905R80118
NOISE IMPACT ASSESSMENT MANUAL
FOR
SELECTED TRANSPORTATION SOURCES
PRELIMINARY DRAFT
FOR EPA REVIEW ONLY
Office of Noise Abatement and Control
Office of Air and Waste Management.
U.S. Environmental Protection Agency
Washington, D.C. 20460
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TABLE OF CONTENTS
Introduction . 1
Decibel Addition 1
Frequency 5
Sound Descriptors 5
Definitions 6
Highway Traffic Noise Predictor 7
Railroad Line Noise Predictor 18
Fixed Wing Aircraft Noise Predictor 23
Concluding Remarks 32
References 33
APPENDIX A
ROADWAY SEGMENTS
APPENDIX B
BARRIER A.TTENTUATION
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LIST OF ILLUSTRATIONS
Figure Page
1 Chart for Combining Sound Levels by "Decibel Addition" 4
2 Map Showing Roadway and Observer . 8
3 Leq Nomograph for Roadway Noise 10
4 Leq Nomograph for Roadway Noise
5 Variation of Leq with Distance from Roadv/ay Center!ine 14
6 Leq Contour Construction for Highway Noise 16
7 -Noise Levels for Railroad Line Operations 20
8 Graphical Sequence for Calculating SEL 24
9. Ldn Chart for Single Events
10. Location of Points A and B from the Break Release Point 27
*>
11. -Flight Profile for Alpha Takeoff Procedure 28
12. SEL Variation with Distance 29
Bl. Section View Showing Barrier Parameters B-2
B2 Plan View of a Roadway Partially Shielded by a Barrier B-2
B3. Barrier Adjustment in dB from the Barrier Parameters,
DR, DB, and H B-4
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LIST OF TABLES
Table £age_
1 Common Sounds and Their dB Levels . 2
2 Correction values A to be added to Leq to obtain Ldn 17
3 Adjustments to Ldn Noise Contours 20
Bl Barrier Attenuation Corrected for Length of Barrier B-5
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NOISE IMPACT ASSESSMENT MANUAL
FOR
SELECTED TRANSPORTATION SOURCES
INTRODUCTION
This manual is designed for persons who have little or no back-
ground in environmental noise pollution and yet have a need to estimate
the noise impact from sources such as highway traffic, railroad line
operations, and aircraft flight operations. Such persons might include
government officials as well as members of the private sector who are
involved in environmental planning, assessment and enforcement activities,
The materials needed to make these noise impact assessments are arranged
in this manual in the form of tables, nomographs, and curves, so that a
mathematical treatment of the subject matter is deliberately avoided.
References are included for those readers who want more background infor-
mation or who require methods which predict noise levels more accurately.
DECIBEL ADDITION
The magnitude of a sound is measured in decibels (dB). Table 1
shows a range of decibel levels associated with everyday sound sources.
Because decibels are based upon a logarithmic scale, known decibel
levels of separate sound sources cannot be added in the usual way.
For example, suppose the sound intensity from source A is 80 dB and
the sound intensity from source B is also 80 dB. When sources A and B
are operating simultaneously, the resulting sound is not 160 dB, but
rather 83 dB.
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TABLE 1 - Common sounds and their dBA levels
Source .
Rocket launching pad 180
Oet plane - 140
Gunshot blast . 140
Riveting steel tank 130
Automobile horn 120
Sandblasting . 112
Woodworking shop . '100
Punch press 100
Pneumatic drill ' 100
Boiler shop . 100
Hydraulic press 100
Can manufacturing plant 100
Subway ' 90
Average factory ' 80-90
Computer card verifier -85
Noisy restaurant 80
Office tabulator 80
Busy traffic ' - 75
Conversational speech 66
Average home 50
Quiet office 40
Soft whisper - 30
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Figure 1 provides a chart for combining sound sources two at a
time. The horizontal scale represents the difference in decibels between
the two levels to be added, while the vertical scale is the decibel
increment which is added to the higher level of the two original levels.
In the above case, the difference between source A and B is zero. From
Figure 1, add 3.0 dB to the higher value, obtaining 83 dB.
