:
cks
^
:ks
67
57
>7
i7
S'
\
^
\
\
^
/
SMTWTFSSM
Dny of the Week
Figure B-6. Hourly and Daily Variations in Intercity Highway Traffic on
Major California Intercity Highways. Expressed in Terms of Vehicles
Per 10,000 Annual Average Daily Traffic.
Data Shown for 1957 and 1967. (From Reference 25)
B-17
-------
level does not vary as much during a 24-hour period as the contribution from automobiles.
The net result is an estimated 5 to 10 dB(A) decrease at night in the residual noise level.
However, the type of truck which tends to dominate the nighttime residual noise level
is the heavy duty transport truck — particularly the 5-axle type. This is illustrated in
Figure B-7 by the hourly variation in percent distribution of truck types by number of.
25
axles observed on major California highways. Heavy-duty 5-axle trucks clearly domi-
nate intercity truck traffic during the nighttime. The same pattern can be expected for
truck traffic mix on the major urban freeways.
During the daytime, the hourly mix of urban vehicle traffic will tend to
vary during the day as indicated in Figure B-8. This is a composite estimate of the
urban traffic mix based on known statistics on vehicle miles in urban areas of automo-
biles and trucks, and on detailed samples of hourly mix of these vehicles in typical
2.9
urban areas.
The detailed mix for truck traffic during the daytime in urban areas can be
estimated from the data in Tables B-4, B-5, and B-6. The first table shows the per-
centage distribution by truck size for three ranges of trip lengths ranging from local
(urban or farm) to long haul (greater than 200 miles). Table B-5 shows the distribution
by type of trip for the same range of truck sizes, while Table B-6 indicates the distri-
bution of truck trips in urban areas according to the type of land use at the starting
and termination points.
The variation in population density and vehicles per capita will obviously
vary from city to city from the typical values used here for estimating the residual
noise level. Figure B-9 illustrates the general distribution of central city population
1 9
density according to 1960 census data for the 128 largest cities in the United States. '
This shows two general trends in population density. It is generally higher for cities
with a higher total population and, as indicated by the four lines characterizing
regional areas, is higher for older regions. This is simply reflecting the fact that the
population of a geographically fixed urban land area tends to increase gradually with
time. Automobile and truck ownership, on the other hand, tends to decrease with
B-18
-------
_
u
O>
u
£
o
-------
o
w-
E
_X
5
I
M-
o
c
0)
o
o
10
9
8
7
6
5
4
3
2
1
0
12
Total
8^10 12
8 10 12
A.M.
P.M.
Figure B-8. Typical Hourly Distribution of Total. Daily Urban Vehicle Traffic
(Based on data from References 2 and 9)
B-20
-------
Table B-4
Distribution of Annual Truck Vehicle-Miles According to
Truck Size and Type of Trip (From Reference 9)
Type of Trip
Local (Urban or Farm)
Intermediate (<200 miles)
Long Haul £>200 miles)
1
Total -All Trips
Percent by Truck Type
(Gross Weight)
<1 0,000
Pounds
66.8
27.8
5.9
i
54.5
10-26,000
Pounds
2i.2
20.8
4.3
18.4
> 26, 000
Pounds
7.0
42.7
79.0
20.9
Miscellaneous
5.0
8.7
10.8
6.2
Total
100$
100$
100$
100$
Table B-5
Distribution of Type of Truck Travel According to Truck Size
(From Reference 9) ,
Type of Trip
Local
Intermediate
Long Haul
Total
Truck Size (Gross Weight)
<1 0,000
Pounds
87.7
11.1
1.2
100$
10-26,000
Pounds
75.1
22.6
2.3
100$
> 26, 000
Pounds
21.4
40.0
39.0
100$
Total
All Trucks
68
21
11
100$
B-21
-------
Table B-6
Distribution of Truck Trips by Urban Land Use at Start and End of Trip
(From Reference 9)
^^^^ To
From ^^"^^^^
Residential
Non-Residential
Total
Residential
24.1
18.0
42.1
Non-Residential
16.8
41.1
57.9
Total
40.9
59.1
100$
population or population density as indicated in Figures B-10 and B-ll respectively.
This is a reflection of the greater use of urban mass transit in crowded older cities.
The net effect on predictions of residual noise level in urban areas will be a trend to
make the density of highway vehicle sources more nearly constant and roughly inde-
pendent of city size.
Finally, as an indication of the sensitivity of the residual noise level to
changes in the input parameters, the effect of changes in the estimated density of the
operating sources is illustrated by the following alternate cases:
Change in
Residual Noise Level
dB(A)
Increase density of heavy trucks by factor of 4
Increase passenger car density by factor of 2
Increase density of all sources by factor of 2
Increase passenger car density by factor of 4
+2
+4
+5
+8
B-22
-------
28
S
24
20
16
12
S! 4
I I I I I I I 11 I I
i i i i i i i i i
i i i i T'l-V-i"" i i i i i 111 i
i i I I i i l i i I
IOO.OO9 I MILLION
POPULATION OF URBANIZED AREA
Figure B-9. Central City Population Density as Related to
Urbanized Area Population
(From Reference 1, 9)
100
80
60
40
20
TWOORMORE CARS
ONE CAR'
- NO CARS
I960 POPULATION DENSITY
' PERSONS PER SO. MILE (IN THOUSANDS)
12 16
J
Figure B-10. Effect of Central City Population Density
on Automobile Availability
(From Reference 9)
B-23
-------
140
120
£ 100
o
S2
o
o
o
80
a 60
40
20
0
-1 | —T
• Western Cifies
© Eastern Cifies
Q_©
I
I
I
20 50 100 2 5 1000 2
Area Population (Thousands)
5 10,000
Figure B-ll. Registered Trucks/Capita, 1963-65
(From Reference 9)
B-24
-------
B.3 Single Event Noise Levels-for Major Transportation Noise Sources as a
Function of Distance
The evaluation of relative annoyance of single events, presented in
Section 4.3, required prediction of single event noise levels as a function of distance
from the source. This was carried for both measures of single event levels utilized as
follows:
Maximum A-Weighted Noise Level
The reference octave band spectrum for each source of a fixed distance
(50 feet for surface vehicles — 1000 fe t for aircraft) was used as a baseline for pre-
dicting the decrease in octave band levels at greater distances using atmospheric
i
absorption loss coefficients and ground absorption loss values from References 14 and 15.
The attenuated levels at octave band were then recombined after applying the
A-weighting convection to the spectrum to define the new A-weighted noise levels.
Typical results of this process are illustrated in Figure B-120
Single Event Noise Exposure Level (SENEL)
The SENEL for a single event can be expressed as the sum of its maximum
noise level and an effective duration correction factor. The effective duration of noise
for moving sources is a function of the distance from the source (R) and its velocity (V).
For surface vehicles such as automo biles and trucks, the SENEL can be roughly approxi-
mated as follows:
' SENEL « LA(R) + 10 logj"| ^- , dB re 20 MN/m2 and 1 second
(4)
See glossary of terms at end of this Appendix for definition.
B-25
-------
100
90
^E 80
o
CN
£
co
o
o
0)
at
70
60
M^
^ 50
40
30
20
2-Engine Turbofan
(Takeoff)
Passenger
Car
50 100 200 500 1000
Distance in Feet
2000
5000 10,000
Figure B-12. Variation in Typical Noise Levels vs Distance
For Several Transportation System Categories
B-26
-------
where
L.(R) = A-weighted noise level at the distance R
R = Distance in feet
V = Speed of the vehicle in ft/sec
For trains, the duration of the passby noise is essentially equal to the
passby duration (train length/speed) within a distance R equal to L/ir where L is
the train length. Thus, within this range, the second term in Equation (4) is replaced
with 10 log L/V . At greater distances, Equation (4) is used since the long train
(line) source begins to act like a point source at these distances.
For aircraft, SEN EL values were predicted in the same manner used
for predicting Effective Perceived Noise Levels (EPNL) as a function of slant
distance io me aircraft. 0/27.28 TUS jotter time-Integrated measures of single-
event noise are used for evaluating noise impact near airports due to aircraft
operations.
B-27
-------
B.4 Noise Impacted Land Areas Around Freeways and Airports.
As indicated above, the methodology for evaluating noise impact near air-
ports is well developed and fully documented by examples such as in Reference 29. The
index used for evaluating the noise impact is the noise exposure forecast (NEF) which is
a measure of the composite time-integrated noise exposure on the ground due to aircraft
operations. The evaluation of noise impacted land area for the total transportation
system dictated the need to apply a similar methodology to highways. The index of
noise impact utilized in this case is the community noise equivalent level (CNEL).
This composite measure of noise, defined in the Glossary at the end of this Appendix,
utilizes A-weighted noise levels as a basic measure of noise magnitude.
As for the NEF scale, a CNEL value accounts for the time-integrated
single-event noise level (expressed by an SENEL), the number of single events in a
24-hour day and, by weighting factors, the time of day in which these single events
occur. These weightings approximate the increased sensitivity of a community to
27
intrusive noise during the evening and nighttime periods. This composite scale can
be used in the same way as the NEF scale to predict reaction of a community to an
accumulation of intrusive noises. The CNEL value at a given point.can be approxi-
mated by:
CNEL » SENEL + 10 log Np - 49.4dB (5)
where
SENEL = average SENEL for each single event
N_ = weighted number of single events equal to Nn + 3 N_ + 10 N
ND/ N_, N^. = number of single events during the daytime (7:00 a.m. -
7:00 p.m.), evening (7:00 p.m. - 10:00 p.m.), and nighttime
(10:00 p.m. -7:00 a.m.), respectively.
For analysis of the CNEL near freeways, an average SENEL is selected for
each type of vehicle, using Equation (4), along with corresponding figures for the
number of vehicles passing by during each of the three time periods. The total CNEL
for this traffic mix is the logarithmic (or energy) summation of the CNEL values for
B-28
-------
each type of vehicle. Typical SEN EL values for each type of highway vehicle at a
reference distance of 50 feet have been specified in Table 4-6 of Chapter 4. The
SENEL at other distances was computed in the manner explained in Section B.3.
Close to a freeway, the propagation loss for the maximum noise level
decreases according to the inverse square law of distance R from the source (i.e.,
2
- 1/R ). However, the time-integrated measure of the single event (SENEL) includes
a correction for duration which increases directly as the distance R. The net result is
that the SENEL decreases according to a first power law with distance from the
vehicle.
This is exactly equivalent to other analytical models for predicting noise
near highways, which show that for high traffic volumes (roughly greater than 1000
vehicles per hour), where the traffic noise can be treated as a line source, the average
noise level near the freeway decreases according to the first power of the distance from
30 31
the traffic lane. ' The average A-weighted noise levels (L,-r\) predicted by these
latter models, for a wide range of traffic volumes and average vehicle speeds, are shown
in Figure B=13. In this figure, the change in slope of the curves with traffic flow rate
is due to the change in character of the noise as traffic volume increases. For low flow
rates, each vehicle is heard as an isolated single event as it passes by. For high flow
rates, the stream of traffic is heard as a nearly continuous quasi-steady state noise
with only minor fluctuations due to the particular traffic mix at any instant.
For evaluation of noise impact on all types of urban roads, the following
additional parameters were required beyond those already described:
• Mileage on each type of road
• Typical vehicle speed by road type
• Typical traffic flow rates
• Typical road right-of-way
These parameters are defined in Table B-7 for 1970 road conditions.
B-29
-------
co
I
CO
o
80
70
CN
Z
a.
o
(N
£
CQ
-------
Table B-7
Highway Mileage and Estimated Average Highway
Speeds for Urban Areas in 1970
Freeway
Major Arterial
Minor Arterial
Collector
Local
Notes
Mileage
Miles
9, 160
38,535
46,991
43,970
351,300
2
Typical
Speed
mph
55
40
40
30
25
3
Typical
Flow Rates
Vehicles/Hr
1,980
735
365
157
43
Typical
Right-of-Way
(No. of Lanes)
Feet
200
175
160
150
125
(8)
(6)
(5)
(4)
(2)
1 For urban areas in 1968 from Reference 32.
2 Estimated based on typical free traffic flow.
3 Computed average (see text).
4 Estimated effective value based on 12 feet per lane and 50-foot setback to
nearest residence from edge of roadway.
