vvEPA
905280003
HANDBOOK FOR
MOTOR VEHICLE NOISE ENFORCEMENT
By Gale R. Hruska, Noise Control Specialist
U.S. Environmental Protection Agency
Region V Noise Program
Chicago, Illinois 6O6O4
U.S. Cnvtrohtnental Protection Agency
Region 5, Library (PU2J)
77 West Jackson Boulevard, 12th Floor
Chicago, It 60604-3590
Illustrations by Robert J. Nagel
U.S. EPA, Region V
Graphic Arts Section
-------
Introduction
777
Often a police officer's only introduction to noise enforce-
ment is when someone puts a sound level meter in one of his
hands and a copy of the town noise ordinance in the other,
and he is told to "go out and quiet those %*!!!&*!
motorcycles."
The purpose of this booklet is to provide some elementary
instruction in the fundamentals of motor vehicle noise
enforcement. The three main topics to be covered are: (1)
sound and its characteristics, (2) the sound level meter, and
(3) enforcement procedures.
-------
Sound Pressure And Wave Motion
What is sound? Let's use a balloon to find out.
1-2 When one blows up a balloon he uses his lungs to force
air into the balloon. This causes the balloon skin to expand
into its stretched out shape. The air in the balloon is now
under pressure.
3 If we squeeze the balloon in the middle, what happens?
The balloon bulges out at the ends and the pressure inside
the balloon increases. When the balloon is released it pops
back to its original shape at its original pressure.
4 Suppose the balloon were very long and someone
squeezed it at one end. What would we observe at the other
end? First we would notice that nothing happened for a short
period after it was first squeezed, then, just like the small
balloon, the pressure would increase. What is happening is
that the excess pressure caused by the squeeze is traveling
down the tube at a speed of about 1 200 feet per second. This
excess pressure is commonly termed a sound wave. If the
squeeze were released, a decrease in the pressure would
travel down the balloon in the same manner. To convince
yourself that these actions actually produce sound waves,
burst the balloon with a pin.
-------
H
Z
111
s
111
u
Q.
(A
Weight
niiiiiiiiiiiiiiiiiiiiiiiii
AMPLITUDE (A) - The Maximum Displacement Of The Weight
FREQUENCY - The Number Of Times Per Second The Period Occurs
How do we describe sound? This time let us look at
something that appears not to have anything to do with
sound at all: a weight hanging from a spring.
If we pull the weight down a distance (A) from the point it
naturally hangs, then release it, we see an interesting
phenomena. The weight starts returning toward the rest
position, and goes through it until it reaches a point as high
above the rest position as it was pulled below it. The weight
then starts down until it reaches the lowest position, where
the process repeats itself again, and again, and again . . .
We call the maximum displacement from the "at-rest"
position the AMPLITUDE, and the time it takes to go
through one complete cycle (from down to up to down) the
PERIOD of the vibration. The number of periods that occur
in one second is called the FREQUENCY. The units of
frequency were once called cycles per second but are now
called Hertz. They are abbreviated "Hz".
-------
Air
••
• •
v.
• w
• • <
•
• •
• •
Loudspeaker
Compression Compression Compression
Expansion Expansion
What does the vibration of a spring have to do with sound in
air? Let's look at a hi-fi loudspeaker emiting a singletone. As
the speaker cone moves forward and backward like the
spring, it alternately compresses and expands the air in front
of the cone. The compression and expansion then moves out
away from the loudspeaker as a sound wave.
-------
Single Frequency Sounds
Electric Motor
Horn ;
Whistle
I
I
10
20 =
5O
100
200 =
5OO
1OOO 20OO
= 5000
10,OOO HERTZ
i
N
X
in
N
0)
N
x
o
o
CM
Piano Keyboard
There are a number of common sources of sound that act
much like a spring because they cause a single frequency
sound to be produced. The keys of a piano are a good
example. Pressing the middle C key causes its string to
vibrate about 260 times per second. The vibrating string and
soundboard cause the air adjacent to it to compress and
expand with the same frequency. Just as with the balloon,
the changing pressure moves outward as a sound wave.
Other examples of tones are the hum of a motor (60 Hz) and
the sound of a police whistle (3500 Hz).
-------
Random Motion - Noise
in
*
in
o
0.
(0
5 _
iiiiiiiiiiiiiiiiiiiiiinii
TIME
Look at the spring again. Suppose that instead of just pulling
the spring down and releasing it, there is an invisible hand
which randomly either pulls or pushes on the weight at
different times. The way the spring moves might be
something like the diagram shows. (Sound can behave in
this random manner also - consider rock and roll music.)
How do we describe its motion?
Certainly there is no single frequency or amplitude with
which to describe the motion as in the previous case.
Fortunately noise such as this can be shown to be composed
of many single frequency components, each having its own
amplitude.
