55019761
about
U.S. ENVIRONMENTAL PROTECTION AGENCY
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Olll "\ ^
SOUN
v_y /
U.S. ENVIRONMENTAL PROTECTIONoiCY
OFFICE OF NOISE ABATEMENT
CONTROL
20460
MAY 1976
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CONTENTS
Page
Basic Properties of Sound Waves 1
Intensity and Loudness 3
Human Hearing and Acoustics 9
BIBLIOGRAPHY OF SOUND AND NOISE 15
GLOSSARY 26
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ABOUT SOUND
This booklet is Intended for anyone requiring a knowledge of, the
fundamentals of acoustics and noise. It provides enough detail to allow the
reader to become familiar with the physical phenomenon of sound and how
it is propagated, described, and, to a certain degree, perceived. A bibliog-
raphy Is provided for those requiring more detailed technical information
on specific aspects of this expansive subject.
Basic Properties of Sound Waves
Sound waves occur in a medium having the properties of mass and
elasticity. For our purposes, this medium is considered to be air.
Air consists of gas molecules that are distributed fairly evenly and
that move around in a random fashion. Air exerts a pressure (atmospheric)
of about 14.7 Ib/in.2, which is roughly equivalent to 106 dynes/cm2. This
atmospheric pressure is directly related to the density (mass per unit vol-
ume) of the air.
Because air possesses both inertia and elasticity, sound waves can be
propagated in it. The inertia of air is due to its weight, 0.075 pound per
cubic foot. Elasticity is the characteristic that tends to pull a displaced
particle (the molecule) back to its original resting position. The transfer
of momentum, through molecular displacement, from the displaced mole-
cule to an adjacent one is the mechanism of sound wave propagation.
When a vibrating object moves outward it compresses a layer of air
surrounding it. This compression travels outward, dissipating in relation
to the energy that created it. As the vibrating object moves inward, the
surrounding air is rarefied. This rarefaction travels outward in a manner
similar to the compression. The result is therefore a series of alternating
compressions and rarefactions, in sympathy with the vibrations. This is
illustrated in figure 1.
The number of times per second that the wave passes from a period
of compression, through a period of rarefaction, and starts another com-
pression period is referred to as the frequency of the wave. Frequency
1
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:.t-..-v-
• • *«.fc • .
.-i:5.-.*.V
-Avjv;.
:&&'•
• "•«•>•
A
\7 Y7
Distance •
Figure 1
The upper curve in the figure shows how pressure varies
above and below average with distance at a given time.
The lower curve shows how velocity varies, above zero
(that is, molecules moving to the right) and below zero
(that is, molecules moving to the left). The distance (A)
between crests of both curves is the wavelength of the
sound.
is expressed in cycles per second, or hertz (Hz). The distance traveled by
the wave through one complete cycle is referred to as the wavelength
(Figure 2).
Wavelength in Air vs. Frequency
Wavelength U) in Feet
100
III
I
50
II 1 i 1
I |
20
I
1 I 1 1
10
li 1
1 1 II
5
iiil ii
1 I '
2
IMI
1 0.5
li i ii 1 i
II 11
0.2
i 1
1 I 1 1 1 1
0.1
1
10 20 50 100 200 500 1000 5000 10.000
Frequency in Cycles per Second (CPS)
Figure 2
2
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The rate at which the molecular displacement occurs is termed the particle
velocity of the wave, or displacement per unit of time. This is illustrated in
Figure 3. Under normal conditions of temperature and pressure, the velocity
of sound in air is 1100 feet per second.
Note: Wavelength vs. Frequency. The higher the frequency, the
shorter the wavelength and vice versa. A tone of 20 Hz has a wave-
length of 55 feet. (Wavelength x frequency = 1100 ft/sec in air).
14.0 16.8^ 19.6 22.4
Distance from Source in Feet
Figure 3
Intensity and Loudness
Sound propagated from a simple source radiates more or less equally
in all directions from that source, forming what might be called a sphere of
acoustic power. The power is expressed as watts/m2. Since the power
sphere increases proportionally with the increase in distance from the
source, the power per unit of area, or intensity, decreases because the con-
stant power quantity is being distributed over an expanding area. Figure 4
provides an illustration of this principle.
The acoustic power radiated from a source cannot be conveniently
measured in watts by instruments. However, the changes in atmospheric
pressure caused by the sound wave can be determined so as to provide
a meaningful measure. The fluctuation above and below the normal atmo-
spheric pressure is called the sound pressure and is the most common
measure of strength of a sound or noise. Sound pressure measure-
ments and the units of specifying intensity are the bases for:
1. Human hearing levels, since the ear is most sensitive to sound
propagated in air.
2. Noise levels of various noise producing sources.
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Loud ness
^•Point of Measurement
^ Direction of Travel
___ Unit of Area
Sound Wave
Distance from Source
Figure 4
3. Reference and calibration levels (pressures) for audiometers
and other such equipment.