B-31
-------
Effect of Vehicle Speed
Varying the average vehicle speed has two effects. The average maximum
noise level for most highway vehicles increases approximately according to the cube
o i o o *\A
of the vehicle speed. ' ' Although noise levels of heavy (diesel) trucks increases
less with speed, due to their small contribution to the total highway noise impact,
the cube rule for vehicle speed was assumed for all vehicles.
The second influence of vehicle speed is that it changes duration of a
single event and hence the SENEL, as indicated by Equation (4). The net result is
that the average SEN EL for each type of vehicle varies by the square of the vehicle
speed, as indicated by the following correction factor:
AS = 20 log -, dB (6)
Forty (40) mph is used as the nominal reference speed corresponding to the reference
SENEL at 50 feet for each type of vehicle.
Effect of Traffic Flow Rates
The average traffic flow rates Q listed in Table B-7 for each road type
were based on an average value computed by:
~ Vehicle - Miles per day ... /,
U = -rrrc - ^ — 75 — jrm — ^r r vehicles/nr
(17 hours) x (Road Mileage)
This provides a highly smoothed average flow rate based on total national figures for
32
traffic volume, road mileage, and an average "traffic day" of 17 hours. (See
Figure B-8.) Actual flow rates on many urban freeways will be substantially greater
than this. However, to counterbalance this unconservative assumption, it was
assumed, when evaluating the noise impacted area near freeways, that the entire
length of the freeway was adjacent to residential land.
The weighted number of single events Np required for computation of
the CNEL was substantially greater than the actual daily total. Using typical hourly
B-32
-------
rates of urban traffic, such as indicated in Figure B-8, and the weighting factors for
time of day indicated for Equation (5), the weighted total number of events (vehicles)
was 2.2 to 3 times the actual daily total. For conservatism, a factor of 3 times the
daily total was used for this analysis.
Additional Factors for Freeway Impact Analysis
An average shielding loss of 3 dB was used to approximately account for
the attenuation effect of barriers at the edge of freeways. In fact, freeway noise can
be changed substantially (up to 10 to 15 dB), depending on the design of barriers and
31 35
elevation of the road. ' The value of 3 dB was considered a reasonable average
for the wide range of freeway design conditions that exist.
No attempt is made to account for the effect of changes in road grade or
conditions of the road surface on traffic noise. Both of these factors can be significant
31
in specific situations..
The effect of varying distances from an observer to each lane of traffic
on a multi-lane highway with two-way traffic was accounted for by a single correction
factor to allow computations of noise impact to be based on an equivalent single-lane
flow of traffic.
The nearest residence to all highways was assumed to be 50 feet from the
edge of the roadway. The width of the roadway itself was assumed to be 12 feet for
each lane, with an average number of lanes varying with road type (as indicated in
Table B-7). Thus, effective right-of-way was equal to the road width plus a 50-foot
set-back on each side.
Noise Impacted Land Area
As discussed in Section 4.4 of the text, a criterion value of CNEL = 65
was selected as the outer boundary of the noise impacted area. The area involved for e
each type of road was equal to:
A = 2 [d-dRl . L/5280, sq. mi. (7)
B-33
-------
where
d = distance in feet from "equivalent single lane" of traffic to
position of CNEL =65 contour
dD = distance in feet from this lane (positioned at center of traffic
K
flow closest to observer) to the edge of the effective right-of-way
L = total mileage for this type of road, miles.
The resulting predictions of noise impacted land area for highways in 1970 has been
presented in Section 4.4 of the main text. For a criterion value of CNEL =65, it
was found that only freeways contributed to the estimated noise impact area. As dis-
cussed in Section 4.4, a lower CNEL criterion would be appropriate in some areas
which have a lower residual noise level. However, since all land adjacent to the
freeways was assumed to be residential, the noise impacted areas estimated are con-
sidered reasonable for ranking the relative contribution of freeways to noise impacted
land of the transportation system.
Future Programs
The growth of freeway mileage has been very rapid over the past 20 years.
However, the growth has slowed down to a current rate of about 5 to 7 percent per
year. The initial rapid growth was responding to the urban expansion in the 1950's
and the need for improved travel facilities. The marked effect of freeway develop-
ment on urban travel efficiency is illustrated in Figure B-14. This shows the change
in the 30-minute radius driving time in Los Angeles for three time periods — 1937,
before freeways were available or urban growth had occurred; 1953, when there were
only 45 miles of freeway in the city; and 1966, when the freeway system had expanded
36 37
to about 340 miles. ' The largest radius (shortest travel time) obviously occurs
for the latter period, while the smallest radius (longest driving time) occurred in 1953
during the beginning phases of urban expansion.
Continued expansion of the freeway system in urban areas is expected to
follow the trends indicated by Figures B-15 and B-16. The first shows the relationship
32
between mileage per capita of various types of urban roads and city population.
B-34
-------
CD
CO
Seventh Street
Broadway
X/C/C/C
Pacific Ocean
Figure B-14. Radius of 30-Minute Driving Time to Downtown Los Angeles
(From Reference 36)
-------
0_
O
eff
8
10
Local
a>
a>
£ 1.0
I
0.1
Average for:
o Major Arterials
o Minor Arterials
o Collector
Freeways
0.05
100,000 250,000 500,000 1 Million 2.5 Million
City Population
Figure B-15. Urban Street Mileage Versus City Population
(From Reference 32)
B-36
-------
0)
0)
III
u-
o
-------
The nearly constant values of highway mileage per capita fora wide range in city
population is clearly evident. Note, however, that the largest number of miles per
capita occurs for all types of roads for the largest cities. Figure B-16 shows that there
is a very high correlation between freeway mileage and passenger vehicle ownership,
with a slight trend toward fewer miles per vehicle for larger cities.
Thus, for projection to the year 2000, it was assumed that freeway miles
would increase in direct proportion to automobile ownership in urban areas. (See
Figure B-3.)
There has been a small progressive increase in average vehicle speeds
38
over main rural highways in the last 30 years, amounting to about 1 percent per year.
24
Engine horsepower has also progressively increased. However, neither of these
effects were considered in forecasting trends in highway noise in the future. There
are, in fact, data to indicate that individual vehicles have become progressively
34
quieter, which would tend to counteract the preceding effects.
Considering that most freeway systems are currently operating near
capacity, flow rates were assumed to remain essentially constant. Based on the pre-
ceding assumptions and methods, predictions of noise impacted land for urban highways
for the years 1985 and 2000 were made for several options of noise reduction for high-
way vehicles. The results have been presented in Table 4-7.
Urban traffic on freeways is predicted to continue to create the only
significant noise impacted land areas. This is due to the inherently high volume of
traffic flow carried on freeways as compared to all other types of urban streets. As
indicated in Figure B-17, even though freeways constitute only about 2 percent of the
road mileage in a typical urban area, they handle 21 percent of the vehicle-miles —
as much as all of the traffic on local streets.
B-38
-------
Collector Streets
Local Streets
71$miles of street
21$ vehicle miles
Minor Arterials
Major Arterials
Freeways
2% miles
21$ vehicle miles
20 30 40 50 60 70 80
Cumulative Percentage of Miles of Urban Streets
Figure B-17. Cumulative Urban Street Mileage Versus Cumulative
Vehicle Miles Served in a Typical Urbanized Community''
B-39
-------
B.5 Noise Impact on Operators or Passengers of Transportation Vehicles and
Infernal Combustion Engines .
The two effects considered in evaluating noise impact on operators or .
passengers were potential hearing damage risk and speech communication. To rank the
various sources in terms of potential hearing damage risk, the actual noise exposure for
a typical operator or passenger was converted to an equivalent 8-hour A-weighted
noise exposure level L(8 hr) with the following expression:
L(8hr) - LA + 16.7 log (T/8), dB(A) (8)
L. = A-weighted noise level at the operator's ear
T = typical exposure time in hours
This is essentially equivalent to the "5 dB per doubling of time" rule that is used in
the Occupational Safety and Health Act for defining allowable noise exposures for
employees. The equivalent 8-hour exposure levels estimated in this way have been
presented in Figure 4-4 for transportation vehicles and in Table 4-9 for internal
combustion engine devices, along with corresponding exposure times.
Impact of transportation systems on speech communication for passengers
39
was evaluated primarily in terms of speech interference effects. Criteria for the
latter are specified in terms of the allowable background noise level as a function of
talker-listener separation distance to prevent interference with significant continuous
speech communication. The criteria allow for the tendency of a talker to raise his
voice level as noise level increases above about 55 dB(A).
Conversely, the same criteria for negligible speech interference can also
be used along with data on normal voice level to estimate the minimum levels desired
in multi-passenger commercial vehicles to provide speech privacy. That is, a lower
bound exists for the desired noise level in such passenger vehicles so that a private
conversation cannot be overheard by adjacent passengers. This minimum internal noise
level can be lowered only by decreasing talker-listener separation, or increasing the
B-40
-------
propagation loss between the talker-listener pair and an observer with a sound barrier.
The criteria for speech privacy is based on maintaining an articulation index (AI) less
40
than 0.05 for the undesired communication path.
Table B-8 summarizes these two criteria for the major passenger vehicles
and is based on the specified typical talker-listener separation distances. In general,
most transportation vehicles meet these two criteria with the exception of multi-
passenger helicopters, which are generally too noisy,and city or highway buses, which
generally have internal levels lower than allowed for speech privacy between adjacent
seats.
B.6 Glossary of Terminology
Sound Pressure Level (SPL)
The sound pressure level, in decibels (dB), of a sound Is 20 times the
logarithm to the base of 10 of the ratio of the pressure of this sound to the reference
pressure. For the purpose of this report, the reference pressure shall be 20 micro-
newtons/square meter (2 x 10 microbar).
Noise Level (NL)
Noise level, in decibels, is an A-weighted sound pressure level as
measured using the slow dynamic characteristic for sound level meters specified in
ANSI SI.4-1971, American National Standard Specification for Sound Level Meters.
The A-weighting characteristic modifies the frequency response of the measuring
instrument to account approximately for the frequency characteristics of the human
-4
ear. The reference pressure is 20 micronewtons/square meter (2 x 10 microbar).
Single Event Noise Exposure Level (SENEL)
The single event noise exposure level, in decibels, is the level of the
time-integrated A-weighted squared sound pressure during a given event based on
reference pressure of 20 micronewtons per square meter and reference duration of
one second.
B-41
-------
Table B-8
Criterion Noise Levels for Speech Interference and Speech Privacy
in Transportation Vehicles in Terms of
A-Weighted Noise Levels
Maximum for No Minimum for
Speech Interference Speech Privacy
Talker-
Listener 9 Observer. Barrier Loss
Vehicle Distance Distance 0 5 dB lOdB
Feet dB(A) Feet dB(A)
Buses, Trains 1-1.7 79-85 2.25-2.7 90-93 84-86 76-79
Commercial Jet Aircraft
-Short Haul 1.1-1.3 82-84 2.7-3.1 89-90 82-84 74-,76
Commercial Jet Aircraft •
- Long Haul 1.2-1.7 79-83 3.0-3.3 88-89 81-89 74-75
For communicating voice (Reference 39).
2
Typical range of side-by-side seat spacing — 5 inches.
3
Typical range of front-to-back seat pitch.
4
Excess loss by barrier or voice directivity.
B-42
-------
Daily Community Noise Equivalent Level (CNEL)
Community noise equivalent level, in decibels, represents the average
daytime noise level during a 24-hour day, adjusted to an equivalent level to account
for the lower tolerance of people to noise during evening and nighttime periods
relative to the daytime period. Community noise equivalent level is calculated from
the hourly noise levels by the following:
CNEL = 10 log
t.,
antllog
antilog + ,0
HNLD
To"
Where
HNLD are the hourly noise levels for the period 0700 - 1900 hours;
HNLE are the hourly noise levels for the period 1900 - 2200 hours;
HNLN are the hourly noise levels for the period 2200 - 0700 hours;
and y means summation.