•a
3
4*
"5.
E
Tone
II
3
4*
"a
Noise
Xlfllfl
A
Frequency
Frequency
-------
Motorcycle At 40 mph
ID
•O 120
Automobile At 4o mph
0)
0)
0
M
M
2
Q.
•D
O
0)
110
100
90
80
70l
0 20O 4OO 6OO 8OO 1000
Frequency (Hz)
0 200 400 6OO 80O 1OOO
Frequency (Hz)
As an example of sound with many frequencycomponents,
let us look at motorcycle and automobile noise. Here we see
typical sound pressure components (which are equivalent to
amplitudes) for motorcycles and autos. Notice that the levels
for motorcycles are greater than for automobiles. They also
contain components that are higher in frequency. These are
two reasons why motorcycles annoy people more than
automobiles.
8
-------
Number
1O
63
1OO
1OOO
NUMBER
O 1 1.8 2 3
Logarithm of the Number
LOGARITHM
RULE
1 )log(a x b) log(a)+ log (b)
2)log(a/b) log(a) - log (b)
EXAMPLE
log(10 100) = log(10) + log(1OO)
-1+2 = 3
= log(1OOO)
log(1000/1OO) = log(1OOO) - log(1OO)
= 3-2 = 1
= log (10)
How do we describe the volume of the sounds we hear in
everyday life ? This can be a problem since the lowest sound
pressures the ear can detect are more than a million times
less than those that we hear when a jet takes off. To make
the numbers manageable we need to consider the concept of
the DECIBEL. To do this we must first look at logarithms.
Instead of defining logarithms to the base 10 as was done in
school, we shall investigate them by examining some of their
properties. As we can see from the chart, the logarithm of
whole numbers that are powers of ten ( 1, 10, 100, 1000,...)
are smaller whole numbers such that they indicate the
number of zeros that were in the original number (0, 1, 2, 3,
...) As one might guess, a number in between two
consecutive powers of ten has its logarithm between the two
corresponding consecutive whole numbers. For example, 63
is between 10 and 100, therefore the logarithm of 63 is
between 1 and 2 (1.8 to be exact). The values of logarithms
are usually found by either looking them up in a table or
using a calculator.
Two important properties of logarithms are given in the
table. When two numbers are multiplied together the
logarithm of the product is equal to the sum of the logarithms
of the individual numbers. When two numbers are divided
the logarithm of the quotient is equal to the difference
between the logarithms of the individual numbers. Thus we
see that the multipying of real numbers is equivelant to
adding their logarithms, and division of real numbers is
equivelant to subtracting their logarithms. 9
-------
Sound Pressure Level (SPL) - Decibels
Definition
SPL=2O Logio (Measured Sound Pressure / Reference Pressure)
Reference Pressure (Pref) = O.OOOO2 Newtons / (meter)2
= O.OOOOOOOO294 Ib./in2
Sound
Whisper
Jack Hammer
Jet Take-Off
Examples
Sound Pressure
31 xRref
12,600 Pref
1,000,000 x Pref
Sound Pressure Level
30 dBA*
82 dBA
12O dBA
decibels using A-weighting Network (to be described later)
Let's get back to sound. Because the sound levels we
encounter in daily life can vary over such a wide range,
talking about sound pressure in units such as pounds per
square inch would be unweildy. To remedy this situation we
define the SOUND PRESSURE LEVEL (SPL) as:
SPL=20x|ogarithmo(measured sound pressure/reference
pressure)
The reference pressure used for environmental noise turns
out to be the lowest level sound that a person with normal
hearing can detect. The unit of sound pressure level is called
the DECIBEL.The figure shows the sound levels produced by
three common sound sources.
Logarithms (?), decibels (?) - does all of this complicated
jargon mean that a police officer will have to have a degree in
mathematics ? NO ! All enforcement equipment is calibrated
directly in decibels, so no calculations are involved. The
police need only know three specific decibel numbers which
will help him in enforcing the noise ordinance.
10
-------
Sound Level And Distance From The Source
( L 6 )dB
RULE OF THUMB: Doubling The Distance From A Source
Decreases The Noise Level By 6dBA; Halving The
Distance Increases It By 6dBA.
QOdBA 84dBA 78dBA
5 Feet
10 Feet
2O Feet
What happens to the sound pressure level as you move
toward (or away from) a sound source. Most people know
that the noise level increases as you get closer and
decreases as you move away. A good rule of thumb to follow
is that the noise level decreases by about 6 decibels each
time the separation between the source and receiver is
doubled; it increases by about 6 decibels each time the
distance is halved.
As an example, suppose you measure some motorcycle
noise at 84 dBA when you are 10 feet from a vehicle. (The
"A" in "dBA" refers to the decibel value measured using a
particular weighting network. This concept will be covered
when we talk about sound level meters.) What levels would
you expect to measure at 5 feet ? At 20 feet ? At 40 feet ?