4. Computation of power wattage levels.
The Decibel
The intensity of the faintest sound that the normal person can hear is
about 0.0000000000001 watts/m2; the intensity of sound produced by a
Saturn rocket at liftoff is greater than 100,000,000 watts/m2, a range of
100,000,000,000,000,000,000. There is a simpler way to represent these
numbers, however: scientific notation. In scientific notation, numbers are
represented as numbers between 1 and 10, multiplied by 10 raised to
some power. For example, the number 10 can be represented as one times
10 (1.0 x 10), the number 100 as 10 squared (1.0x102), the number 1000
as one times 10 cubed (1.0 x 103), and so on. In the same manner, 0.001
can be represented as 1.0 x 10-3 , I y 0.01 as 1.0 x 10"2 dra, etc. The
following list gives examples of numbers represented in scientific notation:
Number Scientific Notation
0.0025 2.5 x10'3
2.5 2.5x10° (10° = 1.0)
250 2.5 x102
2,500,000 2.5 x106
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Using scientific notation, we can represent the intensity of the faint-
est audible sound as 1.0 x 10~12 watts/ m2 and the sound intensity pro-
duced by the Saturn rocket as 1.0 x 108 watts/m2. Now, suppose we divide
all of the numbers in our range of values by the lowest one (1.0 x 10'12).
Our range now extends from 1.0 to 1.0 x 1020. Most of us would find it
difficult to use a measurement scale with such a large range. Fortunately,
we can use another mathematical notation to compress the scale of numbers
into one that is more comprehensible: the logarithmic scale. The logarithm
(log) of a number is the power to which 10 must be raised to produce the
number in question. For example:
103 = 1000 and log 1000 = 3.0
102-5 = 316andlog316 = 2.5
10° = 1.0 and log 1 =0
10°-3 = 2andlog2 = 0.3
By using the logarithms of our ratios, we can form a new measurement
scale in which an increase of 1.0 represents a tenfold increase in the ratio.
All of our ratios can now be represented between the numbers 0.0 and
20.0; i.e., log 1.0 = 0.0 and log (1.0 x 1020) = 20.0. The unit for these
types of measurement scales is the bel, named after Alexander G. Bell,
inventor of the telephone. Each 1-bel increase in the measure represents
a tenfold increase over the last measure. A bel turns out to be a rather
large unit, so for convenience, the bel is divided into 10 subunits called
decibels, abbreviated dB. Using a decibel scale our range is now between
0.0 and 200.0 rather than between 0.0 and 20.0. These kinds of scales
are standard in the electronics and acoustical industries for measuring
ratios of powers or quantities proportional to power such as voltage or
sound pressure. These types of measurements are called levels. By definition,
A level in bels = |og(Measured Quantity \
\Reference Quantity/
To express the level in decibels we multiply by ten. Thus,
. , - . / Measured Quantity \
A level m dec.bels = 10 l°g(Reference Quantityj '
As previously mentioned, sound pressure (SP) can be measured more con-
veniently and accurately than sound power. It can be shown mathemati-
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cally that sound intensity, or sound power, varies proportionally to the
square of the sound pressure.
Mathematically,
(Sound Intensity \
Reference Intensity jdB-
/ SP \2
Sound Pressure Level (SPL) = 10 logfgp—1 dB.
ref/
The log of a quantity squared is equal to twice the log of the quantity;
therefore,
cp
SPL = 20' '
The reference sound pressure (SPref) used in acoustics is 20 micropascals,
which ideally in air is the pressure equivalent of 10-12 watts/m2 (the
threshold of human hearing). Power levels or intensities can be computed
from sound pressure measurements. A 20-dB increase in the sound pressure
level (SPL) represents a tenfold increase in the sound pressure (SP). For
example, compare a measurement of 120 dB overall SPL to 60 dB overall
SPL. Although 120 dB is only twice the numerical value of 60 dB, the
sound pressure required to produce 120 dB is 1000 times the sound pressure
required to produce 60 dB.
There are a few rules to remember when using the decibel.
1. The decibel is used to express ratios. The reference quantity
must be specified.
2. In acoustics, the reference sound pressure is 20 micropascals.
3. Power levels are not easily measured but are usually computed
from measured SPLs.
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Combining Noise Levels
Because they are logarithmic, decibels are not additive. If two similar
noise sources produce the same amount of noise (say 100 dB each), the
total noise level will be 103 dB, not 200 dB. Figure 5 provides a guide
to the addition of decibels. The following example using Figure 5 as a basis,
provides an illustration of the way in which the noise levels of multiple
sources would be added.
J I
I
345678 9I(
Difference in Decibels Between the Two Levels being Added
Source by itself
Figure 5
Difference Amount to be added
New
Total
100dB
+101 dB
+100dB
+ 96 dB
1 dB .
3.7 dB
9.3 dB
2.7 dB
1.6 dB
0.4 dB
103.7dB
105.3 dB
105.7dB
Attenuation With Distance
Sound attenuates according to the inverse square law: sound intensity
decreases inversely with the square of the distance from the source. In other
words, each time the distance from the noise source doubles, the sound
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pressure is halved. This phenomenon produces a decrease of about 6 dB
each time the distance from the source is doubled. When frequency is
taken into account along with attenuation with distance, it is seen that
higher frequency sound dissipates at a greater rate than does low fre-
quency sound (Figure 6). Figure 6 does not take into account attenuation
resulting from wind gradients, temperature gradients, ground cover, etc.