EXAMPLE
Calculate the overall decibel level for the following intensity
levels associated with each source:
Source A - 82 dB
Source B - 76 dB
Source C - 84 dB
Source D"- 71 dB
The levels are first arranged in ascending order before combining
them two at a time using Figure 1.
71.
76-
82-
84
77.3
-83.3
-86.6
Answer
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The 71 dB and 76 dB levels are combined to produce 77.3 dB from
Figure 1. 'Now, 77.3 dB is combined with 82 dB to yeild 83.3 dB.
Finally, 83.3 dB is added to 84 dB to yield 86.6 dB as the overall
dB level.
FREQUENCY
It is known that human hearing is sensitive to the frequency of a
sound. Because of this hearing characteristic, sound measuring equipment
incorporate devices which filter certain frequencies in approximately
the same fashion as the ear. The A-weighting network found on most
sound level meters is such a device. Sounds measured in this manner
are referenced as dBA rather than simply dB. Most federal noise regula-
tions and local noise ordinances express allov.'able noise limits in terms
of dBA levels.
SOUND DESCRIPTORS
While the basic measurements of sounds include frequency and intensity,
the time variation of the sound intensity is normally required for community
noise measurements. This is due to the wide fluctuation in the sound
intensity in most environments. As a truck passes by a quiet street,
the sound level may rise by 30 to 40 dB above the ambient level for a
few seconds. To add this temporal characteristic of typical noise
environments, different noise measurement methods have been developed
over the years. One such method is the energy average equivalent sound
level. This method is described below in simple terms and is used as
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the basic descriptor in this manual. More detailed mathematical
definitions may be found elsewhere.' ' '
DEFINITIONS
Equivalent Sound Level (Leq) - This is the level of a constant sound
which, in a given situation and time period, has the same sound energy
as does a time-varying sound. The symbol Leq is used for the equivalent
sound level. Some common time periods for measuring Leq's include one
hour, eight hours, and 24 hours represented as Leq(l), Leq(8), and Leq(24),
respectively.
Sound Exposure Level - This is the level of sound accumulated over a
given time interval or event and the symbol SEL is used. In contrast
to Leq, which'represents the average energy level, SEL represents the
accumulation of sound energy. Therefore, SEL is always increasing up
to the end of the event. SEL is frequently used in measuring the noise
from a moving source, such as a airplane taking off or landing.
Day-Night Average Sound Level (Ldn) - This is the Leq(24) computed
after 10 dB has been added to the nighttime levels (10 p.m. to 7 a.m.)
The symbol Ldn is used. The 10 dB nighttime penalty is incorporated
in the Ldn since noise can be more intrusive at night.
The Leq, SEL, and Ldn are the noise descriptors used in this
manual. In particular, the Leq and Ldn are cumulative noise
descriptors which have been adopted by EPA as appropriate for describing
environmental noise. ' '
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HIGHWAY TRAFFIC NOISE PREDICTOR
The current methodology (3'4=>5) for predicting highway traffic
noise levels employs a computer program to account for common highway
complexities, such as, roadway gradients, pavement characteristics,
roadway curves, ramps, depressed or elevated roadway sections, and
barriers. By taking these and other factors into account, the computer
program provides predicted sound levels to within +2 dBA of actual
measurements.^ ' A short method which uses only a nomograph rather
than a computer is used in this manual and provides a conservative
estimate of the highway noise. The extent of the over-prediction
by the short method may vary from zero to a few decibels, depending
on the complexity of the real highway relative to the short method
model assumptions.(5'
Figure 2 shows a roadway and a residential development. The
observer is located at point D. Distance DC is the perpendicular
distance to the center line of the roadway. In the short method
traffic is assumed to be freely flowing on a flat straight roadway
with no obstructions between the observer and the roadway. In
addition, the roadway is assumed to extend in both directions a
distance which is at least four times the perpendicular distance
from the observer to the roadway (distances AC and BC must be four
times distance DC). If these conditions do not exist in any par-
ticular situation, corrections must be applied to the final computed
sound level (see Appendices).
The noise emitted from the traffic is viewed in terms of three
source categories which are defined as follows:
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Center!ine of Roadway
Observer,
NResidentia! Development
Figure 2 - Map showing Roadway and Observer
B
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Automobiles: Two axle, four wheel vehicles including light
trucks, such as a panel truck.