Hourly Noise Level (HNL)
The hourly noise level, in decibels, is the average (on an energy basis)
noise level during a particular hour. Hourly noise level is determined by sub-
tracting 35.6 decibels (equal to 10 loglo 3600) from the level of the time-integrated
A-weighted squared sound pressure measured during the particular hour.
B-43
-------
Residual Noise Level
The temporal pattern of an A-weighted noise level measurement of com-
munity noise is generally characterized by two features. The first is the variation in
peak levels caused by street traffic, aircraft, and other single event noises. The
second feature is that noise level characterized by a fairly steady lower level upon
which are superimposed increased levels of the single events. This fairly constant
lower level is called residual noise level. The continuous noise one hears in the
backyard at night when no single source can be identified and which seems to come
from 'bll around" is an example of residual noise.
B-44
-------
REFERENCES
B-l. Statistical Abstract of the United States 1970, Bureau of Census, U.S.
Department of Commerce.
B-2. 1969 Statistical Supplement to the Survey of Current Business, Bureau of
Census, U.S. Department of Commerce.
B-3. Auto Facts, Automobile Manufacturers Association, Inc., Detroit, Michigan,
1971.
B-4. Air Transport 1971, Air Transport Association of Amercia, Washington, D.C.
B-5. 70-71 Transit Fact Book, American Transit Association, Washington, D.C.
B-6. Leisure Time Product Noise, Sub Council Report, National Industrial
Pollution Control Council, May 1971.
B-7. "Joint DOT-NASA Civil Aviation Research and Development Policy Study,"
supporting papers, Department of Transportation and National Aeronautics
and Space Administration, Washington, D.C., March 1971.
B-8. Transportation and Parking for Tomorrow's Cities, Wilbur Smith and
Associates, New Haven, Connecticut, 1966.
B-9. Smith, Wilbur and Associates, "Motor Trucks in the Metropolis, "Automobile
Manufacturer'slAssociation, Detroit, Michigan, August 1969.
B-10. Ganz, A. "Emerging Patterns of Urban Growth and Travel, V
Massachusetts Institute of Technology, Department of City and Regional
Planning, January 1968.
B-ll. "Study on the Effect of Noise Pollution, Section on Community Noise,"
Office of Noise Abatement and Control, Environmental Protection Agency,
Washington, D.C., October 1971.
B-12. Shaw, E.A.G., "Theoretical Model for Steady State Urban Noise," 77th
Meeting of the Acoustical Society of America, Philadelphia, Pennsylvania,
April 9, 1969.
B-45
-------
B-13. Abramowitz, M. and Stegun, I.A. (editor), "Handbook of Mathematical
Functions with Formulas, Graphs and Mathematical Tables," National
Bureau of Standards Applied Mathematics Series 55, June 1964.
B-14. Sutherland, L.C., "Air-to-Ground Sound Propagation — Some Practical
Consideration," 82nd Meeting of the Acoustical Society of America, Denver,
Colorado, October 1971.
B-15. Society of Automotive Engineers, "Method for Calculating the Attenuation
of Aircraft Ground to Ground Noise Propagation During Takeoff and Landing,"
AIR 923, August 1966.
B-16. Sutherland, L.C., "A Review of the Molecular Absorption Anomaly,"
77th Meeting of The Acoustical Society of America, Philadelphia, April 1969.
B-17. Evans, L.B., Bass, H.E. and Sutherland, L.C., "Atmospheric Absorption
of Sound, Theoretical Predictions," 82nd Meeting of The Acoustical Society
of America, Denver, Colorado, October 1971.
B-18. Piercy, J.E., "Role of the Vibrational Relaxation of Nitrogen in the
Absorption of Sound in Air," Journal of the Acoustical Society of America,
Vol. 46, September 1969.
B-19. Harris, C.M., "Absorption of Sound in Air Versus Humidity and Temperature,"
NASA Contractor Report 647, January 1967.
B-20. Society of Automotive Engineers, "Standard Values of Atmospheric Absorp-
tion as a Function of Temperature and Humidity for Use in Evaluating
Aircraft Flyover Noise," ARP 866, August 31, 1964.
B-21. Wyle Laboratories, (unpublished data).
B-22. Stephenson, R.J. and Vulkan, G.H., "Traffic Noise," Journal of Sound
and Vibration, March 1968.
B-23. Delany, M.E., "Range Prediction for Siren Sources, " Special Report 033,
Aerodynamics Division, National Physical Laboratory, November, 1969.
B-24. Dickerson, D.O. (Editor), "Transportation Noise Pollution: Control and
Abatement, " NASA Contract NGT 47-003-028, National Aeronautics and
Space Administration, Lang ley Research Center, and Old Dominion University,
Summer, 1970.
B-46
-------
B-25. Estep, A.C., "1967 Classified Vehicle Study, " Business and Transportation
Agency, Department of Public Works, Division of Highways, Sacramento,
California, December 1970.
B-26. Bishop, D.E.,. etal, "Procedures for Developing Noise Exposure Forecast
Areas for Aircraft Flight Operations, " FAA DS-67-10, Federal Aviation
Administration, August 1967.
B-27. Wyle Laboratories, 'Supporting Information for the Adopted Noise Regula-
tions for California Airports, WCR 70-3(R), Final Report to the California
Department of Aeronautics, January 29, 1971.
B-28. Galloway, W.J. and Bishop, D.E., "Noise Exposure Forecasts: Evolution,
Evaluation, Extensions, and Land Use Interpretations," FAA NO-70-9,
Federal Aviation Administration, August 1970.
B-29. Bishop, D.E. and Horonjeff, R.D., "Noise Exposure Forecast Contours for
Airport Noise Tradeoff Studies at Three Major Airports, " FAA-NO-70-7,
Federal Aviation Administration, July 1970.
B-30. Johnson, D.R. and Sounders, E.G., "The Evaluation of Noise from Freely
Flowing Road Traffic, " Journal of Sound and Vibration, Vol. 7, pp. 287-309,
March 1968.
B-31. Gordon, C.G., etal, "Evaluation of Highway Noise," NCHRP 3-7/1
Highway Research Board, National Academy of Sciences, Washington, D.C.,
January 1967.
B-32. 1970 National Highway Needs Report, 91-28, Bureau of Public Roads,
Federal Highway Adminstration, U.S. Department of Transportation,
September 1970.
B-33. Thiessen, G.J., "Community Noise — Surface Transportation, " 74th Meeting
of The Acoustical Society of America, Miami, Florida, November 1967.
B-34. Passenger Car Noise .Survey, California Highway Patrol, January 1970.
B-35. Beaton, J.L. and Bourget, L.,"Can Noise Radiation from Highways be
Reduced by Design," Highway Research Record No. 232, Highway Research
Board, National Academy of Sciences, Washington, D.C., 1968.
B-36. Pegrum, D.F., "Residential Population and Urban Transport Facilities, in the
Los Angeles Metropolitan Area, " Occasional Paper No. 3, Bureau of Business
and Economic Research, University of California, Los Angeles, California
1964.
B-47
-------
B-37. 1968 Traffic Statistics, Department of Traffic, City of Los Angeles,
California.
B-38. Highway Statistics/1969, Federal Highway Administration, U.S. Department
of Transportation.
B-39. Webster, J.C., "SIL-Past, Present and Future, " Sound and Vibration,
August 1969.
B-40. Kryter, K.D., "Methods for the Calculation and Use of the Articulation
Index, " Journal of The Acoustical Society of America, Vol. 34, 1962.
B-48
-------
APPENDIX C
NOISE GENERATOR CHARACTERISTICS
This appendix provides a detailed analysis of the characteristics of the principal noise
generators in the transportation category. The noise sources to be analyzed are:
• jet engines,
• propellers and rotors,
• internal combustion engines, and
• tires.
C.I Jet Engine Noise
There are three primary sources of noise on a commercial jet aircraft:
engines, boundary layer pressure fluctuations and internal equipment. Engines produce
noise at inlets and at the exhaust regions of fan exit ducts and the primary nozzle.
Pressure fluctuations in the fuselage boundary layer excite structural components that
in turn radiate acoustic energy into the aircraft interior. Equipment such as pumps,
blowers and auxiliary power plants installed on an aircraft create noise problems in air-
craft interiors. The latter two noise sources are the primary contributors to the noise
levels in the passenger cabin during cruise. The major aircraft noise problems, how-
ever, are associated with the noise levels imposed upon communities adjacent to large
airports. Noise generated by the jet engines constitutes the dominant component in
producing this noise impact.
The two principal sources of noise in a jet engine are the jet exhaust and
the fan/compressor. As illustrated in Figure C-l, for the case of a low bypass-ratio
turbofan engine, jet noise radiates mainly toward the rear of the engine. Fan/com-
pressor noise radiates forward out through the engine inlet and aft through the fan
exhaust duct. Figure C-2 shows the effect of engine power setting on the relative
contributions from the jet and fan noise sources. On takeoff, the jet noise contributes
measurably to the overall noisiness. During landing approaches, however, the fan
C-l
-------
Inlet
Fan
Fan Discharge Duct
' / \
I / ' * *
/ x VI /
Takeoff
110 120 dB
Landing Approach
Fan Noise from Inlet
Fan Noise from Fan Exit
Primary-Jet Noise
Overall Sound-Pressure Level
Figure C-l. Turbofan-Engine Noise Sources and Distribution
C-2
-------
120
no
Perceived-
Noise Level
(PNdB)
100
90
80
wtx
Composite
Fan Whine f om Discharge Duct
Fan Whine from Inlet
Takeoff
Thrust Range
Secondary
Jet
Landing-Approach
Thrust Range
6 8 10 12
Engine Net Thrust (1,000 Ib)
14
Figure C-2. Turbofan-Engine Noise at 400-Foot Altitude
C-3
-------
whine from the inlet and discharge ducts is 10 to 20 PNdB higher than the jet exhaust
noise.
Engine design is a critical governing factor in determining the balance
between jet and fan noise. In the early turbojet engines, the jet noise component was
dominant throughout the range of power settings. Subsequent high bypass-ratio turbo-
fan engines generate significantly reduced jet noise levels. However, as the fan noise
radiation is reduced through improved fan design technology and fan duct noise attenu-
ation, both sources of noise retain their significance in determining the total jet engine
noise levels. The following two sections contain brief accounts of the generation and
radiation characteristics of these sources of jet engine noise.
C.I.I Jet Exhaust Noise
The noise generation processes in the exhaust wakes of current and antici-
pated future turbofan engines are dominated by quadrupole noise radiation. This
mechanism is caused by the turbulent mixing that occurs along the boundary between
the high-velocity exhaust jet and the quiescent atmosphere. The mixing process
generates a series of flow fluctuations with small turbulent eddies formed close to the
j
nozzle orifice. Increasingly larger eddies are generated within the developing-
mixing layer progressively farther downstream, as illustrated in Figure C-3. However,
these fluctuations degenerate into smaller scale structures. They also interact with
each other and with the mean flow to form both larger and smaller eddies. This inter-
action results in a distribution of turbulence scales at any location within the mixing
layer, with the mean turbulence scale proportional to the local mixing layer width.
The acoustic pressure fluctuations associated with the turbulence fluctu-
ations are distributed in a corresponding manner, with the peak frequencies generated
varying continuously from high frequencies in the thin mixing layer close to the
nozzle exit to low frequencies in the wide mixing layer far downstream. However,
once generated, the acoustic waves interact with other turbulent structures (diffraction)
and mean flow gradients (refraction) to emerge from the jet flow with different
directional and physical characteristics than originally emitted. These phenomena
are qualitatively illustrated in Figure C-3.