-10
-20
.30
S
= -40
S
-50
-60
-70
Inverse Square
Octave
Bands
4800 2400 12001 600 300 150M5N37.5
9600 4800 2400 1200 600 300150. 75
V \ . \ \ \\V
100 200
500 1000 2000 5000 10,000 20,000 50,000 100,000
Distance from Source in Feet
Figure 6
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Human Hearing and Acoustics
In laboratory experiments, it was found that the "absolute" threshold
of hearing in young adults corresponded to a pressure of about 0.0002
dyne/cm2. This reference level was determined in a quiet ambient noise
atmosphere and at the most acute frequency range of human hearing,
between 1000 and 4000 Hz. The general range of human hearing is usually
defined as being between 20 and 20,000 Hz. Frequencies below 20 Hz
are callled infrasonic, with frequencies between 20 and 20,000 Hz called
sonic or referred to as the audible frequency area. Frequencies above
20,000 Hz (some texts refer to above 15,000) are called ultrasonic.
The loudness of sound (sensation) depends upon the intensity, but
it also depends upon the frequency of the sound and the characteristics of
the human ear. The intensity of sound is a purely physical quantity, where-
as, the loudness depends also upon the characteristics of the ear. Thus, the
intensity of a given sound striking the ear of a normal hearing person and
of a hard-of-hearing person might be the same, but the loudness sensation
would be quite different. Again, a 100-Hz tone that is barely audible has
about 5000 times the intensity of a 1000-Hz tone that is equally loud,
i.e., barely audible, whereas 100-Hz and 1000-Hz tones would sound equally
loud at a 100-dB level. The relationship between frequency intensity and
loudness is quite involved. We do have, however, a sense of relative loudness
so that there is a fair measure of agreement among trained observers in their
judgments as to when one sound is one-half, one-third, and so on as loud as
another. The question is often asked, "Suppose we reduce the intensity
level of a noise by 10 decibels, what percentage reduction in loudness
have we obtained?" The answer is that it depends on what the initial level
was. Figure 7 can be used to give an approximate answer for different
values of the original level.
When human ear response to frequency and intensity is plotted, we
find that the response is not linear and that it varies with sensation level.
Figure 8, an equal loudness chart, demonstrates this response characteris-
tic. The equal loudness levels in Figure 8 were defined as the intensity
required to make a given test tone seem equally as loud as the reference
tone, which was the 1000 Hz reference. The unit of loudness level that is
used to plot the data is called the phon. Thus, the loudness level in phons
of any sound of that frequency is equal to its intensity level in decibels.
Generally, the following is noted from the equal loudness curves:
1. At low intensity levels, high frequency tones sound louder than
low frequency tones of the same intensity.
9
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80
70
•2 60
u
3
•
0)
50
40
§ 30
20
10
0
4 6 8 10 12
Loudness Level .Reduction (dB)
14
16
Figure 7
120
m
c 100
<2 80
I 60
f «
O)
£ 20
0
Loudness Level
50 100 500 1000 5000 10,000
Frequency in Cycles Per Second
Figure 8
10
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2. At high intensity levels, all tones of the same intensity sound
almost equally loud, regardless of their frequency.
3. At low intensity levels, a given change in intensity level produces
a larger change in loudness at low frequencies than at high fre-
quencies.
4. At high intensity levels, a given change in intensity level produces
practically the same change in loudness regardless of frequency.
Audiometry
The "Absolute Zero" level of hearing determined in the laboratory
was found to be 16 to 20 dB too low for a "Normal Population" Audio-
metric Zero. Thus, the American Audiometric Zero is about 20 dB above
the zero based on the 0.0002 dyne/cm2 reference.
The intensity (loudness) scale on most audiometers has a range of
from -10 dB through 0 dB and up to 100 dB, with the maximum intensity
range dependent on the particular test frequency. Frequencies below
1000 Hz and above 4000 Hz usually do not have a full 100 dB (on audio-
meter intensity scale). These maximum intensity limits should always be
noted whenever testing the hearing of a person having a severe loss of
hearing. The maximum iritensity limits, if they exist, are usually marked
below the frequency indicator scale on the audiometer.
The majority of audiometer intensity scales are calibrated in 5-dB
steps: 5, 10, 15, 20, 25, etc. And, even if the audiometer has a variable
intensity control, usually the 5-dB steps are still recorded.
The speech reception threshold was also found to be approximately
16 to 20 dB above the 0.0002 dyne/cm2 reference. Thus, the audiometric
zero for speech material is about the same as the audiometric reference
for 1000-Hz test frequency area.
Human Response To Noise
Generally, any unwanted sound is referred to as noise. Thus, noise
does not necessarily imply that the sound field is loud. It is the attributes
making up a noise that determine whether it is annoying. Some of the
main attributes are:
1. The frequency spectrum, broadband or narrowband
2. Intensity levels
11
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3. Modulation characteristics
4. Time and place of the occurrence of the noise
5. Duration of the noise (short or continuous)
6. Individual background.
The frequency spectrum of sound refers to the breakdown of acoustic
energy from 20 through 10,000 Hz. This breakdown is usually accomplished
by measuring acoustic energy, or sound pressure levels, in eight octave
bands, the most common of which are (in Hz):
Band Number 1 20 through 75
Band Number 2 75 through 150
Band Number 3 150 through 300
Band Number 4 300 through 600
Band Number 5 600 through 1200
Band Number 6 1200 through 2400
Band Number 7 2400 through 4800
Band Number 8 4800 through 9600 (10,000)
The most significant bands regarding possible hazardous effects on man
are bands 4, 5, 6, and 7 (300 through 4800 Hz).
Obviously, intensity is a factor in determining deleterious effects
on humans, as indicated in the table below. But frequency, too, must be
considered, especially where one is trying to gauge characteristics of annoy-
ance. Generally, higher frequency noise is more annoying at equal inten-
sities, than lower intensity noise.