Medium Trucks: Gasoline-pov/ered two axles six wheel vehicles,
such as a city truck, and busses.
Heavy Trucks: Diesel-powered, three or more axle vehicles,
such as a tractor trailer.
Requi red 1nformati on
Obtain from the Highway Department the following information:
1. The volume flow for each of the above source categories
during the busiest hour of the day.* The symbol V represents the
number of vehicles per hour (veh/hr).
2. Determine the average speed, S, in miles per hour (mph)
for each source category.
3. Select the distance, DC, in feet for each observation
point of interest.
Nomograph Procedure
Figure 3 shows the nomograph which is used to estimate the Leq
level in dBA in the following manner:
1. Draw a line from the pivot point at the left end through the
heavy truck average speed in mph to intersect at a point on line A.
2. Draw a line from this point on line A to the heavy truck
vehicle volume in veh/hr. This produces an intersecting point on
line B.
*For predicting noise impact from proposed roadways, the design
hourly volume should be used.
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3. Draw a line from the point on line B to the distance to
observer, DC. This line intersects the Leq scale which approximates
the Leq level during the busiest hour due to the heavy truck traffic.
4. Repeat steps 1, 2, and 3 for the information on automobile
traffic. However, in step 1 draw the line from the pivot point through
the appropriate lower set of crosses associated with the average speed
of the automobiles.
5. Multiply the medium truck volume flow by 10 to account for
the inherent noisiness of these vehicles in comparison with automobile
noise. Having this adjusted vehicle volume flow, repeat step 4.
(Should the speed limit for automobiles and medium trucks be the same,
steps 4 and 5 may be combined into one step. Simply add the flow
volume for automobiles to the adjusted medium truck flow volume and
follow step 4 to account for both sources simultaneously.)
6. Having the decibel values due to the separate noise sources,
add them using decibel addition (Figure 1) to obtain the Leq associated
with the busiest hour of the day.
Example
Calculate the Leq value for the following traffic data which
correspond to the busiest hour of the day. Assume DC equals 200 ft.
Vehicle Category
Heavy truck
Automobile
Medium truck
Volume V
(veh/hr)
150
4000
200
Adjusted V
(veh/hr)
150
4000
2000
i
Speed S
(mph)
40
50
40
Distance DC
(ft)
200
200
200
Leq
(dBA)
68
64
59
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Note that the third column incorporates the multiple factor of
ten for medium truck volume flow. The Leq values listed in column 6
are obtained from the marked-up nomograph in Figure 4.
These Leq levels are now added with the aid of Figure 1 as follows
59
65.3-
64"
-69.8 dBA answer
68
Noise Contours
To plot the Leq contours for the area near a highway, proceed
as follows:
1. At any distance DC from the highway center!ine, compute the
Leq by the method already described.
2. Plot this point on Figure 5 (Leq vs. DC) and through this
point draw a line parallel to the heavy dashed lines.
3. On the vertical axis, locate the noise level valve for each
contour desired, move horizontally to the constructed line and then
vertically down to the horizontal axis to determine the corresponding
distance DC.
4. On a map or scale drawing of the roadway, draw the contours
parallel to the roadway at the specified distances measured perpen-
dicularly from the roadway center!ine.
Ex amp 1 e
Plot the 75, 70, 65, and 60 dBA contours for the highway noise
example previously considered.
13
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It was determined that Leq was equal to 69.8 dBA when DC was 200 ft.,
so this point is plotted on Figure 5. (Shown with an X). Through
this point a line is drawn parallel to the heavy dashed lines. Now the
distances DC corresponding to the 75, 70, 65, and 60 dBA levels are
determined to be 87, 182, 402, and 880 ft. respectively. On the site
plan of the highway, Figure 6, construct a perpendicular to the
highway center!ine. Lay off the distances DC to scale along this
perpendicular as shown. Nov.1 construct the contours through these
points and parallel to the highway.
Ldn
In order to approximate the Ldn values from the computed busiest
hour I.eq levels, obtain the percentage of day and nighttime percentages
of the 24-hour traffic volume and apply correction values from Table 2.