C-4
-------
TURBULENCE NOISE
n
i
Ul
u(x,y)
L(P,T)
Ambient
Layer
(Mixing Flow)
DM~.»-,I« roreriTiai v-ore • —• __
Nozzle m-.rrr^ ••"- =
Small Scale
Shorter Lived Eddies
Velocity Fluctuation P> Pressure Fluctuation
(Acoustic)
Energy
(Eddy)
Source Distribution
Eddy Degeneration
Eddy Interactions
Eddy Convection
Energy
V
• x/D
x For Max Level of f
Large Scale
Longer Lived Eddies
Sound Path Effects
Diffraction Refraction
Velocity and Temperature Gradients
f = frequency
D = nozzle.diameter
x = distance downstream from nozzle
-L(P,t) = length of potential core
= jet spreading angle
Figure C-3. Jet Noise Generation
-------
The basic mathematical model of this noise generation process was first
1 2
formulated by Lighthill ' who combined the equation of continuity and momentum
into the inhomogeneous Lighthill wave equation:
2 2 ^ T
^ r\ ^ o i • -
-H - % -H -
ax.- 3xi 3xj
T.. = Lighthi 11's turbulence stress tensor
'J 2
- nu.u. + P.. - a ofi.-
p i J ij o M uij
u. = velocity in the i direction
a = speed of sound in the uniform medium
o r
p = density of the flow
P.. = tensor incorporating pressure and viscous terms
6.. = Kronecker delta
The well-known solution of this differential equation may be written:
Numerous theoretical investigations of jet noise generation have arrived at analytical
3—6
results by means of careful manipulation of this solution. The approach has been
developed to a high degree of sophistication and permits qualitative estimates of the
noise radiation as a function of the flow velocity and Mach number. However, the
Lighthill theory has its basic limitations. It is not well suited to describe flows in
which the speed of sound and the mean density vary. Therefore, the effects of temper-
ature and temperature gradients, although implicit in the formal solution, are usually
neglected. In addition, the connected path of sound through the mean shear layer is
not readily accounted for. These restrictions on the application of the Lighthill theory
7-9
have resulted in the formulation of new mathematical approaches by various investigators.
C-6
-------
These will not be discussed here, since they have not brought on improved techniques
for the quantitative evaluation of jet noise radiation.
The application of dimensional analysis to Lighthill's solution yields the
dimensional law for the intensity of the acoustic radiation from a jet flow to:
I
/ £\
5 V / 5
pa V (1 - M cos 6)
'o o
where
I = acoustic intensity
U = flow velocity
p = atmospheric density
D = flow dimension (jet diameter)
M- = flow Mach number
r = source-observer distance
0 = angle subtended by source-observer with respect to
flow direction
o
This result has been well substantiated by experiment, although the U
law somewhat overestimates the intensity at high jet velocities., Furthermore, the
empirical dependence of intensity on the jet density p is a function of the jet velocity.
2
Thus, p appears to correlate the experimental data at jet velocities greater than
1800 feet/second, whereas at lower speeds, Q provides the better correlation parameter.
With certain similarity assumptions concerning the variation of the peak frequency f,
the mean velocity U, and the wake diameter D with distance along the jet axis, the
above equation leads to expression for the power spectral density of the sound power
emitted by the jet flow. At high frequencies well above the typical frequency
f = 0.2 U /D defining the peak of the power spectrum, the asymptotic expression
becomes:
C-7
-------
where
W = acoustic power
f = frequency
subscript e = nozzle exit
At the low frequency limit, the corresponding variation is:
dW ,2
~sr ~ f
Figure C-4 shows a normalized sound power spectrum obtained from measurements on a
wide range of jet engines and scale model air jets. The spectrum shows the theoreti-
cally predicted trends at the low and high frequency limits.
Figure C-5 presents a generalized correlation of the peak polar sound
pressure levels generated by jet engine exhausts and scale model air jets. The
correlation'on the basis of p yields acceptable data scatter, although the increasing
spread of the data at the low velocity end is of particular note. The, cause of this
anomaly is additional noise generated upstream of the nozzle exit and propagating out
through the jet wake into the far field. This noise is generally dipole (U ) in character,
hence its dominance at low velocities. Referring to Figure C-5, line B-B shows the
peak polar sound pressure levels measured with a.noiser-generating obstruction installed
in the upstream pipe; line C-C shows the same measurements with the obstruction
removed. Line D-C is obtained if the upstream pipe is carefully treated to eliminate
all possible upstream sources of noise. The immediate conclusion from this is that the
line A-B, which represents data from a wide range of jet engines and model rigs, is
influenced by sources other than the jet mixing noise. In the case of scale model air
jets, these sources may be simple upstream obstructions. For the full-scale jet engines,
i •' . .
however, there are numerous additional and as yet incompletely defined sources. These
additional sources are often termed engine core noise and are of significance for new
technology engines which have subsonic primary exhaust velocities.
Current and advanced technology turbofan engines are characterized by
having a lower velocity fan exhaust stream surrounding the primary jet exhaust. The
generation of the noise field by these multiflow jets is even more complicated than for a
C-8
-------
-10
-20
-30
.01
I ' ' I • i
f
W(f)
wt
Ue
• ( =
I =
frequency
acoustic power/cps
total acoustic power
exit velocity
effective exit diameter : '
de for subsonic jets
sonic velocity at nozzle throat
ambient velocity of sound '
I I I
1
r2 -i
.05 .1 .2
;5 1.0 2.0
I I I I
5.0
10
Modified Strouhal Number
fda.
Jea0
Figure C-4. Normalized Power Spectrum for Axisymmetric
Jets Issuing from Convergent Nozzles
(Data from Eldred, Reference 10)
C-9
-------
180
—T " 170
-N
_o|cs_2 160
^ 150
<
in
~ 140
O
dT
.3
o
I
_l
Q_
I/)
<
o
130
120
110
100
-0.6
500
1000
R = djstqnce From jet exit
a = ambient speed of sound
o
T = ambient temperature
p = ambient density
A. = nozzle exit area
J
p. = jet density
,. Subscript ISA% refers to standard
day ambient condition
, Standard Day Jet Velocity
1500 ft/sec
-0.4
-0.2
Lo
0;2
0.4
Figure C-5. Jet Noise Correlation Peak Polar OASPL
(Data from Bushel I, Reference 11)
C-10
-------
single jet. One significant aspect of jet noise generation is that the sound results
from an extended source volume over the length of the mixing flow. Variations to
this mixing flow caused by the interaction of the two jets will result in different noise
12
characteristics. A recent experimental study included a detailed examination of
the effects of the bypass flow on the total noise radiation, resulting in the following
conclusions:
• The reduction in jet noise due to shrouding of primary flow by
secondary flow is maximum for a secondary to primary velocity
ratio near 0.57 on a constant thrust basis.
• "The reduction in jet noise increases with increasing area ratio.
• The noise reduction is independent of the pressure ratio of the
priiTiory r.czzlc end the tcto! t6r™r>6rc!t|Jr(? of the nrimarv flow.
These concepts are illustrated in Figure C-6. A noise reduction of
10 PNdB at a 1500-foot sideline is achieved at an area ratio of 10 as compared with
a single nozzle jet (area ratio zero) having the same thrust.
C.I.2 Fan/Compressor Noise
Compressors generate two distinct types of sound, broadband and harmonic.
The random broadband sound extends over a very wide range of frequencies. The har-
monic sound has one or more fundamentals corresponding to the blade passage fre-
quencies of the compressor stages, together with associated harmonics. A third type
of compressor noise, combination tones, is important in high bypass ratio turbofan
engines operating at takeoff power. A typical compressor noise spectrum is shown in
Figure C-7.
Broadband Noise
Broadband noise is attributable to the action of turbulence and other
13
irregular flow disturbances upon the compressor blades. Sharland studied compressor
broadband noise radiation both theoretically and experimentally, and his work forms
much of the basis for present knowledge. Basically, there are two primary mechanisms
for broadband noise generation. The first is associated with the passage of a blade into
C-11
-------
Ratio of Secondary Velocity to Primary Velocity
1.0
0.8
0.6
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Ratio of Primary Velocity to Secondary Velocity (V,/V«)
i 0
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• 5
10
| «•— IU
3
Area Ratio
Figure C-6. Effect of Velocity Ratio on Maximum Perceived Noise Level
on a 1500 ft Sideline Using Secondary Velocity as Reference at Constant Thrust
(Data from Eldred, Reference 12)
C-12
-------
Sound-
Pressure
Level
Fundamental
Blade-Passing
Tones
Third Stage •
Second Stage -
First Stage -
60
200 600 2000
Frequency (Hz)
Second Harmonic
of Blade-Passing
Tones
First Stage
Second Stage
I— Third Stage
Combination Tones
6000 20,000
Figure C^-7. Typical Compressor-Noise Spectrum
C-13
-------
an existing region of turbulent flow caused by upstream disturbances, which in
practice may be various compressor stages of rotor, stators, inlet guide vanes, struts,
et cetera. The velocity fluctuations in the oncoming flow generate lift fluctuations
at the blade surface, which in turn radiate noise. The second mechanism is essentially
self-generated and arises even for a blade operating in undisturbed air. At sufficiently
high Reynolds numbers, typical of compressor blading, the boundary layer becomes
unstable. In addition to generating direct pressure fluctuations on the blade surface,
it gives rise to an unsteady wake which in turn induces pressure fluctuations at the
blade trailing edge.
.For an infinite rigid surface, the monopole and dipole terms in the
14
equation for sound radiation by turbulence disappear, while the quadrupole term
must be integrated over all space including an image turbulence behind the surface.
That is, the quadrupole radiation, which is doubled due to reflection, is the only
source of noise in this case. However, for a finite surface (e.g. a compressor blade),
the reflection is not complete and a magnification of the quadrupole field results.
In this case, it is convenient for analysis purposes to regard the surface pressure fluctu-
ations as the source of noise, which is therefore of a dipole nature. These observations
g X
suggest that large surfaces will radiate as U while small ones will radiate as U .
Whether the surface is "large" or "small" depends upon its size relative to both acoustic
(\) and turbulence (&) wavelengths.
Since the latter quantities are related by the flow Mach number
N = i/\r one can state that for a surface of dimension d, the radiation is quad-
rupole if M » jj/d, and dipole if M << A/d. Since f, is also proportional to
frequency, higher frequencies are more likely to radiate as quadrupoles.
The two sources of random sound discussed above can be related to two
basic situations. The first is the production of noise on a blade due to the boundary
layer actually set up on that blade. This is termed "self-generated" noise; it is small-
scale (i.e., high frequency), and it quadrupole in nature. The second is the noise
produced by passage pf the blade through turbulence generated upstream of the blade.
C-14
-------
This is "externally generated," has a much larger scale (lower frequency), and is dipole
in character.
A dimensional analysis of the various sources yielded the following relation-
ships for the total power (W) radiated from an area (S):
External Turbulence
4 2
... 1 IT MS ,,. , ,
W = -7- p (dipole)
o
Self-Induced Boundary Layer Pressure Fluctuations
-7 PC0'3 3
W = 10 — (dipole) a 3
Reflection of Boundary Layer "Self-Noise"
-8 Pb^5 5
W = 6x10 (quadrupole) a
o
It turns out that for turbulence levels in excess of 0.001, the first
equation dominates and suggests that "external turbulence" is a very significant noise
source.
The prediction of broadband noise radiation, although analytically
straightforward, is critically dependent upon the nature of the spatial correlation of
the surface pressure fluctuations. The problem reduces to that of estimating the
variation of spanwise and chordwise correlation lengths with the compressor operating
configuration.
Harmonic Tones
The main difference between harmonic and broadband noise generation is
that the former is associated with periodic rather than random flow disturbances in the
compressor duct. Otherwise, the mechanisms are very similar. The word "periodic"
C-15
-------
here essentially includes the fundamental or steady velocity terms which are associated
with the steady thrust and torque forces acting upon the blades. Since this is the basic
mechanism of propeller noise radiation, this source of compressor sound is sometimes
referred to as the propeller mode. Its origin was first analyzed by Gutin, whose
theory is still widely used for propeller noise prediction.