Intensity Levels and Human Speech-Hearing
dB
140 Threshold of pain
130 Feeling of tickle
120 Average threshold of discomfort for pure tones
110 Loud shout at 1 ft distance
100 Discomfort for speech begins around this level
90
80 Loud speech
70
60 Average speech conversational level
50
40 Faint speech at 3 ft distance
30 Whisper (average)
12
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20
10
0
Very quiet speech (faint)
Threshold of hearing (young adult)
Speech Interference Level (SIL)
The numerical average of the sound pressure level (SPL) readings
in the 600-1200, 1200-2400, and 2400-4800 octave bands has been em-
pirically shown to correlate with the subjective level of speech interference.
Figure 9 indicates the communication interference caused by various SILs.
Distance Between Talker & Listener — ft
Figure 9
13
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BIBLIOGRAPHY OF SOUND AND NOISE
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SOME CURRENT BOOKS ON NOISE AND VIBRATION
Academic Press, Inc., 111 Fifth Avenue, New York, NY 10003
1. Mason, W. P., "Physical Acoustics," 1964, $18.00.
2. Mason, W. P. (Editor), "Physical Acoustics, Principles & Methods,"
Vols. 1 and 2.
3. Brekhovskikh, "Waves in Layered Media."
4. Robinson, D. W., "Occupational Hearing Loss," 1971, $14.50.
5. Kryter, K., "Effects of Noise on Man," 1970, $19.50.
6. Mason, "Physical Acoustics," Volume 10, 1973, $39.50.
7. Meyer/Neumann, "Physical and Applied Acoustics," 1972, $18.50.
8. Tobias, J. V. (Editor), "Foundations of Modern Auditory Theory,"
Vols. I & II, 1972.
9. Welsh, R., "Physiological Effects of Noise."
10. Dalos, P., "The Auditory Periphery—Biophysics and Physiology,"
1973, $32.50.
Addison-Wesley Publishing Co., Inc., Jacob Way, Reading, MA
1. "Vibrations: Theoretical Methods," 1966, $9.75.
2. Towne, D. H., "Wave Phenomena."
American Elsevier Publishing Co., Inc., 52 Vanderbilt Avenue, New York,
NY 10017
1. Taylor, C. A., "The Physics of Musical Sounds," 1965, $9.50.
2. Smith, B. J., "Environmental Physics: Acoustics," 1970, $9.45.
3. Petursewicz, "Industrial Noise," 1974, $19.50.
4. Smith, "Acoustics," 1971, $9.45.
5. Ford, R. D., "Introduction to Acoustics," 1970.
6. Gayford, M. L., "Electroacoustics—Microphones, Earphones, and
Loudspeakers," 1971, $15.00.
American Society of Mechanical Engineers, 345 East 47th Street, New
York, NY 10017
1. Snowdon, J. C. and E. E. Ungar, "Isolation of Mechanical Vibration,
Impact, and Noise," 1973, $25.00.
16
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American Speech & Hearing Association, 9030 Old Georgetown Road,
Washington, DC 20014
1. "Noise as a Public Health Hazard," Proceedings of the American
Speech and Hearing Association National Conference, June 13-14,
1968, Washington, DC, W. D. Ward, J. E. Pricks, editors (1969).
Allyn & Bacon, Inc., Rockleigh, NJ 07647
1. Tse, F. S., I. E. Morse, and T. H. Hinkle, "Mechanical Vibrations,"
1963.
Ann Arbor Science, P. O. Box 1425, Ann Arbor, Ml 48106
1. Cheremsisnoff, P. N., and R. A. Young, "Pollution Engineering
Practice Handbook," $29.50.
2. Plant Engineering and Maintenance Handbook, $20.00.
3. Deininger, R. A., "Models For Environmental Pollution Control,"
$24.50, Cat. No. 0069-2/501.
A. S. Barnes and Company, Inc., Cranbury, NJ 08512
1. Albers, Vernon M., "The World of Sound," $5.95.
G. Bell & Sons, London, England
1. Wood, A. B., "A Textbook of Sound."
Bureau of National Affairs, Dept. NRR-504, 1231 25th Street, NW, Wash-
ington, DC 20037
1. Noise Regulation Reporter.
Chemical Publishing Co., Inc., 200 Park Avenue S., New York, NY 10003
1. Rettinger, M., "Acoustics-Room Design and Noise Control," 1968,
$17.50.
2. Rettinger, M., "Acoustic Design & Noise Control," 1973.
17
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Crane, Russak & Company, Inc., 347 Madison Avenue, New York, NY
10017
1. Haines, G., "Sound Under Water," 1974, $8.25.
2. Taylor, C. A., "Physics of Musical Sounds," 1965, $9.50.
CRC Press, Inc., 18901 Cranwuod Parkway, Cleveland, OH 44128
1. Goodfriend, L. S., "Noise Pollution," Cat. No. 5003/306, $26.00.
Marcel Dekker, Inc., 270 Madison Avenue, New York, NY 10016
1. Ensminger, "Ultrasonics: The Low and High Intensity Applications,"
1973, $24.50.