(Answer: 90 dBA, 78dBA, and 72 dBA respectively)
Notice also that the sound pressure level rises at a faster rate
as you move closer to the vehicle. It takes only 5 feet to
increase the level by 6 dBA (starting at a 10 foot separation),
but it takes a 10 foot change for the level to decrease by 6
dBA. The importance of this observation is that the police
officer should make sure he is at least as far away from the
vehicle as the ordinance requires. It is better to be a little too
farthan a little too close; less error will result, and what error
does occur will favor the violator.
Remember: 6dBA PER DOUBLING OF DISTANCE
-------
Two
Vehicles
Each
Vehicle
Alone
Produces
SOdBA
Both
Vehicles
Running
Together
Produce
83dBA
SOdBA
Rule of Thumb: Each time the number of (identical) noise
sources is doubled the SPL is increased by 3dBA. Each time
the number is halved, the SPL is decreased by 3dBA.
Suppose we have two identical automobiles, each alone
producing 80 dBA noise levels. What sound pressure levels
do we measure if we run them at the same time ? One might
naively reason that two times eighty dBA is 160 dBA.
Unfortunately this answer is wrong; we cannot add decibels
directly to get the overall effect. The correct answer is 83
dBA.
The correct answer is obtained by using the following rule of
thumb: Each time the number of (identical) noise sources is
doubled, the sound pressure level increases by 3dBA; each
time the number is halved, the sound pressure level is
decreased by 3dBA.
12
-------
1 Source
SOdBA
2 Sources
83 dBA
4 Sources
86 dBA
8 Sources
89 dBA
How does the 3dBA rule help the police officer ? Consider
this example. Suppose your noise ordinance has an 80 dBA
noise limit. You cite a violator for causing an 89 dBA noise
level and the case comes to court. The judge asks you how
loud 89dBA is. Knowing this rule of thumb you are able to tell
him that 89dBA is the same noise level that would be
produced by 8 vehicles, each one producing the maximum
allowable limit of SOdBA. Cased closed!
Remember: 3dBA FOR EACH DOUBLING OF IDENTICAL
SOURCES.
13
-------
Effects Of Additional Noise Sources
Violator Level
9O dBA
V N
Ambient Level
(all other noises)
80 dBA
Ambient & Violator Level
9O.4 dBA
RULE OF THUMB: Violators Shall Not Be Given A Citation For A Noise
Violation Unless The Level Measured When The Violation Occurs Is At
Least 10dBA Above The Ambient Noise Level.
Athird rule of thumb to remember involves the contribution
to the overall level of all of the other vehicles and noise
sources present at the time a violator is cited. This
extraneous noise is called the AMBIENT LEVEL. A violator
might asK'There were a lot of other cars on the road when
you caught me, so how do you know that they didn't cause
the reading to be too high ?" The rule that applies here is: a
violator shall not be cited unless the level measured when
the violation occurs is at least 10 dBA above the ambient
noise level immediately before the violation.
If the above condition is satisfied, then the additional noise
caused by all the other sources producing noise will add less
than 0.4dBA to the level produced by the violator.
The rule also implies that if there are several noisy vehicles
traveling together and you can't separate the noise made by
each separately, you will have to let them all go free and
hope to catch them singley some other time.
Remember: THIS AMBIENT LEVEL MUST BE AT LEAST
10dBA BELOW THE LEVEL MEASURED WHEN THE
VIOLATOR PASSES BY
14
-------
Reflection Of Waves
Reflected Wave
Transmitted Wave
Incident Wave
The final rule to remember deals with the reflection of sound
from large objects. Let us look at what happens when a
water wave encounters an obstacle.
Throw a stone into a pond. This wave acts very much like a
sound wave as it travels outward in an everwidening circle. If
the water wave encounters an obstacle, you see part of the
wave reflected back in the direction it came from. When we
throw stones into the pond one after another we see some of
the incoming waves interacting with the reflected waves. If
we were to make measurements of the wave heights (which
would be equivalent to the sound pressure in air) we would
find that the height at a point close to the rock would be
quite different than if this obstacle were not there. Asimiliar
phenomena occurs in air when large objects are situated
near a point where measurements are being taken.
-------
Sound
Source
2d
Sound
Level
Meter
Sound
Source
3dBA Error
Sound
Level
Meter
Approximately 1 /2 dBA Error
RULE OF THUMB: Keep At Least As Far From Any Reflecting
Object As You Are From The Vehicle Being Measured.