Of much more importance is the noise radiated by the action of fluctuating
forces upon the blades due to the presence of "higher harmonic" flow fluctuations.
These flow fluctuations are due to the presence of obstructions such as struts, guide
vanes, or stators, which produce velocity defects and potential flow interactions.
Figure C-8 shows the basic geometry for stator-rotor interaction. Several alternative
theories are available for the prediction of harmonic noise generation. Lowson's
theory will be used to describe the problem.
Like Curie's theory of sound radiation by surfaces, Lowson's analysis
starts with Lighthill's basic equation for aerodynamic sound generation:
2 9 2
o p _a2 5 P =
°
st
The left-hand side is the acoustic-wave equation, and on the right-hand side are the
three source terms corresponding to monopole, dipole and quadrupole radiation,
respectively. The only term of significance for most applications is the second, which
corresponds to noise radiation by surface forces. Retaining only this term, the solution
may be written:
-^rff
4ira J J
(x,
' " " ~ i* ~ii
where the integration must be performed over the entire source region.
C-16
-------
Stator
Stator
R = rotor radius .
fi = rotational frequency in radians/sec
d = stator-rotor spacing
a = stator exit swirl angle
p = rotor turning angle
w = flow velocity
Figure C-8. Basic Wake Geometry
(Data from Lowson, Reference 16)
C-17
-------
If one of the aerodynamic force components acting on the blade, namely the thrust,
is written in its Fourier form, the nth harmonic can be written:
T = T cos
n
Upon substitution, the integral solution can be integrated for one revolution to yield:
c -
where
m
B
= sound pressure ampli-
tude of the nth sound
harmonic
= order of the harmonic
= number of blades
= rotational speed
= Bessel function of
M
g
6
X
= blade Mach number
= speed of sound
= , distance from blade
= angle from fan axis to
field point
= multiple of load harmonic
II.IIUII Ul
order mB
When n = 0, this reduces to the Gutin equation. The important thing to note about this
equation for any value of m is that any thrust harmonic can generate any sound harmonic.
Outside a certain range of frequencies, however, the acoustic efficiencies
of these harmonics are low; for practical purposes, only values of the load harmonic
number (k) need be considered lie in the range:
-Msine)
-------
Such equations, although based upon a number of simplifying assumptions,
give convenient solutions for the sound radiated by rotors and stators, which have been
demonstrated by Ollerhead and Munch to be remarkably accurate for the first har-
monic radiation by typical fan configurations. However, these computations, like all
solutions to compressor noise generation, are only as accurate as the force-input terms.
These correlations were obtained with use of airfoil-wake data to estimate the harmonic
content pf velocity profiles behind compressor stages. The scatter in the data shown in
Figure C-9 reflects this uncertainty in the force-input terms. Typical velocity profiles
were assumed for the theoretically determined line in the figure. Improved knowledge
of these profiles should close the discrepancy between the theoretical and experimental
noise levels.
Combinarion Tones
Combination tone noise is radiated from the inlet of turbofan engines
having fan blades rotating with supersonic tip speeds; hence, it is a prominent type of
noise from the current high bypass ratio turbofan engines at takeoff power. This effect
18
is illustrated in Figure C-10. Unlike the sound field produced by fans at subsonic
operation, where discrete tones are produced at harmonics of blade passage frequency,
fans at supersonic tip speeds generate a multiplicity of tones at essentially all integral
multiples of engine rotation frequency.
The essential features of combination tone generation are well established. '
Shock waves are produced at the leading edge of each blade and spiral forward of the
fan, conveying sound energy out of the inlet to the far field. The waveforms are fairly
uniform close to the fan, both in shock amplitude and in spacing between shocks.
Farther forward of the fan, however, much of the blade-to-blade periodicity is lost and
variations in shock amplitude and spacing become prominent. Since the shocks form a
fairly steady but irregular pattern rotating with the fan, the corresponding noise spectrum
is composed of a series of tones at harmonics of the shaft rotation frequency.
This loss of blade-to-blade periodicity can be explained on the basis of
finite amplitude wave theory. Close to the fan, the intervals between shocks are quite
C-19
-------
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Figure C-9. Comparison of "Theory" and Experiment. Maximum Fan Discharge Noise
in Octave Band Containing Fundamental Blade Passage Frequency
(Data from Lowson, Reference 16)
C-2C
-------
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-------
uniform due to the regular spacing of the blades. Some variation in shock amplitude,
however, is inevitable because of small manufacturing variations in the incidence
angles and other geometric properties of the blades. As the waves spiral forward of the
fan, this amplitude variation creates significant interval variation because of the
influence of shock strength on the rate of propagation. Strong shocks travel faster
than relatively weak shocks; thus an initial small variation in shock strengths of two
consecutive shocks will cause the spacing between these shocks to vary with distance
away from the fan. At the same time, both shocks are decaying and they eventually
reach a stable situation where the spacing is unchanged with further propagation.
A feature of the combination tone noise spectra in the engine far field is
that two fans, although identical in design, produce different spectral signatures. The
fact is that each blade is slightly different within well defined manufacturing tolerance
bands. When the blades are assembled to produce fans, the small deviations of each
fan from design will be different from fan-to-fan. A deterministic prediction procedure
for a given design would thus require knowledge of the variation in manufacture of all
blades on each fan. Since this is impractical both in advance of and during production,
an estimate of the average spectrum for a given fan design represents the practical
limits in combination tone prediction. This average will not only depend on relevant
geometric blade parameters, but also on their standard deviations from design.
C.2 Propeller and Rotor Noise
The mechanisms by which rotors, propellers and fans produce intense sound
pressures have been the subject of much work, especially in recent years. Traditionally,
noise generated by propellers has been separated into two parts called the rotational and
vortex components. Rotational or periodic noise here describes all sound which is identi-
fied with discrete frequencies occurring at harmonics of the blade passage frequency.
Vortex or broadband noise describes the modulated sound produced by the
unsteady pressure field associated with vortices shed from the trailing edge and tips of
the blades, as well as some of the noise sources associated with turbulence effects in
the air stream. The helicopter rotor deserves separate consideration. Although much
C-22
-------
of its noise can be explained in terms of propeller noise sources, there are a number of
other sources exclusive to that device which make significant contributions to the over-
all levels.
Rotational Noise '
All real rotating airfoils, i.e., those having thickness, have a pressure
distribution when moving relative to the surrounding medium. This pressure distribution
can be resolved into a thrust component normal to the plane of rotation and a torque
component in the plane of rotation. This pressure field on the air is steady relative to
the blade and rotates with it if operating under conditions of uniform inflow. For non-
uniform inflow, such as a helicopter rotor in steady forward flight, the difference in
relative blade speed during forward and backward motion of the blade relative to the
flight path require? a cy.lie incidence variation to provide a reasonably uniform lift
over the disc. To a first approximation, the forces on the air next to the disc would be
constant under these conditions; the effects of incidence changes would appear only as
variation of chordwise loading over the blade. From a fixed point on the disc, the
rotating field appears as an oscillating pressure. The frequency of the oscillation is the
frequency with which a blade passes that point (blade passage frequency) and the wave-
form of the oscillating pressure is determined by the chordwise distribution of pressure
on the blades.
In addition to experiencing a fluctuating force, an element of air in the
disc will be physically moved aside by the finite thickness of the blade. In a fixed
frame of reference, this displacement is equivalent to a periodic introduction and
removal of mass at each element of air near the disc. The rate of mass introduction at
a point, which is determined by the blade profile, incidence and speed, can then be
expressed as the strength of a simple source. Up to values of resultant tip speed
approaching sonic, thickness noise is generally found to be small compared with ttie
noise arising from torque and thrust. At higher tip speeds, however, it may assume
equal importance.
C-23
-------
Interaction and Distortion Effects
Certain periodic effects are usually identified with helicopter rotors, but
may occur to a lesser degree in propellers. Impulsive noise, blade bang or blade slap
may consist of high-amplitude periodic noise plus highly modulated vortex noise caused
by impulsive fluctuating forces on the blades. The mechanisms by which these forces
may arise are: (1) blade-vortex interaction, (2) periodic stalling and unstalling of a
blade, and (3) shockwave formation and collapse due to unsteady periods of local
supersonic flow. The first and second conditions (and possibly the third) may occur .
(1) when a blade passes through or near a tip vortex, or (2) when an unsteady wake is
generated by a preceding blade. Operation in this unsteady flow condition leads to
strong fluctuating forces. Here, aeroelastic properties may become significant
parameters. The third mechanism may also result directly from operation of a blade at
high tip speed (such as an advancing helicopter blade during high speed flight). When
high tip speed occurs, blade slap is by far the dominant source of aerodynamic noise.
Vortex Noise
The dominant source of broadband noise is called vortex noise, which has
been defined as that sound which is generated by the formation and shedding of vortices
in the flow past a blade. For an infinite circular cylinder, normal to the flow and in
2 5
the range of Reynolds numbers from 10 to 10 , it is well known that the vortices are
shed in an orderly vortex street which is a function of cylinder diameter and flow
velocity. The process in the case of a rotating airfoil is similar and since there is a
different velocity associated with each chordwise station along the span, a broadband
of shedding frequencies results. This produces a dipole form of acoustic radiation in
which the strength of the source is proportional to the sixth power of the section velocity.
Hence the frequencies associated with the area near the tip tend to be of greatest
amplitude. Also, since a blade develops lift (thrust), tip and spanwise vorticity of
strength proportional to the thrust gradients are generated and shed. Their dipole
acoustic radiation combines with that from the trailing edge vortices to make up the
so-called vortex noise.
C-24
-------
C.2.1 Propeller Noise
As discussed above, the noise produced by an operating propeller has been
an object of scientific interest for many years. All of the early work in the aeronautical
noise field, both analytical and experimental, was concerned with the propeller noise
20
problem or with allied configurations such as Yudin's work with rotating rods.
Although closely related to the noise produced by rotors and fans, the
problem of propeller noise is simpler in some respects because of the configuration and
operating conditions of the propeller. The small number of blades in a normal pro-
peller, together with the flow velocity through the propeller disc, minimizes the inter-
ference effects due to operation in the wake of preceding blades. The structure and
location of the propeller is such that noise due to blade flutter and asymmetrical induced
flow are not normally encountered. At moderate tip speeds, i.e., slightly below the
onset of compressibility effects, both vortex noise and rotational noise due to thickness
are lower than the rotational noise due to thrust and torque. Consequently, most of
the noise work on propellers, of both a theoretical and experimental nature, has con-
centrated on the effects of thrust and torque. In studies dealing with the reduction of
overall propeller noise, however, vortex noise has been shown to be an important
contributor and in the case of high-speed flight, the level of thickness noise may
exceed that of thrust and torque noise.
Rotational Noise
The theoretical work of Gutin provides the equation for the sound
pressure of the'mth harmonic tone; ,
169.3rr.BRM,
p -
m ~ SA
where
- T cos 6
2
P = rms sound pressure level (SPL) in dynes/cm
m = order of the harmonic
S = distance from propeller hub to observer, ft
C-25
-------
R = propeller radius, ft
2
A = propeller disc area, ft
P, = absorbed power, horsepower
T = thrust, Ibs
B = number of blades
M = tip Mach number
Jmg = Bessel function of order mB
x = argument of Bessel function 0.8 M mB sin Q
Q = angle from forward propeller axis to observer
The expression gives reasonable agreement with experimental results for the
first few harmonics of conventional propellers operating at moderate tip speeds and for-
ward velocities. In these circumstances, summation of the square root of the sum of
the squares of the solutions to the above expression for m = 1, 2, 3, 4 will yield an
adequate approximation of the overall sound pressure of the thrust and torque com-
ponents. Under such conditions, it is a suitable estimate of the total noise as well.
As tip Mach number is reduced to the range between 0.5 and 0.3, experi-
mental results begin to diverge from the predicted values in the direction of higher
levels. In this region, vortex noise, which originates in the variable forces acting on
the medium during flow past the blade, makes itself known.