Dover Publications, Inc., 180 Varick Street, New York, NY 10014
1. Sabine, W. C., "Collected Papers on Acoustics," 1964, $2.00.
2. Lowery, H., "A Guide to Musical Acoustics," 1966, $1.00.
3. Olson, H. F., "Music Physics and Engineering," 1967, $2.75.
4. Kolsky, H., "Stress Waves in Solids."
5. Love, A. E. H., "A Treatise on the Mathematical Theory of Elastic-
ity," 1944.
6. Lamb, H., "Hydrodynamics."
7. Lamb, H., "The Dynamical Theory of Sound."
8. Rayleigh, Lord, "Theory of Sound," Vols. 1 & 2.
9. Rayleigh, Lord, "Scientific Papers," Vols. 1-6.
Dowden, Hutchinson & Ross, Inc., 523 Sarah Street, Stroudsburg, PA
18360
1. Albers, V. M. (Editor), "Underwater Sound," Pennsylvania State
University, 1972, $20.00.
2. Lindsay, R. B. (Editor), "Acoustics: Historical and Philosophical
Development, Brown University, 1973, $24.00.
3. Flanagan, J. L. and L. R. Rabiner (Editors), "Speech Synthesis,"
Bell Laboratories, Murray Hill, NJ, 1973, $22.00.
4. Lindsay, R. B. (Editor), "Physical Acoustics," Brown University,
1973, $24.00.
Duxbury Press, 10 Davis Drive, Belmont, CA 94002
1. Stevenson, Gordon M., "The Politics of Airport Noise," 1971.
18
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Elsevier/Excerpta-Medica/North Holland, Associated Scientific Publishers,
P. O. Box 211, Jan Van Galenstraat 335, Amsterdam, The Netherlands
1. Lawrence, Anita, "Architectural Acoustics," 1970.
2. Bushel, R. G., "Acoustic Behavior of Animals," $45.00.
3. Richardson, E. G. and E. E. Meyer, "Technical Aspects of Sound,"
Vols. 1-3.
Gale Research Co., Book Tower, Detroit, Ml 48226
1. Bragdon, C. R., "Noise Pollution," A Guide to Information Sources,
Edited by Dr. C. R. Bragdon.
General Radio Company, 300 Baker Avenue, Concord, MA 01742
1. Peterson, A. and E. Cross, "Handbook of Noise Measurement."
Grozier Publishing Co., Warren Avenue, Harvard, MA 01451
1. Lyon, R. H., "Transportation Noise," 1973.
Halsted Press, 605 Third Avenue, New York, NY 10016
1. Lindsay, R. B., "Physical Acoustics," (Benchmark Papers in Acous-
tics, #4), 1974, $24.00.
2. Kuttruff, H., "Room Acoustics," 1974, $29.75.
3. Flanagan, J. L. and L. R. Rabiner, "Speech Synthesis," 1973, $22.00.
4. Albers, V. M., "Underwater Sound," 1972, $22.00.
5. Lindsay, R. B., "Acoustics: Historical and Philosophical Develop-
ment," 1973, $24.00.
6. King, "The Measurement & Suppression of Noise with Special Refer-
ence to Electrical Machines," 1965.
Harper and Row, 49 E. 33rd Street, New York, NY 10016
1. Baron, R. A., "A Tyranny of Noise," 1971.
Hastings House Publishers, Inc., 10 East 40th Street, New York, NY 10016
1. Mankovsky, V. S., "Acoustics of Studios and Auditoria," 1971,
$16.50.
2. Oringel, R. S., "Audio Control Handbook," 1970, $10.00.
19
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3. Nisbett, A., "The Use of Microphones," 1974, $10.95.
William S. Hein & Co., Inc., 1285 Main Street, Buffalo, NY
1. Hildebrand, J. L, "Noise Pollution and the Law," 1970.
Holt, Rinehart and Winston, 383 Madison Avenue, New York, NY 10017
1. Davis, H. and S. R. Silverman, "Hearing and Deafness," 1970, $13.00.
The Kent State University Press, Kent, OH 44240
1. Sevarie, S. and E. Levy, "Tone: A Study in Musical Acoustics,"
1968, $7.95.
Robert E. Krieger Publishing Co., Inc., P. O. Box 542, Huntington, NY
11743
1. Green, D. M. and J. A. Swets, "Physiological Acoustics: Signal Detec-
tion Theory & Psychophysics," 1974, $18.00.
Lexington Publishing Company, 98 Emerson Gardens, Lexington, MA
02173
1. Jensen, P., and G. Sweitzer, "How You Can Sound-proof Your
Home," 1974, $7.95.
J. B. Lippencott, E. Washington Square, Philadelphia, PA 19105
1. Sataloff, J., "Occupational Hearing Loss."
MacMillan Company, 866 Third Avenue, New York, NY 10022
1. Rschevkin, S. N., "A Course of Lectures on the Theory of Sound,"
1963, $12.50.
2. Milne-Thomson, "Theoretical Hydrodynamics."
McGrath Publishing Company, 821 Fifteenth Street, NW, Washington,
DC 20005
1. Swenson, G. W., "Principles of Modern Acoustics," 1965.
20
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McGraw-Hill Book Company, 1221 Avenue of the Americas, New York,
NY 10020
1. Morse, P. M. and K. U. Ingard, "Theoretical Acoustics," 1968,
$23.00.