The Table gives corrections to apply to Leq for various percentages of
daytime and nighttime traffic. As an example, suppose the Leq is 65 dBA
and 85% of the traffic occurs during daytime hours. From the Table,
A = + 1 dB. Therefore Ldn ^ 65 + 1 = 66 dBA.
15
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TABLE 2 - Correction values A to be added to Leq to
obtain Lcin. (Source: "Design Guide for
'Highway Noise Prediction and Control,"
Bolt, Bersnek and Newman Report 2739,
Vol. 1, pg. 0-17}
'Percentage of traffic
during daytime hours
62.5
75
85
90
95
100
Percentage of traffic
during nichttime hours
37.5
25'
15'
10
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Correction valued
(dB)
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17
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RAILROAD LINL^NOISE^RCDICTOR_
Recent studies (6,7,8) s'no-vi ^nc± cumulative noise exposure
levels may he approximated for both railroad line operations and
railroad yard operations. Due to the complexities involved in
railroad yard operations, only railroad line operations are con-
sidered in this manual.
The noise generated by line operations consists of two major
sources: the noise generated by the locor.otive and the noise
generated by the passing cars. Analysis of field measurements shows
that the locomotive noise does not vary appreciably with speed, while
car noise is directly related to both the train speed (the faster the
speed the higher the noise) and Uio train pass-by time (the longer
the pass-by time the higher the noise). Since faster trains have
lower pass-by times, the two components of the car noise tend to
compensate each other. Therefore, the "train speed may be neglected
when estimating railroad line noise.
Required Information
The railroad line noise predictor requires the following:
1. Determine the number of daytime (t^) and/or nighttime (Nn)
train operations. Daytime operations are from 7:00 a.m. to 10:00 p.m.
and nighttime operations are from 10:00 p.m. to 7:00 a.m. N
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Procedure
Figure 7 is the graph which is used to estimate the noise levels
in the following manner:
1. Calculate the equivalent number of operations !! for which
the Equivalent Sound Level (Leo) is sought.
(a) For Loq (day), N = Nd .
(b) For Leq (night), N = Nn
(c) For Leq (24), N - Nd + Nr
(d) For Ldn, N = Nd + 10 Nr
2. Having N, enter Figure 8 at the prescribed distance and
proceed vertically to the appropriate N contour. Kove across
horizontally and read the Leq level.
3. If necessary, enter Table 3 to add corrections to the Leq
level obtained in step 2. Note that in the case o^ I'rjltiple occurrences
of the variables shown in the table, only the larger of the odiugrient
values should be used.
Example
Calculate the Ldn level at distances of 100, 203 and 1000 ft from
the track center!ine for the typical 24-hour train operations shewn
below:
Type
Freight
Passenger
Freight
Passenger
Direction
Eastbound
Eastbound
Westbound
Westbound
Total
»D
10
6
8
J5
30
NN
6
3
5
_3
17 i
-------
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The equivalent number of operations is calculated as
N = ND + 10 NN
+ 30 + 10(17) = 200
Entering Figure 2, the following Ldn levels are obtained:
At 100 ft, Ldn = 77 dBA
At 200 ft, Ldn = 72.5 dBA
At 1000 ft, Ldn = 59.5 dBA
Example
'Develop Ldn contours for the train operations in previous example
assuming the presence of a grade crossing.
The equivalent number of operations remains at 200 as already cal-
culated. To obtain the Ldn contours,^) the following table is
constructed:
Desired Contour
Value, dBA
80
75
70
65
60
Adjusted Factor
from Table 3
4
4
4
4
4
Adjusted Contour
Value, dBA
76
71
66
61
56
Distance to
Contour Val
125
260
460
850
1500
Desired
ue, ft.
Due to the presence of the grade crossing, the desired contour
distances, which would normally be obtained directly from Figure 8,
are spread out further from the track center!ine. Consequently,
column 3 lists the adjusted contour values which are obtained by
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substracting the adjustment factor in column 2 from the desired contour
value in column 1. Now enter Figure 8 with these adjusted contour values
to obtain the distance to the desired contour values as listed in column 4.
For example, the 80 dBA contour is 125 ft. from the track center!ine, the
75 dBA contour is 260 ft. from the track center!ine, etc.