Look at what happens when we make measurements near a
large wall. If we put the sound level meter right next to the
wall, the value we read will be approximately 3dBA greater
than if the wall were notthere. However, ifwemovethewall
back until it is the same distance from the SLM as the SLM is
from the sound source, we find that the reflected wave will
now only cause about 0.5 dBA error. Thus we have a rule of
thumb: when making measurements keep at least as far
away from any large reflecting objects as you are from the
vehicle being measured.
How large is a "large object"? It depends on the frequency of
the sound. For automobile noise anything larger than your
body can be considered as a large object.
When a SLM is mounted above a patrol car window it will
measure some of the reflected from the patrol car itself.
Since we are "stuck" with the position of the SLM, we will
have to give a little extra leeway to the enforcement sound
levels. The exact amount will be discussed later.
Remember: KEEP AT LEAST AS FAR FROM ANY LARGE
REFLECTING OBJECTS AS YOU ARE FROM THE VEHICLE
YOU ARE MEASURING
16
-------
Ground
BAD
RULE OF THUMB: Measurements Should
Be made At Least Three Feet
Above The Ground.
RIGHT
When considering sound reflection you may ask, " What
about the reflection from the ground ?"
The noise level limit stated in an ordinance takes into
account the fact that the noise heard by the receiver consists
of sound that is reflected from the ground to the receiver as
well as the direct wave. Normally there should be no
concern; the exception is when the SLM is close to the
ground. In this situation sound from the exhaust pipe, which
is also close to the ground, travels close to the ground along
the whole path. The result is that the sound waves become
distorted producing unexpected results. Therefore a good
rule to remember is:
ALL MEASUREMENTS SHALL BE MADE WITH THE
MICROPHONE AT LEAST THREE FEET ABOVE THE
GROUND.
WRONG
17
-------
The Sound Level Meter - Introduction
Sound Source
Sound Level Meter
I
Microphone
Electronic
Network
Visual
Display
Transforms Sound Modifies Signal Communicates Results
Signal Into Electrical To Give Meaningful To Person
Signal Results
Now that you know all about how sound behaves, it is time to
learn how it is measured. The most common device used in
noise ordinance enforcement is the SOUND LEVEL METER
(SLM). This device performs three basic operations. First it
uses a microphoneto convert the energy in the sound into an
electrical signal. An electronic network then conditions the
signal to provide meaningful results. That's all that you will
probably need to know about how the SLM physically
operates. More important to the enforcement officer is that
he must know how to use the instrument correctly. This is
what we will concentrate on.
1ft
-------
Sound Level Meter Features
Microphone
Meter Display
Power Switch.
Battery Test.
Meter Response.
Weighting Network.
Maximum Hold •
ON DiOFF
BAT TEST
RESPONSE
SLOWBI FAST
ATTN
50 6O 70
WEIGHTING
LIN
MAX HOLD
•* CAL
120
Attenuator
Calibration Adjustment
This illustration depicts a sound level meter having features
found on many SLMs used in motor vehicle enforcement.
Your particular SLM may not have some of the features,
and others of them may be incorporated into the instrument
so that no switch is necessary. You may also have some
special features not discussed here.
-------
Sound
Waves
Thin
'Diaphragm
Back
Plate
Drum
Microphone
Electrical
Signal
Microphone
An important part of the SLM is the MICROPHONE. The
microphone works like a drum in reverse. When you hit a
drum the drumhead vibrates causing the air in front of it to
alternately compress and expand. As you know from the
previous discussion, this causes a sound wave to be
propagated outward toward your ear.
When a sound wave hits a microphone it causes the
diaphragm (which is usually a metal foil only a few
thousandths of an inch thick tightly stretched over the front
of the housing) to vibrate like the drumhead. The diaphragm
also happens to be part of an electrical circuit; its vibration
produces electrical signals in the SLM circuitry which are
proportional to the sound pressure causing the vibration.
20
-------
Normal Incidence Sound
• Grazing Incidence
Sound
n
o
EH
o
RULE: Always follow Manufacturer's
recommendations with regard to the
preferred orientation of the Sound Level
Meter.
Auto
Readings Almost
Identical
Whistle
Readings
Different
An important question to answer is " How should I hold the
SLM ?" Should the microphone be pointed at the noise
source (normal incidence) or should the face of the
microphone be oriented at some other angle such as at a
right angle to the sound wave (grazing incidence)? The rule
here is to follow the manufacturer's recommendations as
stated in the operator's manual. For most (but not all) SLM's
the preferred direction is at grazing incidence.
For low frequency sounds such as those produced by
automobile exhaust systems, the readings at the two
orientations will vary hardly at all. However, if you try
measuring the sound level of a high frequency source such
as a siren or a whistle, the sound level measured will depend
very much on the angle at which the sound hits the
microphone.
Here is an experiment you should try. Place your SLM off to
the side of the patrol car and then change the orientation of
the microphone while the car is idling. See what changes
you can detect in the reading asthe microphone is pointed in
different directions (while keeping the distance from the
tailpipe the same of course).