Vortex Noise
2j
An equation developed by Hubbard, which was based on Yudin's original
22
work, additional work by Stowell and Deming, and others, is frequently used to
calculate vortex noise in terms of SPL:
kA (V )6
SPL = 10 log U./ (dB at 300 ft)
io16
v/here
k = constant of proportionality
C-26
-------
2
A, = propeller blade area, ft
V-. 7 = velocity at 0.7 radius
The expression indicates that vortex noise is a strong function of blade
velocity; doubling the blade velocity increases the SPL by 18 dB. The effect of
doubling blade area is less severe; the SPL is increased by 3 dB. This suggests that the
way to reduce vortex noise is to minimize the tip velocity and to make up the required
thrust by increasing blade area as far as possible within the constraints of efficiency
and structure. It should be remembered, however, that the vortex noise of propellers
does not become significant until the blade velocity is already below normal operational
values.
C.2.2 Rotor Noise
Rotational Noise
The study of rotor noise has had the advantage of drawing on the knowledge
gained from earlier interest in the propeller. It was found, however, that although
propeller noise theory was fairly accurate in describing the sound level of the first
harmonic of rotors, it was grossly in error for the higher harmonics. This is not alto-
gether surprising when one considers the relative complexities of the two systems. The
propeller that Gutin described was a rigid device rotating in steady, uniform flow.
The modern rotor is quite a different system. The main feature of the rotor aerodynamics
is the lack of symmetry. In transitional and forward flight, the rotor disc encounters
highly nonuniform inflow, and the mechanism by which forward thrust is obtained gives
rise to cyclic pitch and fluctuating airloads. Under these operating conditions, velocity
fluctuations are induced which give rise to a multitude of blade-loading harmonics.
The calculation or experimental determination of these higher harmonic blade loads is
23-25
extremely complex and has met with only limited success. Many authors are of the
opinion that all the significant higher harmonic sound effects (except possibly at
transonic or supersonic speeds) can be attributed to these unsteady higher harmonic
loadings and, further, that any sound harmonic receives contributions from all loading
harmonics.
C-27
-------
23
Lowson and Ollerhead have undertaken to avoid the problem of theoreti-
cally determining the blade-loading harmonics by deriving empirical harmonic decay
laws. A study of the available full-scale blade-loading data revealed that the amplitude
of the airload harmonics decayed approximately as some inverse power of harmonic
number, at least in the range which covered the first 10 harmonics. For steady flight
out of ground effect, the optimum value for the exponent was found to be -2.07 so
-2.0
that the amplitude of the mth loading harmonic was proportional to m . This law
was then extrapolated indefinitely to higher frequencies in order to provide some esti-
mate of the higher harmonic airload levels. However, before this could be used as a
basis for noise calculation, account had to be taken for phase variations around the
rotor azimuth and along the rotor span. It was assumed that the phases could be ran-
domized. In the case of the spanwise loading variations, this was accomplished by the
introduction of a "correlation length" concept such as commonly used in turbulence
theory. By assuming that the correlation length was inversely proportional to frequency,
this resulted in an approximate net effect of adding a further -0.5 to the exponent of
the loading power law. Also, an effective rotational Mach number concept is introduced
which enables the.effects of forward speed to be calculated directly from results for the
hover case.
Using these approximations, the rotational noise spectrum for the Bell UH-1
helicopter was calculated for comparison with available measurements. The comparison
is shown in Figure C-l 1. Because of uncertainties regarding the overall levels, they
were normalized on the basis of power in the third and higher harmonics. Although for
this reason, nothing can be said about overall levels, the agreement, insofar as spectral
shape is concerned, is good up to the thirtieth harmonic.
Broadband (Vortex) Noise
The fundamental generation mechanism of broadband and, more particularly,
vortex noise from rotors is not yet fully understood. In Yudin's early work with rotating
rods, vortex noise was considered to be a viscous wake-excited phenomenon and indeed
it must be in that case. However, in the case of a lifting airfoil such as a rotor, the
C-28
-------
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to
30
20
10
0
g -io
-20
-30
-40
-50
— — — Theory, M = 0.5,
O Elevation - 10 deg
High Collective
A Elevation = 5 deg,
V Elevation - 10 deg
— — -^
<
]
J^x.
' N
Elevation = 5 deg
, r = 100 ft, Ground Running at
Pitch over Concrete
r = 600 ft, Hover over Grass
, r = 200 ft, Tiedown over Grass
^
%*
\
C
^J
[
7 V
r >
V
t
*
^
O^AX. I
00^5^
( °
^Qtoo
^\
1 2 4 6 10 10 20 4<
Harmonic Number
Figure C-ll. Noise Spectrum; Comparison of Theory and Experiment
For a Two-Blade Rotor (UH-1A and UH-lB)
(Data from Lowson/Ollerhead, Reference 23)
C-29
-------
experimental evidence could support equally well the contention that it is caused by a
random movement of the lifting vortex in the tip region. Quite likely, both the tip
vortex and the vortex sheet shed from the upper surface of the airfoil contribute in
varying degrees, depending on the configuration and operating conditions. There is
evidence, however, that a portion of what was originally identified as broadband
vortex noise may, in fact, be higher harmonic rotational noise. Lowson and Ollerhead
report that the rotational noise of rotors may dominate the noise spectrum up to 400 Hz
and higher. At any rate, broadband noise is generated and can be dominant under
some rotor operations, e.g., at very low rotational velocities with two-bladed or
three-bladed rotors, where even higher harmonics of the blade passage frequency may
26
be inaudible. Hubbard and Regier extended the work of Yudin and postulated that
for propellers with airfoil sections, as for rotating circular rods, the vortex noise energy
was proportional to the first power of blade area and to the sixth power of the section
velocity. Experimental measurements, where they are available and reliable, should
be used to evaluate the constant for a particular set of conditions.
Modulation (Blade Slap) Noise
Rotors suffer more from distortion noise than any other aerodynamic noise
generator. Blade slap is the colloquialism that has been applied to the sharp cracking
sound associated with helicopter rotor noise sources. To date, the only attempt at a
quantitative study of the problem seems to be the papers published by Leverton and
27 28
Taylor. ' In the latest, Leverton lists the three main mechanisms generally
postulated for blade slap in the literature:
• Fluctuating forces caused by blade-vortex interaction.
• Fluctuating forces resulting from stalling and unstalling of the
blade.
• Shock wove formation due to local supersonic flow; it is suggested
that this is either (a) a direct result of operating a blade at a high
tip speed, or (b) caused by a blade-vurtex interaction.
C-30
-------
At the present time, detailed information on these mechanisms is still
limited; therefore, it is almost impossible to state which is the most likely mechanism.
However, a blade intersecting the tip vortex shed by a preceding blade could itself
cause the other two mechanisms to occur. Leverton assumes that blade slap is the
direct result of the fluctuating lift caused by the interaction of a blade and a vortex
filament. .This can be either an actual intersection when a blade cuts a vortex fila-
ment or the effect of a blade passing very close to a vortex filament.
Although it is easy to imagine a blade and a tip vortex intersecting, it is
extremely difficult to visualize the details of such an encounter and practically
impossible to describe it mathematically. As a blade intersects or comes near a
vortex filament, the blade circulation and hence the lift profile will become severely
distorted. On a single rotor lift system, a blade will most likely pass near, or cut
through, a tip vortex shed by a preceding blade (Figure C-12 (a) ). On a tandem
rotor lift system, it is more likely that one rotor will cut the vortex filament generated
by the other disc (Figure C-12 (b) ). The fact that large fluctuations in lift occur
when a blade passes close to a vortex filament is obvious.
C.3 Internal Combustion Engine Noise
The externally-radiated noise from internal combustion engines results from
a multitude of noise-generating mechanisms. Unlike the jet engine, for which one or
two sources of noise dominate the noise-radiation characteristics, several noise sources
contribute measurably to the noise signatures of internal combustion engines. The
following major source categories are commonly recognized:
• exhaust noise
• intake noise
• engine-radiated noise due to cylinder pressure development
(combustion noise)
• engine-radiated noise due to mechanical components
(mechanical noise)
• cooling fan noise
C-31
-------
(a) Single Rotor System
(b) Tandem Rotor System
Aircraft
Figure C-12. Typical Blade-Vortex Intersections for a Single
Rotor System (a), and a Tandem Rotor System (b)
(Data from Leverton, Reference 28)
C-32
-------
Several specific subsources are distinguished in the engine-radiated noise
category. These will be discussed below in the detailed evaluation of the separate
noise categories.
C.3.1 Exhaust and Intake Noise
Exhaust noise is potentially the greatest noise source of the automotive
engine. It is produced by the sudden release of gas into the exhaust system by the
opening of the exhaust valve. The closing of the valve produces only minor effects.
The fundamentals and harmonics of the firing frequency are the principal components
which have to be dealt with in the exhaust-muffler system. At high speeds, the
individual frequency components are masked by a more continuous spectrum attributed
to turbulence noise associated with the high velocity of the exhaust gases over the
valve seat.
Intake noise is produced by both the opening and the closing of the inlet
valve. At opening, the pressure in the cylinder is usually above atmospheric and a
sharp positive pressure pulse sets the air in the inlet passage into oscillation at the
natural frequency of the air column. The oscillation is rapidly damped by the changing
volume caused by piston motion downward. Closing of the inlet valve produces similar
oscillations, which are relatively undamped. In practical installations, measurements
indicate that intake noise is not fully silenced and in some vehicles it is the pre-
29
dominant source of noise.
Figure C-13 shows spectra of the noise radiation from a diesel engine
running at 1500 rpm with (a) open exhaust' and inlet, (b) silenced exhaust, and (c)
silenced exhaust and inlet. Comparison of spectra (a) and (b) shows that exhaust
noise predominates by about 10 dB over the whole frequency range. Comparison of
spectrum (b) with the spectrum with the air inlet silenced (c) shows that the next
greatest noise source i: the air inlet. The remaining noise, spectrum (c), is emitted
by the engine structure itself from vibration of the external surfaces and by the cooling
fan. In the diesel engine, air inlet noise generally predominates only in the low and
middle frequencies, up to 1000 Hz. In the gasoline engine, this inlet noise may also
C-33
-------
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100
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(a)
Open Exhaust and Inlet
90
80
70
Open Inlet
Silenced Exhaust
Silenced Exhaust and Inlet
60
I I I I I
J I
100
000
10,000
Frequency (Hz)
Figure C-13, Octave-Band Spectra of Inlet and'Exhaust Noise of
2-Litre Diesel Engine at 1500 rpm Full Load
(Data from Priede, Reference 30)
C-34
-------
predominate in the high frequency range owing to the "hissing" noise produced in the
carburetor.
Both exhaust and intake noise show the same dependence on engine
speed:
Sound Level dB(A) = 45 Log]()N + k
where
N = engine speed - rpm
k = undetermined parameter
The noise levels increase with increasing engine load; from no-load to full-load, the
intake noise increases between 10 and 15 dB for diesel engines and between 20 and
25 dB for gasoline engines. Intake noise is also affected by the construction of the
exhaust system; restrictions in the exhaust markedly increase the intake noise. Both
exhaust and intake noise are greatly influenced by design variables such as the size
of the valves, their timing and the construction of the parts.
Automotive engines are normally equipped with exhaust mufflers and intake
silencers. In some cases these are inadequate because of space and cost I imitations,
even though techniques for silencing to any desired degree are well known.
Large mufflers must be used to obtain adequate silencing with low back
pressures. Two mufflers in series are sometimes used (for example, on bus engines).
The ratios of net muffler volume to engine displacement volume of a group of typical
older passenger cars indicate values between 1.5 to 4.2. For some quieter mufflers,
the volume ratio is double these values.
The location of the muffler along the exhaust pipe is important, especially
with the simpler mufflers, because of pipe resonances. The most advantageous muffler
location for single-muffler systems is indicated at the center of the exhaust pipe, which
32
allows for cancellation of pipe resonances.