2. Beranek, L. L, "Noise and Vibration Control," 1971, $29.50.
3. Doelle, L. L., "Environmental Acoustics," 1972, $18.50.
4. Urick, R. J., "Principles of Underwater Sound for Engineers," 1967.
5. Stratton, J. A., "Electromagnetic Theory."
6. Beranek, L. L., "Acoustics."
7. Beranek, L. L. "Noise Reduction."
8. Morse, P. M. and H. Feshbach, "Methods of Theoretical Physics,"
Vols. 1 & 2.
9. Morse, P. M., "Vibration & Sound."
10. Moore, R. K., "Wave and Diffusion Analogies."
11. Lindsay, R. B., "Mechanical Radiation."
12. Harris, C. M., "Handbook of Noise Control," 1957.
13. David and Denes, "Human Communication," 1973, $25.00.
14. Keast, D. N., "Measurements' in Mechanical Dynamics," 1967.
15. Phelan, R. M., "Dynamics of Machinery."
16. Egan, M. D., "Concepts in Architectural Acoustics," 1972, $16.50.
17. Sataloff, J., "Industrial Deafness: Hearing, Testing and Noise Measure-
ment," 1957.
18. Harris, C. M. and C. E. Crede, "Shock and Vibration Handbook,"
3 Volumes, 1961.
Nelson-Hall Company, 325 W. Jackson Blvd., Chicago, IL 60606
1. Lipscomb, D. M., "Noise: The Unwanted Sounds," 1974, $15.00.
Penguin Books, 7710 Ambassador Road, Baltimore, MD 21207
1. Taylor, R., "Noise," 1970.
University of Pennsylvania Press, 3933 Walnut Street, Philadelphia, PA
19174
1. Bragdon, C. R., "Noise Pollution: The Unquiet Crisis," 1971, $15.00.
Pennsylvania State University Press, 215 Wagner Building, University
Park, PA 16802
1. Albers, V. M., "Underwater Acoustics Handbook," 1965, $19.50.
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2. Skudrzyk, E., "Simple and Complex Vibratory Systems," 1968,
$24.50.
3. Albers, V. M., "Suggested Experiments for Laboratory Courses in
Acoustics and Vibrations," 1972, $7.00.
Pergamon Press, Inc., Maxwell House, Fairview Park, Elmsford, NY 10523
1. Rzchevkin, "The Theory of Sound."
2. Tucker, D. G. and B. Gazey, "Applied Underwater Acoustics,"
1966, $8.50.
3. Malecki, I., "Physical Foundations of Technical Acoustics," 1969,
$47.00.
4. Bugliarello, G. et al, "The Impact of Noise Pollution: A Socio-Techno-
logical Introduction," Polytechnic Institute of New York, 1974,
$11.50.
5. Koren, H. et al, "Environmental Health and Safety," Indiana State
University, 1974, $16.00.
Philosophical Library, 15 East 40th Street, New York, NY 10016
1. Duerden, C., "Noise Abatement," 1971, $25.00.
Plenum Publishing Corporation, 227 West 17th Street, New York, NY
10011
1. Viktorov, I. A., "Rayleigh and Lamb Waves."
2. Welch, B. L. and A. S. Welch, "Psysiological Effects of Noise," 1970.
3. Stroke et al, "Ultrasonic Imaging and Holography," 1974, $37.50.
4. Green, "Acoustical Holography," Volume 5,1974, $32.50.
5. Rozenberg, "Physical Principles of Ultrasonic Technology," Volume
1,1973, $27.50.
Prentice-Hall, Inc., Englewood Cliffs, NJ 07632
1. Thomson, W. T., "Vibratory Theory and Applications," 1965,
$11.25.
2. Van Der Ziel, A., "Noise: Sources, Characterization, Measurement,"
$12.00,
3. Kinsman, B., "Wind Waves."
4. Berland, T., "The Fight for Quiet," 1970.
5. Thomson, W. T., "Theory of Vibrations with Applications," 1973,
$15.95..
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Purdue University, Office of the University Editor, South Campus Courts—
D, Lafayette, IN 47907
1. Crocker, M. J., "Noise and Vibration Control Engineering," 1971.
Sagamore Publishing Co., Inc., 980 Old Country Road, Plainview, NY
11803
1. Burroughs, L, "Microphones: Design and Application."
Sams & Company, 4300 62nd Street, Indianapolis, IN 46268
1. Davis, D., "Acoustical Tests & Measurements, 1965, $4.95.
The Scarecrow Press, Inc., P. O. Box 656, Metuchen, NJ 08840
1. King, R. L., "Airport Noise Pollution," 1973, $10.00.
Spartan Books, 50 Essex Street, Rochelle Park, NJ 07662
1. Blanke, M. P. and W. S. Mitchell, "Vibration and Acoustic Measure-
ment Handbook," 1972, $30.00.
Springer-Verlag, 175 Fifth Avenue, New York, NY 10010
1. Roederer, J. G., "Introduction to the Physics and Psychophysics of
Music," 1973, $5.90.
2. Skudrzyk, E., "The Foundations of Acoustics: Basic Mathematics and
Basic Acoustics," 1971, $72.50.
3. Krautkraemer, J. and H. Krautkraemer, "Ultrasonic Testing of
Materials," 1969, $32.40.
4. Cremer, L. and M. Heckl, "Structural Vibrations and Sound Radiation
at Audio Frequencies," $38.90.