22
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FIXED VJING AIRCRAFT NOISE PREDICTOR
With the advent of jet engine aircraft in the late 1950's, noise
exposure levels increased appreciably from aircraft operation around
air installations. This led to the development of cumulative noise
exposure ratings, such as the community noise rating (CNR) for air-
craft, the noise exposure forecast (NEF), and the community noise
equivalent level (CNEL). Each of these methods essentially add the
noise effects on an energy basis and include penalties for sensitive
periods of a 24-hour cycled '
Computer programs exist ^ ' ' which generate noise level contours
due to flight operations from fixed wing aircraft around airports.
Likewise, there exists (2,11) a nancj calculation method, to be described
in this manual, which provides approximate Ldn levels at fixed points
around the airport. It is emphasized that this short method of pre-
dicting aircraft noise does not include many complicating factors
which are built into the computer programs. However, while the short
method provides only approximate Ldn levels, it does allow relative
comparisons among different sets of flight operations around an
airport. This information can be useful to a planner who needs a
quick evaluation of possible noise strategies related to aircraft
operational changes.
23
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ft
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. .... " "(c) ... . '
Figure 8 - Graphical Sequence for Calculating SEL
24
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Reguired In fo rma ti on
The aircraft noise predictor requires the following information:
1. The flight track locations around the airport.
2. The relationship between the aircraft altitude and the
distance along the flight track (the projection of the aircraft position
onto the ground plane) for each aircraft operation.
3. The relationship between the SEL and the slant distance for the
point location in question.
4. The average daily number of each aircraft operation, by daytime
and nighttime operations.
5. Runway utilization and flight path utilization in terms of
percentage for each operation.
Proc ed u_r_e
Figure 8 shows the geographical procedure for establishing the SEL
for a given class of aircraft at a selected location.
1. Establish the distance along the flight track location and
enter the appropriate altitude profile curve, such as Figures (a).
Read the corresponding altitude.
2. Having the altitude, a, and knowing the sideline distance,
b, as shown in Figure 8(b), calculate the slant distance, c, as follows:
c = a2 + b2
(The altitude shown in Figures (b) is actually an approxi-
mation of the true altitude which is on a line perpendicular
to the flight path. For purposes of this manual, the approxi-
mate altitude is adequate.)
25
-------
i :
Vi
S~° 109
NUMBER OF SIMILAR OPERATIONS PER 24-HOUR PERIOD
FIGURE 9
Ldn CHART FOR SINGLE EVENTS
(Source: "Task II - Joint Services Noise Planning Manual,"
by Wilsey & Ham prepared for the Washington Area
Procurement Center. Nov. 1974.) '
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3. Enter the SEL - slant distance curve, such as Figure 8(c),
with the slant distance to obtain the corresponding SEL value.
4. Knowing the number of similar flight operations and the
number of daytime and nighttime flights, enter Figure 9 to obtain the
difference between SEL and Ldn. Knovn'ng SEL from step 3, the Ldn con-
tribution for the set of flight operations at the location in question
and for the particular flight track is established.
5. Repeat steps 1 through 4 for the other classes of aircraft
using the same flight track.
6. Repeat steps 1 through 5 for the remaining flight tracks
around the air installation.
7. Add all Ldn levels using Figure 1 to obtain the overall Ldn
level.
Example
Calculate the Ldn at points A and 8 in Figure 10 due to an alpha
takeoff procedure by a Boeing 707-3003 at 320,000 Ib. gross weight.
Assume 50 daily flights of which 20% occur at night.
Figure 11 shows the altitude profile for this aircraft under this
take-off procedure, while Figure 12 is the corresponding SEL variation
with distance. To calculate the SEL for points A and B, the following
steps are made:
Point A: Altitude a = 1170 ft. (from Figure!! )
Sideline Distance b = 200 ft.
Slant Distance c = (1170)2 + (200)2 = 1187 ft.
SEL = 108 dB (from Figure 12) -
30
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SEL - Ldn = 27.7 dB (from Figure ' 9)
Ldn = 108 - 27.7 = 80.3 dB Answer
Point B: Altitude a = 2200 ft (from Figure 11)
sideline distance b = 200 ft.
slant distance c - (2200)2 + (200)2 = 2209 ft.