32
According to Martin, it has been demonstrated by experiment and theory
that the direct gas flow through a muffler can considerably affect the silencing effect.
Considering, first, a reactive muffler with resonant chambers and flow interruptions
C-35
-------
(staggered tubes in successive bulkheads), let D be the attenuation in decibels
through the muffler without gas flow and D the practical silencing of the various
frequencies with superimposed gas flow through the muffler, as measured on the actual
engine. Then, as a first approximation, the following relation between D and D
is given:
D
D = T-^TI dB
r 1 - a M
where
M = Mach number of the mean gas flow in the muffler
a = nondimensional coefficient whose value falls between 1.0 to
1.2, depending on the muffler design
Thus, muffling improves with Mach number within the engine operating
range and full-throttle operation is better silenced than idling operation. On the
other hand, absorption type mufflers with a straight-through passage in a perforated
pipe surrounded by a concentric container filled with fibrous sound-absorbing material
show better silencing, D , at idling than at full-throttle, according to the relation:
t
D = D (1 - pM1//3) dB
a o
Again, P is a nondimensional constant, dependent on muffler design, and falling
between 1.0 and 1.2 in magnitude.
Intake silencers are usually of the straight-through design, using resonant
side chambers to control both low frequency and high frequency noise, and a "hiss
felt" for control of the high frequency noise spectrum. Figure C-14 shows octave band
intake noise spectra at 3 feet for two diesel engines at full power, 2000 rpm operation,
with normal inlet silencers and with completely silenced inletsp^ The upper set of
curves, (a), is for a 2-liter engine and includes no-load intake noise spectra. The
lower set of curves, (b), is for a larger 4.2-liter engine. Intake noise on both engines
reaches a substantial 95 to 97 dB peak in the low frequency octave bands at about
120 to 250 Hz at full load. The no-load intake noise octave band peak for the smaller
C-36
-------
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-a
90
* 80
70
"? 90
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I 80
70
i i
Full Load
Normal Inlet
Silencer
(a)
y ^— No Load
y Normal Inlet
/ Silencer
Full Load
Completely
Silenced Inlet
Full Load
Normal Inlet
(b)
Full Load
Completely
Silenced Inlet
00
Frequency (Hz)
Figure C-14. Octave Band Spectra (5) of Noise of Two Diesel Engines
With Normal and With Fully Balanced Air Inlets; (a) 122 cu in
Engine, 2000 rpm; (b) 256 cu in Engine, 200 rpm
(Data from Soroka/Chien, Reference 33)
C-37
-------
engine is about 10 dB lower (88 dB) than the full-load peak (97 dB), with a slight up-
ward shift in frequency. With complete silencing of the intake noise under full load,
the predominant noise is in the upper frequency range from 1000 Hz and up.
C.3.2 Combustion and Mechanical Noise
The noise from the structure of an internal combustion engine is produced
by forces of mechanical origin and by gas forces acting on the pistons resulting from
compression and subsequent combustion. Both produce vibrations of the external sur-
faces which emit the noise. Noises of mechanical origin are those due to operation of
the piston-crank system, valve-gear mechanism, various auxiliaries and their drives.
In practice, mechanical noise is defined as that of the motored engine. This definition,
however, includes the effect of gas forces developed during compression; but the con-
tribution from the compression pressure is rather small. The noise of the running engine
(addition of the gas forces due to combustion) is invariably greater than that of the
motored engine. Thus, combustion is the major noise source in an internal combustion
engine.
The effect of combustion on engine noise is illustrated in Figure C-15
which shows spectra for diesel and gasoline engines, both motored and running, with
33
different forms of cylinder pressure development. In both types of engines, the noise
can be varied some 10 dB by changing the form of the cylinder pressure. Hence, worth-
while reductions of engine noise may be attainable if the effect on noise of the form of
cylinder pressure development is known. Figure C-16 shows some examples of cylinder
33
pressure spectra from a gasoline and a diesel engine at full and no load.
Both diesel and gasoline engine cylinder pressure spectra show a high level
for the first few harmonics, followed by a steady decrease of the level of higher order
harmonics by some 30 to 50 dB per decade.
The low frequency parts of the spectra, up to about 300 Hz or 20th har-
monic, are hardly influenced by the form of pressure diagram, but are largely determined
by the peak pressure. A large reduction of the level of this part of the spectrum is
observed only with a considerable reduction of peak pressure such as occurs with a
C-38
-------
CM
I
X
-
-o
O
lf\
I
a>
I
o
O
90
80
X
CM
-------
Gasoline Engine
Full Load
Diesel Engine
\( Ful1 Load
Gasoline Engine
No Load
///////// it/// / / / / / / r// ///////
100
1000
10,000
Frequency (Hz)
Figure C-16. Cylinder Pressure Spectra for Diesel Engine C and
Gasoline Engine at 2000 rprp Full ,Load and No, Load •
(Data from So rok a/Chi en, Reference 33)
C-40
-------
gasoline engine on no-load. The level's of the harmonics above 20th order are
affected more and more by the actual form of the pressure diagram; thus, at higher
frequencies, the spectra of diesel engines diverge from those of gasoline engines and
have higher levels, particularly in the range from 800 Hz to 3000 Hz.
This difference is ascribed to the different mechanism of ignition. In the
gasoline engine, the flame is initiated from a spark (that is, a point source) from which
the flame gradually propagates until the whole charge contained in the chamber is
burned. Thus, a very smooth blending with the compression is obtained. In the diesel
engine, on the other hand, ignition is spontaneous and an appreciable volume of pre-
mixed fuel and air burns extremely rapidly. This rapid combustion results in the
marked discontinuity (that is, rapid initial pressure rise), invariably observed on the
cylinder-pressure diagrams of diesel engines.
Noise measurements on a large number of automotive diesel engines (with
inlet and exhaust silenced) have shown a striking simi.larity in shape of noise spectrum.
All spectra show a bro'dd.peak in the frequency range from 800 to 2000 Hz, similar to
that of the octave band spectrum (c) of Figure C-13. From oscillographic investigation,
it has been shown that the noise is emitted in impulses coinciding with the rapid increase
of cylinder pressure. It is the objectionable hard "knock" characteristic of diesel
engines.
! ,
the spectrum of'the gasoline engine is different. The components in the
frequency range 800 to 2000 Hz are of lower intensity and the largest peaks in the
spectrum are in the frequency range 400 to 600 Hz. These differences correspond
exactly to the differences previously noted in cylinder pressure spectra. The different
noise characteristics of diesel and gasoline engines therefore are due not to any
differences of the structure but to differences in excitation due to cylinder pressure.
The effect of load on the cylinder pressure spectra (Figure C-16) is very
marked in the gasoline engine, but is very slight in the diesel engine. This is due to
throttling the gasoline engine intake at no-load. These observations again are found
to be in agreement with noise measurements as shown in Figure C-17, where the overall
C-41
-------
i r
100
2 Liter Diesel
90
X
CN
0)
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C
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0)
I
v>
80
TJ
C
D
O
to
70
I
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I I
10
30 50 100
Cylinder Pressure Diagram Loading — psi
200
Figure C-17, Effect- of Load on Overall Calculated Sound Pressure Level
Fora Diesel Engine and a Gasoline Engine
(Data from Soroka/Chien, Reference 33)
C-42
-------
sound pressure levels are plotted against load for a diesel engine and a gasoline engine
at 2000 rpm?3 In the diesel engine, the sound pressure level at no-load differs only
slightly from that at full-load; whereas in the gasoline engine, the sound pressure level
at no-load is less than that at full-lead by some 10 dB.
The relationship between the cylinder pressure spectrum and the engine noise
radiation depends on the relative levels of the combustion and mechanical noise com-
ponents. Smoothing or reducing the cylinder pressure belowa certain "critical" value
will have only a negligible effect on the engine noise because of the constant level of
the mechanical noise.
If the cylinder pressure level is above the "critical level," the level of the
emitted noise is proportional to that of cylinder pressure. This makes it possible to define
the vibrational and radiating properties or the "noisiness" of an engine structure by a
quantity:
attenuation in decibels = cylinder pressure level - sound pressure level
The attenuation is represented by a single curve covering the audio frequency range
which is independent of engine operating conditions — speed, timing and load.
Figure C-18 shows the attenuation curves of four diesel engines and a
33
gasoline engine of similar size (2-liter capacity). As can be seen, variation of
attenuation among the diesel engines of current design is not very large and the curves
are found to lie within a range of some 6 dB. Also, the attenuation curve of the
gasoline engine lies mainly within the group of curves for the diesel engines, which
indicates that its structure is not dissimilar, as regards noise, from that of the diesel
engines.
The attenuation is high at low frequencies and declines at a steady rate by
about 50 dB/decade up to about 1000 Hz. Above 1000 Hz, attenuation declines at a
considerably lower rate, by about some 10 dB/decade. Investigations have shows that
the high attenuation at low frequencies is partly due to higher attenuation of vibration
by the stiffness of the structure and partly due to higher radiation attenuation, since
the wavelength of the sound exceeds that of the linear dimensions of the engine. At
C-43
-------
CO
.2
"o
0)
120
no
100
90
80
70
60
Gasoline
I I I I
I I I I I I I
I I I I I I I
100
1000
10,000
Frequency (Hz)
Figure C-18. Attenuation Curves for Gasoline Engine and Diesel Engines
(Data from Soroka/Chien, Reference 33)
C-44
-------
'high frequencies, from 800 Hz upwards, the noise is due to vibration of resonating
sections of engine surfaces, generally of the crankcase, resulting from transmission of
forces due to cylinder pressure both directly from and via the crankshaft.
The pressure diagram in an engine tends to remain of simil.ar form with
change of speed; therefore, to a first approximation the cylinder pressure spectra will
be geometrically similar at different speeds but with a shift parallel to the frequency
axis corresponding to the change of speed. Thus, the increase of engine noise will
depend on the general slope of the cylinder pressure spectrum. For example, with
cylinder pressure spectra having a slope of 30 dB/decade (corresponding to the slope
of cylinder pressure spectra in most diesel engines), one can expect an increase of
engine noise by 30 dB for the tenfold increase of engine speed.
33
This is confirmed by the test results shown in Figure C-19. The
straight lines give a good fit in the case of aM four diesei engines. The noise of the
gasoline engine increases in speed at a higher rate; this corresponds to the greater slope
of the cylinder pressure spectrum of this engine (Figure C-16). Thus, the engine noise
31
levels may be expressed by the simple relationships:
. Sound Level dB(A) = 30 Logir.N + k for diesel engines, and
Sound Level dB(A) = 50 Loglf.N + k for gasoline engines.
The effect of the engine size is also clearly seen from Figure C-19. If the
amplitude of vibration of engine surfaces does not vary with engine size, the increase
in intensity of sound radiated should be due only to the increase in the radiating sur-
face area and the noise would increase by 13.3 dB for a tenfold increase of engine
capacity. This can be seen from the data on the few diesel engines; an increase of
about 14 to 16 dB is obtained, which is very close to the above value. In general
this gives the result that, power for power, a large engine running slowly is quieter
than a smaller one running faster.
C.3.3 Cooling Fan Noise
Cooling fan noise is a nuisance noise in the automobile. Aerodynamic noise
is generated directly through vortex formation by the fan blades. The most common type
C-45
-------
100
4
* 90
CN
0)
5
co
-o
c
Diesel Engines
0)
I
2
a
£
O
-to
80
70
Liter
t Gasoline Engine
I
I
I I
I I I I
500
1000
2000
Engine rpm
4000
Figure C-19. Calculated Overall Sound Pressure Level versus Speed
for Four Diesel Engines and Gasoline Engine
(Data from Soroka/Chien, Reference 33)
C-46
-------
of engine cooling fan is the axial flow type. This is used invariably to draw air through
the radiators of water-cooled engines. Centrifugal fans are sometimes used with air-
cooled engines. The mechanisms of noise generation by axial flow fans have been dis-r
34
cussed by Sharland and others. These mechanisms are identical to those described
in Section C.I .2 for jet engine fans and compressors and in Section C.2 for aircraft
propellers and helicopter rotors, and will not be discussed separately in this section.