21
-------
Sound Level Meter Weighting And The Sensitivity Of The Ear
-5-
-10-
<•"»-21
m
>-30-
t -40-
M
SJ-50.
More Sensitive
10
Less
Sensitive.
10
I
20
I
50
I I
100 200
I I I I I
500 1000 2000 5000 10,000
FREQUENCY (Hz)
The Ear
Let us consider the switches on the SLM. The use of two of
them, the "on-off" switch and the "battery-test" switch, are
self explanatory.
Consider the "weighting" switch. Some SLM's give you a
choice of A,B,C,D, and linear settings. Which one should you
use? The answer is, "The one your ordinance says to use".
Virtually all motor vehicle noise ordinances today use A-
weighting network, so A-weighting is most likely the one
you'll be interested in.
What is an A-weighting network? Consider the ear. The
graph on the left side shows that the ear is not equally
sensitive to sounds of all frequencies. For example, people
are much more sensitive (and therefore much more
22
-10-
m
TJ
o
-20-
-30H
O
ui
-40
Linear Weighting
A-Weighting
10
20
50 100
FREQUENCY (Hz)
^ I I I 1 I
200 500 1000 2000 5000 10,000
The Sound
Level Meter
annoyed) by noise having significant high frequency
components than they are by lower frequency sound. This is
why a siren is designed to have such a high pitch. The A-
weighting network shown by the second graph virtually
duplicates the ear's sensitivity by descriminating against
low frequency noise. At 100 Hz the network subtracts 20 dB
from the incoming signal before sending it through the rest
of the SLM. At 1000 Hz nothing is subtracted.
The linear weighting network considers all frequencies
equally important and does nothing to alter the signal. It is
used in situations such as when low frequency noise causes
building vibrations. The B,C,and D networks are not
commonly used in enforcement situations.
-------
Errors Caused By Using The Wrong Weighting Network
Example: The Automobile
-------
k.
(A
" O\ ^"jh
ArS ~ /cS
7 ..____. f . 2
F ' f " | ' P
! 1 i lnS*«
\ ' \
/ X Time-^-
^ f
Instantaneous SPL
o
\
Averaging Time
RMS Average SPL
Time
Another setting on the SLM is "meter response". If theSLM
responded to instantaneous sound pressure what would
happen? Looking at a 1000 Hz tone, the meter pointer would
rise and fall 1000 times per second as it followed the sound
pressure. All we would see is a blur.
In a real sound level meter the instrument displays an
"average" sound pressure level by smoothing out the
pointer motion. The technical term for the way the averaging
is done is "RMS averaging". The "fast" and "slow" positions
determine over what length of time the averaging occurs.
24
-------
Fast
Response
Slow
Response
-Xf"^ I ITfy^f
/ A £ A
f^ \ /^ \
True Value
Of Maximum
Reads Lower
Than True
Maximum
How does the meter response affect the SLM reading during
a typical vehicle pass-by? As the vehicle approaches the
microphone, the pointer moves up with the increasing noise
level. As the vehicle moves away from the observer, the
pointer moves down in response to the decreasing noise
level. The enforcement officer notes the maximum level
reached by the pointer. This procedure works reasonably
well with the "fast" setting. If we are in the "slow" response
mode and the vehicle is traveling fast, the pointer may never
reach the true maximum level before it starts decreasing.
The slow response is used when we measure more or less
stationary sources that do not change greatly in level over
short periods of time.
The important points to be made are that (1) fast response is
preferred for motor vehicle enforcement since it best
corresponds to the true maximum noise level that a violator
causes, and (2) the level that is measured with the slow
response will either be less or equal to that measured with
the fast response. Thus if you are citing a violator and
discover that the SLM is set for a slow response, give the
ticket out. The true level was at least as great as what you
read on the meter.
25
-------
Impulsive Sound Measurements
Balloon
Burst
Auto
Backfire
3
(A
(A
0)
•o
c
3
O
Less Than
One Second
Measured
Sound
Pressure
Level
Impulsive Sounds
True Peak
Sound
Pressure
Level
26
RULE: The sound level meter will
always read less than the true
maximum value for an impulse noise.
This is a good time to discuss impulsive sounds, such as a
balloon bursting, a gun firing, or a automobile backfiring.
These sounds are characterized by a very rapid increase in
the sound level and only a slightly longer time for the level to
decrease back to the ambient level. Are SLM's able to
measure these sounds? The answer is "not very well". As
with the vehicle pass-by, the duration of the sound is too
short to allow the SLM to reach the true level, even on the
fast response. It will always read less. You can still issue a
citation in this situation because the level you are reading is
less than the true maximum level that is occuring. In other
words, the violator is causing a greater amount of noise than
you are actually citing him for.