SEL - 95 dB (from Figure 12)
SEL - Ldn - 27.7 dB (from Figure 9)
Ldn = 95-27.7 - 67.3 dBA Answer
31
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CONCLUDING REMARKS
Each noise predictor method contained in this manual has been presented
in an abbreviated form to be used by non-technical individuals who need to
estimate the noise impact from transportation sources.
It is emphasized that these methods provide only approximate Leq and Ldn
levels for any given situation. Yet, this information is useful to
the planner to identify potential noise problems and to assess relative
comparisons among different sets of conditions. References on more
complex noise predictor methods are included for those readers who
have need for a "more complete treatment of their noise problems.
32
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i..;
i
REFERENCES
1. "Information on Levels of Environmental Noise Requisite to
Protect Public Health and Welfare with an Adequate Margin
of Safety," U.S. Environmental Protection Agency, March 1974.
2. "Task II - Joint Services Noise Planning Manual /'(Preliminary), Wilsey & Ha:
prepared for Washington Area Procurement Center, November 1974.
3. Gordon, C.G., Galloway, W.J., Kugler, B.A., and Nelson, D.L.,
"Highway Noise - A Design Guide for Highway Engineers," National
Cooperative Highway Research Program Report 117, 1971.
4. Wesler, J E., "Manual for Highway Noise Prediction" Report No.
DOT-TSC-FHWA-72-1, March 1972.
5. Kugler, B.A., Ccmmins, D.E., and Galloway, W.J., "Design Guide
for Highway Noise Prediction and Control," Bolt, Beranek & Newman,
Report'No. 2739, November 1974.
6. Swing, J.W., and Pies, D.B., "Assessment of Noise Environments
Around Railroad Operations," Wyle Laboratories, Report WCR 73-5,
- July 1973.
7. Swing, J.W., and Inman, D.J., "Noise Levels in Railroad Operations,"
A.S.M.E. Rail Transportation Division, No. 74-WA/RT-8, November 1974.
8. Swing, J.W., "Simplified Procedure for Developing Railroad Noise
Exposure Contours," Sound and Vibration, February 1975.
9. Reddingius, N.H., "Community Noise Exposure Resulting from
Aircraft Operations: Computer Program Operator's Manual,"
Air Force Report AMRL TR-73-108, 1974.
10. "Community Noise Exposure Resulting from Aircraft Operations:
Computer Program Description," Air Force Report AMRL TR-73-109.
11. "Community Noise Exposure Resulting from Aircraft Operations:
. Application Guide for Predictive Procedure," Air Force Report
AMRL TR-110.
33
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APPENDIX A
ROADWAY SEGMENTS
CORRECTION TABLE FOR ROADWAY LENGTH
AC or BC
DC DC
Correction
(dB)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.75
' 0.5
0.25
0
- 0.3
- 0.4
- 0.5
- 0.6
- 0.7
- 0.9
- 1.3
- 1.5
- 1.9
- 2.4
- 3.0
Instructions for use of table:
Distances AC, BC and DC are shown in Figure 3. Determine these
distances with a site plan and a scale or estimate them in the field.
Compute the ratio AC divided by DC. The table gives the correction
in decibels for the length of roadway to the left of the observer.
Compute the ratio of BC divided by DC and obtain a correction for the
length of roadway to the right of the observer. Add the two corrections
and apply this to the levels obtained by the short method of highway
noise prediction.
Example
The perpendicular distance from a roadway to an observation
point is 200 ft. (DC = 200 ft.) The roadway extends 400 ft. to
A-l
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APPENDIX A
ROADWAY SEGMENTS
CORRECTION TABLE FOR ROADWAY LENGTH
AC or BC
DC DC
Correction
(dB)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.75
0.5
0.25
0
- 0.3
- 0.4
- 0.5
- 0.6
- 0.7
- 0.9
- 1.3
- 1.5
- 1.9
- 2.4
- 3.0
Instructions for use of table:
Distances AC, BC and DC are shown in Figure 3. Determine these
distances with a site plan and a scale or estimate thorn in the field.
Compute the ratio AC divided by DC. The table gives the correction
in decibels for the length of roadway to the left of the observer.
Compute the ratio of BC divided by DC and obtain a correction for the
length of roadway to the right of the observer. Add the two corrections
and apply this to the levels obtained by the short method of highway
noise prediction.