A general empirical expression for the noise levels generated by cooling fans may be
written in the form:
Sound Level dB(A) = 60 log^N + k
where
N = fan rotational speed
k = undetermined parameter
At present, the method of testing fan acoustic performance consists of
installing various designs of fans which will give the required cooling, then road-
33
testing the car at different speeds and selecting by ear the most satisfactory fan.
Design constraints on the fan covering space occupied, rotational speed, amount of
airflow, position in car, and other factors cause difficulty in making a quiet fan.
Blade spacing can be used successfully to distribute the level of harmonics
over the operating range. Four blade fans with 76 degree blade spacing have been
found to be a good choice. On the average, slow running fans are the quietest. For
automotive fans, noise is increased by 60 dB per tenfold increase of tip speed. Inten-
sity is proportional to about the 6th power of rpm and therefore a special coupling to
reduce fan speed at high engine speeds is one of the most effective ways of con-
trolling the noise. Fan noise has the peculiar quality that sometimes it is difficult to
mask below other car noises, particularly when the other engine noises are suppressed.
C.4 Tire Noise
Noise generated by tire-roadway interaction is one of the prime sources of
annoyance for several classes of road vehicles. For example, at vehicle speeds above
C-47
-------
30 to 35 mph, tire noise may be the principal component of the overall automobile
and small truck noise spectrum. Figure C-20 shows how the engine noise levels compare
35
with the noise levels produced by various classes of tires for a single-axle truck.
Although the "quiet" tires fall below the engine noise levels over the entire range of
vehicle speeds, the difference in levels is sufficiently small that a moderate reduction
in engine noise would leave tire noise as the principal component even for these tires,
especially at the higher velocities. /
The important source mechanisms in the tire/roadway interaction are:
• "Air pumping" from tread and roadway activities — the sudden out-
flow of air trapped in the treads or roadway cavities when the tire
contacts the road surface, and the sudden inflow of air when the
tire lifts away from the contact area.
• Casing vibration — excitation of the casing and tread by roadway
roughness or by the tire itself.
• Aerodynamic— (a) "spinning disc" noise, (b) impingement of
turbulence upon all or parts of the tire, (c) impingement of dis-
placed air on the roadway surface.
oz
Hayden has made a detailed analytical investigation of these noise
sources and concludes that the third mechanism is negligible except at very high
vehicle speed. Thus, aerodynamic noise mechanisms may be considered to represent
a lower bound for the tire-roadway noise.
The first two noise source mechanisms are discussed below, on the basis
36 - \ •
of Hoyden's analysis.
Air Pumping from Tire and Roadway Cavities
When a section of the tire tread contacts the road surface, some of the
air in the spaces between the treads is displaced, thus creating a locally-unsteady
volumetric flow. Similarly, when the tire rolls over and partially fills cavities in the
roadway, some of the air is squeezed out of these cavities. Finally, when tire seg-
ments leave the contact area, spaces enclosed by the tire and roadway expand rapidly
C-48
-------
90
80* _
CD
T3
"» 70 -
-a
c
60 -
Retread
1/2 Worn Crossbars
New Crossbars
Uneven Wear Rib
New Rib
30 40
Speed - mph
Figure C-20. The Significance of Tire Noise Relative
to Engine Noise for a Typical Single Axle Truck
(Data from Tetlow, Reference 35)
50
C-49
-------
and a volumetric flow transient is created by the air rushing to fill the expanding
cavity. Such fluctuations in volumetric flow rate characterize the driving mechanisms
of the acoustic monopole (or "simple source"). The narrow-band mean-square acoustic
pressure due to the "point" monopole source may be written:
o M =
P \r;
2 2 T
Oo Q 0 In5
22 C IU
16 IT r
u Q
2
r
where
p = the acoustic pressure
p = the ambient density of the medium
r = the radial distance from the source
Q = the volumetric flow rate from the source
o = the circular frequency
Thus, to estimate the overall sound pressure, one needs only to estimate
Q and u. Such a procedure will now be demonstrated for a tire whose geometry is
shown in an exaggerated fashion in Figure C-21.
The mean-flow rate from a single cavity is estimated to be:
rT - Volume change _ (f.c.)gwS _ ,f
__ ^_. (tmt
for the geometry shown (where f.c. is the fractional change in the cavity volume).
The characteristic frequency of occurrence of the flow pulse is:
u = 2irV/S .
By substituting these relationships into the first equation and taking the logarithm with
respect to the reference pressure 0.0002 jjbar, the following "engineering equation" is
derived (for n cavities per tire width):
SPL(r) = 68.5 + 20 log (gw/S) + 10 log n + 20 log (f.c.) + 40 log V - 20 log(r)
C-50
-------
Direction
of Travel
Observation
Point
CD
CU
cm
EH)
tz
U-l
V = forward velocity
W = width of a single cavity or groove in tread
g = depth of groove = tread depth
S = circumferential distance between tread grooves
s = circumferential dimension of tread grooves
R = tire radius
r = distance to observation point
(Note: The respective values of W, g, S, and s on a given tire may be different
for individual cavities.)
Figure C-21, Tire Terminology
(Data from Hayden, Reference 36)
C-51
-------
This equation is valid for the case of a non-directional sound source and hemispherical '
spreading. •'
A similar equation may be derived for the sound due to a smooth tire
rolling over cavities in the.roadway. If there are m cavities in the roadway per
width of the tire (m = W /S ) and the cavities are d deep, w wide and have a
spacing of S , then the sound pressure level is:
SPL(r) = 68.5 + 20 log (d^/SJ + 10 log n + 20 log (f.c.) + 40 log V - 20 log r
Tire Vibration
The excitation of tires by road roughness and resultant tire vibration is
complex, making the prediction of associated sound radiation somewhat difficult.
Reasonable analytical formulations of tire vibration and the resultant sound radiation
would require much presently unavailable knowledge about tire dynamics and dynamic
behavior of the tire/roadway interface. With so much of the needed information
lacking, an experimental approach may be taken to determine the roadside noise due
to tire casing vibrations. The empirical curve for predicting sound radiation from the
acceleration input spectrum shown in Figure C-22 was obtained by placing a tire in a
reverberant chamber and measuring the sound power level spectra for various vertical
36
input acceleration levels and spectra. It may be noted that the tire responds strongly
within a range of frequencies from 125 to 1000 Hz. The cut-on at 125 Hz corresponds
roughly to the fundamental resonances of the tire. Above 1000 Hz, the input acceler-
ation levels are strongly damped.
Vibrational sound from a passenger car tire operating on a granite chip
road surface has been predicted with the use of Figure C-22 from indirectly-measured
36
acceleration spectra. The resultant sound power spectra are shown in Figure C-23
and the overall levels at various speeds in Figure C-24.
The relative importance of each of the previously discussed source
mechanisms to the overall noise radiated by a rolling tire may be estimated from the
relationships developed above. For several different tires and road surfaces, the
C-52
-------
70
O
0)
3 60
£
i
£
2 50
o
ID
CO
TJ
Q_
40
30
I
I
I
Highly
Damped
Regime
_L
I
I
I
16 31.5 63 . 125 250 500 1000 1600 4000
One-Third Octave Band Center Frequencies (Hz)
A.
8000 16,000
Figure C-220 Empirical Curve for Predicting Sound Radiation from Acceleration Input Spectrum
(Data from Hayden, Reference 36)
C-53 .
-------
100
I ' ' I ' ' I ' ' I ' ' I
90
Estimated Contribution of Tire Vibration to Tire Noise Spectra
(Tire Rolling on Granite Chip Surface)
80
o
£
0)
0>
70
60
50
40
30 mph
I
I
I
I
I i i I i
_L
31.5 63 125 250 500 1000 2000 4000 8000
One-Third Octave Band Center Frequencies, Hz
Figure C-23. Estimated Contribution of Tire Vibration to Tire Noise Spectra
(Tire Rolling on Granite Chip Surface) •
(Data from Hayden, Reference'36)
16,000
C-54
-------
90
80
I I
o
2
CO
70
60
o
IO
4-
D
£ 50
"Air
Pump- 4
ing"
40
30
15
20
I
1 Truck Crossbar or Auto Snow
2 Motorcycle Crossbar
3 Passenger Rib
4 Roadway Noise - Concrete
5 Roadway Noise - Asphalt
6 Aerodynamic Sources
7 Vibration - Passenger Tire on Concrete
30 40 50
V = Vehicle Speed (Miles/Hour)
60
70 80 90 100
Figure C-24. Predicted Contribution of Various Source
Mechanisms to Roadside Noise
(Data from Hayden, Reference 36)
C-55
-------
appropriate geometries have been determined for predicting roadside noise from the
f\f
"air pumping" mechanism. The results are shown in Figure C-24. Curves 1 to 5 all
exhibit the 40 log V speed dependence. In each of these cases, it was assumed that
the fractional volume change (f.c.) is 0.1 and that the dynamics of the air pumping
process'are similar in all instances. The latter assumption is undoubtedly too general,
as one intuitively expects air to be squeezed from rib tires in a different manner than
from crossbar treads or road cavities. Crossbar type treads are predicted to.be noisier
than ribs; the concrete road surface examined was rougher than asphalt and thus pre-
dicted to be noisier. . ; '
Comparison of the vibrational sound levels estimated by'the empirical
method with those estimated for the "air pumping" mechanism-tends to indicate that
tire vibration is not a dominant sound-generating mechanism in tires. However, in -
view of the uncertainties involved in the input acceleration calculations, this pre-
diction must be regarded as tentative and somewhat inconclusive. It may be noted
that measurements of tire noise on rough roads suggest that tire vibration noise can be
31
significant. The noise spectra measured on rough roads showed a nearly constant
spectrum level up to 800 Hz, followed by a strong decrease at higher frequencies.
Changes in vehicle speed were found to result in no significant change in the spectrum
shape. This behavior agrees with the tire vibration mechanism, whereas the air
pumping mechanism predicts a linear dependence of frequency on the vehicle speed.
The data obtained on relatively smooth road surfaces, however, appear to
agree with the predictions of the air pumping model in several respects. Evaluation
35
of Tetlow's data> shown in Figure C-20, confirm the following trends:
• Speed dependence of the measured sound approached 40 log V,
especially at the higher speeds.
• Crossbar treads were found to be noisier than rib-type treads.
• Cup-type treads which completely seal upon contacting the road
were the noisiest; this suggests that the u Q term is the greatest
for treads which completely seal, thus the higher acoustic intensities
from the monopole or "air pumping" sound.
• C-56
-------
35
These conclusions are further supported by the data of Figure C-25,
which'show that a 15 dB drop in noise level resulted when a single-axle truck was un-
loaded; load per tire was decreased from 4550 to 1240 pounds. This effect is simply
explained: with the truck unloaded, the sides of the tire tread do not touch the
ground an'd hence the cups in the tread cannot seal against the road surface. Recent
36
data obtained by Hciyden for the tire noise generated by a coasting automobile show
both the 40 log V shift in'overall level and the linear shift in frequency with vehicle
o
speed predicted by the air pumping model.
Hence, it appears that this mechanism of noise generation may be adequate
to explain the tire noise radiation measured in tests over relatively smooth road sur-
faces for a considerable range of vehicle speeds and tire configurations. The importance
of tire vibration noise has not been satisfactorily resolved.
C-57
-------
90
80
co
t>
J. 70
3
O
to
60
-0
A Retread
O 1/2 Worn Crossbars
New Crossbars
20
30 40
Speed (mph)
50
Figure C-250 The Effect of Load on Tire Noise. Solid line
represents maximum tire load (4500 Ibs); dashed line
represents minimum tire load (1240 Ibs). (Data from
Tetlow, Reference 35)
C-58
-------
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U" , .
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C-60
-------
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C-61
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C—62 *U.S. GOVERNMENT PRINTING OFFICE:1973 5M-153/ZZ3 1-3
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