-------
Sound Level Meter Displays
Analog
Digital
Moving Pointer
6O
SO
Changing Display
Max Hold
Switch Which
Allows Holding
Of Maximum
Noise Level
Two types of SLM display are available on SLM's today -
digital and analog. Each has its own advantages and
disadvantages.
The analog display consists of a pointer which moves across
a scale corresponding to the sound level being measured.
This type of display allows one to keep tab on the ambient
level. Its primary disadvantage is that you have to watch it
continuously in order to obtain the maximum level.
The digital display presents the levels directly in decimal
numbers and doesn't require one to read scales. Most digital
displays update their readings five or more times a second,
which makes it hard to read the numbers. Thus a necessity
with the digital display is a maximum hold option which
displays the maximum value the SLM has measured during
the time in which you were interested in.
27
-------
Without
Attenuator
With
Attenuator
60™ 80
Attenuator Switch
A switch found only on analog SLMs is the attenuator. This
switch changes the range of the SLM measurements.
Without an attenuator the display might have to cover
perhaps 70dBAon one scale. Because of the large range,
accurate readings would not be possible.
The attenuator allows the same scale to measure only 10
dBA. Then instead of being able to readthe levels to within 1
dBA, reading to within 0.1 dBA could be possible. The price
that is paid for this precision is that one has to manually
change the attenuator so that the reading stays on scale.
Many measurements have been lost when the readings
went off the scale before the attenuator setting was
switched. Digital displays do not have attenuators, which is a
point in their favor.
28
-------
Set Sound Level
Exceedance Value
Monitoring
Traffic Inside
Vehicle
Ordinary
Traffic -
No Response
Loud Vehicle
Sets Alarm Off
Store Noise
Level; Stop
Further
Reading
Stop Vehicle.
Issue
Summons
Clear Reading.
Reset SLM
To Read Levels
New innovations are appearing in SLMs, making their use
easier and more convenient for motor vehicle enforcement.
Several SLMs now allow monitoring from inside a patrol car
at the same time an officer is doing radar speed enforce-
ment.
One SLM works inthefollowing manner. The officer sets the
level that is going to be enforced into the SLM memory. With
the microphone mounted outside and above the patrol car
window, he monitors his radar or does some other non-noise
related task such as eating lunch. As ordinary traffic passes
by nothing happens. When the noise enforcement level is
exceeded an audible alarm is triggered. The officer then
throws a switch to store the maximum level that occured and
to stop any new loud levels from being recorded. He chases
the violator and issues him a summons. The officer can even
show the violator the SLM reading that caused the violation.
29
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Sound Level Meter Calibrator
Calibrator
An often asked question is, "How do you know your SLM
was reading correctly ?" In order not to be embarassed you
should calibrate the SLM ahead of time using, not to
anyones surprise, a calibrator.
A calibrator consists of a small cavity at one end of which a
small loudspeaker is mounted. At the other end of the cavity
there is a hole in which the SLM microphone can be
inserted. When the calibrator is turned on, the loudspeaker
produces a tone of known frequency and sound pressure
Tone
Generator
Loudspeaker
Air Volume
Microphone
level. To calibrate the SLM, the SLM calibration screw is
turned until the SLM reads the same sound pressure level
that the manufacturer states the calibrator produces.
This is the most convenient check of SLM operation. If the
SLM can be calibrated with the calibrator you can be almost
certain that it is operating correctly. However it is
recommended that both calibrator and SLM be returned to
the manufacturer or to a calibration laboratory every year or
two for a complete check.
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Sound Level Meter Calibrator Use
YES
OK Company
Calibrator
OK Company
Sound Level
Meter
Fly-By-Night
Company
Calibrator
OK Company
Sound Level
Meter
RULE
Use only calibrators approved by
the manufacturer.
There are two basic rules to follow when calibrating a SLM.
First, use only a calibrator that is approved for your particular
SLM. One company's calibrator does not necessarily
produce the correct sound pressure when used with a
different company's SLM. Secondly, be sure that the SLM
YES
Sound Level
Meter
Sound Level
Meter
RULE
Make sure the calibrator is correctly coupled
to the sound level meter.
and the calibrator are correctly coupled. If the calibrator
is not inserted correctly, the air volume connecting the
loudspeaker and microphone may be too large and cause the
sound level in the cavity to be lower than the correct value.
•91
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Except under extreme conditions, Temperature, Barometric Pressure, and
Humidity have little effect on the accuracy of the SLM AS LONG AS IT IS
CALIBRATED UNDER THE SAME CONDITIONS IT IS USED IN.
How do the temperature,barometric pressure,and humidity
affect the SLM operation and accuracy? As long as the SLM
is used under the same conditions as it was calibrated at,
and these conditions are within the manufacturer's
specifications, no significant errors are to be expected.