Example
The perpendicular distance from a roadway to an observation
point is 200 ft. (DC = 200 ft.) The roadway extends 400 ft. to
A-l
-------
the left and 600 ft. to the right of point C (AC = 400 and BC = 600 ft),
Thus AC/DC = 2 and BC/DC = 3. The corrections for the length to the
left is -0.7 dB, and -0.5 dB for the length to the right. The total
correction for roadway length is -0.7 -0.5 = -1.2 dB. Thus, subtract
1.2 dB from the noise level computed for the highway.
A-2
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APPENDIX B
BARRIER ATTENUATION
The following is a description of a short method of computing
barrier attenuation that will give approximate results. The method
is generally applicable to roadways, railroad, and most other noise
sources. For more accurate calculations and for situations not
covered here, the reader is referred to previously mentioned references.
(3,4,5)
It will be necessary to have a scaled sectional drawing of the
barrier, observer, and source as shown in Figure Bl. Elevations and
distances needed to construct this drawing can be obtained from the
site plan drawing for the highway or railroad in question.
In Figure Bl, distance OR is the observer's ear height above the
ground. While this is often about five feet, the observer may be at
a second story window. Distance SN is the height of the noise source
above the roadbed or pavement. For automobiles, assume the noise source
to be at the pavement level, or SN - 0. For trucks and railroad line
operations, the distance SN can be taken as eight feet or the approximate
height of the exhaust stack.
1 - Connect the observer and the source with a line called the
line of sight as shown in Figure Bl. Scale the distance H, which is
the perpendicular distance from the line of sight to the top of the
barrier (not necessarily the barrier height). -
2 - From the section drawing (Figure Bl), scale distances D_,
B
the horizontal distance from the observer to the barrier and DR, the
horizontal distance from the roadway centerline to the barrier.
B-l
-------
Observer
C/
ine of Sight
Existing
rouna
.^Barrier
Source
FIGURE Bl - Section Viev/ showing Barrier Paranoters
Centerline of Roadway
Observer
FIGURE B2 - Plan View of a Roadway Partially Shielded by a Barrier
B-2
-------
3 - Compute the factors H2/DR and H2/DB.
4 - With the computed value of H^/D^, enter the horizontal
axis of Figure B3. Now move upward (vertically) to the proper
H^/DB curve. Note that it may be necessary to interpolate between
the curves given in Figure B3, How slide horizontally to the vertical
axis and read the adjustment in decibels for the barrier.
In the case of highway noise it will be necessary to determine
separately an adjustment for the automobile and truck traffic. This
is because trucks and automobiles have different heights SN above the
pavement. So the adjustments are different and they have to be separately
applied to the truck noise and automobile noise before the latter are
added to give the total roadway noise.
In the previously described method, it was assumed that the
observer was shielded from the entire roadway. Figure B2 is a plan
view of a barrier that is not long enough to shield the observer from
the entire length of roadway. In this case it will be necessary to
obtain the angles a and 3 from the site plan using a protractor. Often
it will be sufficient to estimate the angles in the field, Then Step 5
is completed as follows:
5 - Compute the ratio, a divided by 3 . Enter Table Bl with
this ratio and the adjustment in decibels from Step 4. This table
gives the final barrier attenuation in decibels. Note that if the
ratio of a/3 is 1.0, or the barrier runs the full length of the
roadway, then the adjustment determined in Step 4 does not change.
B-3
-------
DO
h-
O)
Q
10
0
-10
-20
i
0.01
Figure'BS - Barrier Adjustment in dB from the Barrier
- Parameters, DR, DB, and H. '
B-4
-------
Adjustment ]
from Step 4 ' 0
0.1
0.2
0.3
Ratio
0.4
d/B
0.5
0.6
0.7
0.8
0.9
1.0
-5dB
-lOdB
-15dB
; 0
0
i
i
0
0
0
0
-1
-1
_-j
-1
-1
-2
-1
-2
-2
-2
-3
-3
-2
_3
-4
-2
-4
_5
-4
-6
-7
-4
7
10
-5
-10
-15
Table Bl.- Barrier Attenuation corrected for
Length of Barrier
B-5
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