Stackpole Books, Cameron & Keller Streets, Harrisburg, PA 17105
1. Still, H., "In Quest of Quiet," 1970.
St. Martin's Press, 175 Fifth Avenue, New York. NY 10010
1. Stephens, R. B., and A. E. Bate, "Acoustics & Vibrational Physics."
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Tap Books, Blue Ridge Summit, PA 17214
1. Everest, H. A., "Acoustic Techniques for Home & Studio," 1973,
$7.95.
Time-Life Books, Rockefeller Center, New York, NY 10020
1. Stevens, S. S., and F. Warshofsky, et al, "Sound and Hearing," Life
Science Library, 1965, $3.95.
University of Toronto Press, 33 E. Tupper Street, Buffalo, NY 14203
1. Ribner, H. S., "Aerodynamic Noise," 1969, $15.00.
United States Gypsum, 101 S. Wacker Dr., Chicago, IL 60606
1. "Sound Control Construction: Principles and Performance," $1.50.
Van Nostrand Reinhold Co., 450 West 33rd Street, New York, NY 10001
1. Cremer, L. (Editor), "Proceedings of the Third International Congress
on Acoustics," Stuttgart, 1959, Vol. 1-Principles.
2. Waldron, R. A., "Waves and Oscillations," 1964.
3. Josheps, J. J., "The Physics of Musical Sounds," 1967.
4. Olsen, H. F., "Acoustical Engineering," 1957.
5. Olsen, H. F., "Modern Sound Reproduction."
6. Swenson, G. W., "Principles of Modern Acoustics," 1953.
7. Yerges, L. F., "Sound, Noise and Vibration," 1969.
8. Fletcher, H., "Speech and Hearing in Communication," 1953.
University of Washington Press, Seattle, WA 98105
1. Chalupnik, J. D. (Editor), "Transportation Noises: A symposium on
Acceptability Criteria," 1970.
John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10016
1. Pain, H. J., "Physics of Vibration and Waves," 1968, $5.95.
2. Snowden, J. C., "Vibration and Shock in Damped Mechanical Sys-
tems," 1968, $22.50.
3. Bendat, j. and A. Piersal, "Measurement and Analysis of Random
Data."
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4. Kock, W. E., "Seeing Sound," 1971, $7.95.
5. Coulson, C. A., "Waves."
6. Kinsler, L. E. and A. R. Frey, "Fundamentals of Acoustics."
7. Eisberg, R. M., "Fundamentals of Modern Physics," 1963.
8. Hueter, T. F. and R. H. Bolt, "Sonics."
9. Auld, B. A., "Acoustic Fields and Waves in Solids," Vols. 1 & 2,
1973, $49.50 set.
10. Diehl, G., "Machinery Acoustics," 1974.
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GLOSSARY
A-WEIGHTED SOUND LEVEL-The ear does not respond equally to
frequencies, but is less efficient at low and high frequencies than it
is at medium or speech range frequencies. Thus, to obtain a single
number representing the sound level of a noise containing a wide
range of frequencies in a manner representative of the ear's response,
it is necessary to reduce the effects of the low and high frequencies
with respect to the medium frequencies. The resultant sound level is
said to be A-weighted, and the units are dB. A popular method of
indicating the A-weighted units is dBA. The A-weighted sound level
level is also called the noise level. Sound level meters have an A-
weighting network for measuring A-weighted sound level.
ABSORPTION—Absorption is a property of materials that reduces the
amount of sound energy reflected. Thus, the introduction of an
"absorbent" into the surfaces of a room will reduce the sound pres-
sure level in that room by virtue of the fact that sound energy strik-
ing the room surfaces will not be totally reflected. It should be
mentioned that this is an entirely different process from that of
transmission loss through a material, which determines how much
sound gets into the room via the walls, ceiling, and floor. The effect
of absorption merely reduces the resultant sound level in the room
produced by energy that has already entered the room.
ABSORPTION COEFFICIENT-A measure of sound-absorbing ability
of a surface. This coefficient is defined as the fraction of incident
sound energy absorbed or otherwise not reflected by the surface.
Unless otherwise specified, a diffuse sound field is assumed. The
values of sound-absorption coefficient usually range from about
0.01 for marble slate to almost 1.0 for long absorbing wedges such as
are used in anechoic chambers.
ACCELEROMETER (ACCELERATION PICKUP)-An electroacoustic
transducer that responds to the acceleration of the surface to which
the transducer is attached, and delivers essentially equivalent electric
waves.
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ACOUSTICAL POWER-See sound power.
ACOUSTICS—(1) The science of sound, including the generation, transmis-
sion, and effects of sound waves, both audible and inaudible. (2) The
physical qualities of a room or other enclosure (such as size, shape,
amount of noise) that determine the audibility and perception of
speech and music.
AIRBORNE SOUND—Sound that reaches the point of interest by propaga-
tion through air.
AIR FLOW RESISTANCE-See flow resistance.
AMBIENT NOISE LEVEL-The ambient noise level follows the usage of
the word "ambient" throughout the environmental sciences (except
acoustics). That is, the ambient noise level is that level that exists
at any instant, regardless of source.
ANALYSIS—The analysis of a noise generally refers to the examination of
the composition of noise in its various frequency bands, such as
octaves or third-octave bands.
ANECHOIC ROOM—An anechoic room is one whose boundaries have
been designed (with acoustically absorbent materials) to absorb
nearly all the sound incident on its boundaries, thereby affording
a test room essentially free from reflected sound.
ANTINODE (LOOP)—A point, line, or surface in a standing wave where
the vibration or sound pressure has maximum amplitude.