However, if conditions do change between calibration and
use, significant errors can result.
The above statement implies that you should not calibrate
the SLM in Death Valley and then take it up to the top of Mt.
Whitney to make measurements and expect accuracy.
Similarly in a more common situation, you should not
calibrate the SLM in your nice warm patrol car and then use
it outside in the damp cold. Bring the SLM and calibrator
outside and let them reach the same conditions as the
environment; then proceed with the calibration.
32
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1OO
m
•o
80
> 60
•O
§
O
40
_ Wind Noise
Without
Windscreen
Wind Noise
With
Windscreen
20 4O 6O
Wind Speed (mph)
so 100
Does wind affect SLM accuracy ? Yes! For example, a 25 mph
breeze blowing across a typical SLM microphone can cause
readings of SOdBA. It is for this reason we place a plastic
foam ball called a windscreen over the microphone. The
windscreen attenuates the noise caused by wind by
approximately 25 dBA, yet it does not noticeably affect the
level caused by noise. The windscreen also protects the
microphone from rain, dirt, birds, etc; it should normally be
used at all times.
33
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Sound Level Meter Classification (ANSI)
TYPE
Designation
Estimated overall
field accuracy
Cost
Suitabilty for
motor vehicle
enforcement
Precision
1dBA
Expensive
Over-engineered
for most community
problems involving
vehicle noise
General Purpose
3dBA
Modest
Adequate for
motor vehicle noise
and most other
community
problems
Survey
5dBA
Cheap
Not sufficiently
accurate
How accurate are sound level measurements ? To answer
this question we refer to the American National Standard
Institute (ANSI) which has set (voluntary) standards for
three types of sound level meters. This chart demonstrates
some of the characteristics of those SLMs that meet ANSI
specifications. When buying a SLM you should make sure
that the manufacturer certifies that the SLM meets the
specifications of the type which you are interested in.
The SLM type most commonly used in motor vehicle
enforcement is Type 2. We see that when used for motor
vehicle enforcement the estimated accuracy is 3dBA. In
other words, if alt of the procedures recommended in the
manual are followed, it is extremely unlikelythatthe reading
will be in error by more than 3dBA.
In order to take into account such possible errors, we
suggest that actual enforcement of the noise ordinance
begin at sound levels greater than 3 dBA above the legal
limit. This is similiar to enforcing a 25 mph speed limit only
when the violator exceeds 35 mph; a margin of error is
allowed. If the SLM microphone is mounted on a patrol car,
you should allow another 2 dBA leeway and enforce the
ordinance 5 dBA above the legal limit.
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Noise Violation Form (Sample)
NOISE VIOLATION FORM
Date
Officer
Equipment: Sound Level Meter
Procedure
Set meter response to "Fast"
Set meter to A-weighting
. Calibrator.
Check calibrator battery: Before
Check sound level meter battery: Before
Calibrate sound level meter: Before
Attach windscreen
Describe weather
Ambient Noise Level
After.
. After.
. After.
_dBA
Noise Violation
Time
Violator's Name.
Vehicle
. Driver's License Number.
Maximum Measured sound pressure level
Location:
-License Plate Number_
_dBA (Ordinance Limit 80 dBA)
Distance from center of traffic lane
Distance to nearest large reflecting object
A strong case can be presented in court if the violation is well
documented. This figure illustrates a comprehensive
violation form. It is a good idea to include a few words on
what in your opinion causedthe violation. For example "hole
in muffler", "glass packs", "heavy acceleration caused tire
squeal" are phrases that can be used. If your noise
ordinance has exhaust system regulations in addition to
noise limits, you will often be able to cite a violator under
both provisions. This can be especially effective when you
come up before the judge.
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ON
Allow the SLM to come to the same temperature as the
surroundings. Do not make measurements when it is
raining or snowing, or when the temperature is lower
than the manufacturer's recommendation.
Turn the SLM on. Let it warm up for 30 seconds.
Check the Battery.
Fast
Response
Set SLM to fast response.
Battery Check Swjth
A
Weighting
Set .SLM to A-weighting.
*- Correct
Mounting
6
Turn on calibrator. Check Battery.
: 0
Calibration
^, Adjustment
7
Set SLM attenuator to correct range.
Make sure calibrator is mounted
correctly. Adjust SLM to give correct
reading.
The following four pages illustrates a procedure for making
motor vehicle noise enforcement. It's simple, isn't it ?
36
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windscreen
8
Attach windscreen
Measure the correct distance from the SLM
measurement point to the traffic lane (A) and the
distance to the nearest large reflecting object (B). Make
sure that (B) is greater than (A).
10
The SLM may be held at arms length, or
11
The SLM may be attached to a tripod, or
The SLM microphone may be mounted on a boom
on a patrol car.
37
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Enforcement Procedure Using The Sound Level Meter (continued)
more than
3 feet
13
Orient the SLM according to the manufacturer's
instructions so as to give a uniform frequency response.
For most SLMs, this occurs when the microphone
diaphragm is parallel to the ground.
14
Make sure the SLM microphone is at least 3 feet above
ground.
Enforcement
Noise
Level
Correct
Attenuator
Setting
15
Measure the noise levels that occur under normal traffic
conditions. These ambient levels should be at least
10 dBA less than the level to be enforced.
16
If the SLM has a manual attenuator set it so that
enforcement level noise will register on the meter face.
oo
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Enforcement Procedure Using The Sound Level Meter (continued)
Hand Held (or Tripod Held)
SLM Enforcement
Level
Ordinance Level
Patrol Car
Mounted SLM
Enforcement Level
17
Enforcement levels are 3dBA above the ordinance
limits for handheld SLMs, and BdBA above the limits
for patrol car mounted SLMs. This leeway will take
into account all common sources of error.
18
The noise level for which a violator is cited will be
the maximum value observed.
19
Record all pertinent information on a noise violation
form, including comments on the reason the vehicle
made so much noise. This is a necessity if the case
comes to court.
20
Recalibrate SLM after giving a citation to insure that
SLM was functioning correctly when violation was
measured.
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Don't enforce the motor vehicle noise
ordinance on grades exceeding 5%.
Don't make measurements when the
pavements are wet.
Don't use a hand held SLM by
sticking one's arm out of a vehicle
window.
The following three "don'ts" also apply to motor vehicle
measurements:
(1) Don't enforce the noise ordinance on steep grades.
Although you will catch more violators, it is not fair to
borderline vehicles that are forced to run at higher than
normal engine speeds just to make it up the hill.
(2) Don't make measurements when the pavements are wet.
Under these conditions tire noise becomes an important
factor in the total noise produced. The motorist does not have
much control over this type of noise.
(3) Don't be lazy and make noise measurements by sticking
the SLM out of the patrol car window with your hand.
Because of reflections from the car body and the effect of the
open window, you will leave open the possibility of
unaccounted errors.
40
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Tachometer
Sound Level Meter
Now that you have ticketed the violator, what happens next ?
The violatorcan go to court,pay the fine and then go on his
merry way. However, there are communities that feel the
main purpose of the motor vehicle ordinance is to quiet the
town and not just to collect fines. In this situation the judge
may say, "Get your vehicle into good shape, let the police
department test it, and if it passes I will throw out the ticket."
Now your problem begins; how would you test the vehicle ?
A simple method of exhaust system testing utilizes a
stationary test which can be performed in a very short time.
The only equipment needed is your sound level meter, a
small tripod, and an inductive tachometer. Since the test is
stationary, it can be administered in any parking lot.
This type of testing can be put to use in a public relations
program by offering "free" tests to any person wants to
check his vehicle for noisiness.
41
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Positioning Of Sound Level Meter - Stationary Test
SLM microphone at same
height as tailpipe
SLM at 45° angle to tailpipe
exhaust axis and 2O inches
from outlet
To administer the stationary test set your SLM up on a small
tripod with the microphone at approximately the same
height as the tailpipe. (This will save you from breathing the
exhaust fumes.) Position the SLM at a 45 degree angle from
the exhaust flow of the tailpipe and 20 inches from outlet of
the tailpipe.
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Stationary Test
Tachometer
Probe
DISTRIBUTOR
Spark Plug Wire
TACHOMETER
ENGINE SPEED
Autos and Light Trucks - 30OO rpm
Motorcycles - 1/2 red line value
Now place the tachometer probe around one of the spark
plug wires. Letting the driver see the meter face, ask him to
place the vehicle in neutral and then slowly step on the
accelerator until the tachometer reads 3000 rpm. For
motorcycles, which have their own tachometers, you will
ask the driver to run the vehicle up to 1 /2 the red line value.
Based upon the noise level limits in almost all ordinances, a
measured valueof 95 dBA or lesswill insure that the vehicle,
if driven reasonably, will not have any reason to worry about
causing a violation.
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Now you know everything about making noise measure-
ments. (Would you believe almost everything ?)The most
important part of the noise program only begins at this point.
To have an effective program the city should combine a
vigorous public awareness program together with visible
enforcement. Noise enforcement street signs are a
particularly good tool for public awareness. For a full sized
n that,hyasfpy.nd wide acceptance write to
U.S. Environmental Protection Agency, Region V Noise
Program 5AHWM, 230 S. Dearborn Street, Chicago, Illinois
60604, (312) 353-2202.
After the above program has been in effect for a while, you
can expect the environment to be significantly quieter and
the time needed to be spent monitoring for violations can be
relaxed.
44
U.S. GOVERNMENT PRINTING OFFICE: 1980654-110
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