ARTICULATION INDEX (AI)-A numerically calculated measure of the
intelligibility of transmitted or processed speech. It takes into account
the limitations of the transmission path and the background noise.
The articulation index can range in magnitude between 0 and 1.0.
If the Al is less than 0.1, speech intelligibility is generally low. If it
is above 0.6, speech intelligibility is generally high.
AUDIOFREQUENCY-The frequency of oscillation of an audible sinewave
of sound; any frequency between 20 and 20,000 Hz. See also fre-
quency.
AURAL-Of or pertaining to the ear or hearing.
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AUDIOGRAM-A graph showing hearing loss as a function of frequency.
AUDIOMETER—An instrument for measuring hearing sensitivity of hearing
loss.
BACKGROUND NOISE-The total of all noise in a system or situation,
independent of the presence of the desired signal. In acoustical
measurements, strictly speaking, the term "background noise" means
electrical noise in the measurement system. However, in popular
usage the term "background noise" is also used with the same mean-
ing as "residual noise."
BAFFLE-A baffle is a shielding structure or series of partitions used
to increase the effective length of the external transmission path
between two points in an acoustic system. For example, baffles
may be used in sound traps (as in air conditioning ducts) or in auto-
motive mufflers to decrease the sound transmitted while affording
a path for air flow.
BAND—A segment of the frequency spectrum.
BAND CENTER FREQUENCY-The designated (geometric) mean fre-
quency of a band of noise or other signal. For example, 1000 Hz is
the band center frequency for the octave band that extends from
707 Hz to 1414 Hz, or for the third-octave band that extends from
891 Hz to 1123 Hz.
BAND PRESSURE (OR POWER) LEVEL-The pressure (or power) level
for the sound contained within a specified frequency band. The
band may be specified either by its lower and upper cutoff fre-
quencies, or by its geometric center frequency. The width of the
band is often indicated by a prefatory modifier; e.g., octave band,
third-octave band, 10-Hz band.
BOOM CARPET—The area on the ground underneath an aircraft flying
at supersonic speeds that is hit by a sonic boom of specified mag-
nitude.
BROADBAND NOISE—Noise with components over a wide range of
frequencies.
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C-WEIGHTED SOUND LEVEL (dBC)-A quantity, in decibels, read from
a standard sound-level meter that is switched to the weighting net-
work labeled "C". The C-weighting network weighs the frequencies
between 70 Hz and 4000 Hz uniformly, but below and above these
limits, frequencies are slightly discriminated against. Generally, C-
weighted measurements are essentially the same as overall sound-
pressure levels, which require no discrimination at any frequency.
COINCIDENCE EFFECT—The coincidence effect occurs when the wave-
length of the bending wave in a panel coincides with the length of
an incident sound wave at the angle at which it strikes the panel.
At any particular frequency, this effect can occur only if the wave
in air is traveling at a particular angle with respect to the surface
of the panel. Under this condition, a high degree of coupling is achiev-
ed between the bending wave in the panel and the sound in the air.
When the coincidence effect occurs, the transmission loss for the
panel is greatly reduced. See also critical frequency.
COMMUNITY NOISE EQUIVALENT LEVEL-Community Noise Equiva-
lent Level (CNEL) is a scale .that takes account of all the A-weighted
acoustic energy received at a point, from all noise events causing
noise levels above some prescribed value. Weighting factors are in-
cluded that place greater importance upon noise events occurring
during the evening hours (7:00 p.m. to 10:00 p.m.) and even greater
importance upon noise events at night (10:00 p.m. to 6:00 a.m.).
COMPOSITE NOISE RATING-Composite noise rating (CNR) is a scale
that takes account of the totality of all aircraft operations at an
airport in quantifying the total aircraft noise environment. It was the
earliest method for evaluating compatible land use around airports
and is still in wide use by the Department of Defense in predicting
noise environments around military airfields. Basically, to calculate
a CNR value, one begins with a measure of the maximum noise magni-
tude from each aircraft flyby and adds weighting factors that sum
the cumulative effect of all flights. The scale used to describe in-
dividual noise events is perceived noise level (in PNdB); the term
accounting for number of flights is 10 log-|ON (where N is the number
of flight operations), and each night operation counts as much as 10
daytime operations. Very approximately, the noise exposure level at a
point expressed in the CNR scale will be numerically 35-37 dBhigher
than if expressed in the CNEL scale.
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CONTINUOUS SOUND SPECTRUM-A continuous sound spectrum is
composed of components that are continuously distributed over
a frequency region.
CRITERION—A criterion, in Federal environmental usage, is a statement of
the cause and effect relationship between a given level of pollutant
and specific effects on human life.
CRITICAL FREQUENCY-The critical frequency is the lowest frequency
at which the coincidence effect can occur. At this frequency, the
coincidence angle is 90°, that is, the sound wave is traveling parallel
to the surface of the panel. Below this frequency, the wavelength in
air is greater than the bending wavelength in the panel.
CUTOFF FREQUENCIES-The frequencies that mark the ends of a band,
or at which the characteristics of a filter change from pass to no-pass.
CYLINDRICAL DIVERGENCE-Cylindrical divergence is the condition
of propagation of cylindrical waves that accounts for the regular de-
crease in intensity of a cylindrical wave at progressively greater
distances from the source. Under this condition, the sound-pressure
level decreases 3 decibels with each doubling of distance from the
source. See also spherical divergence.
CYLINDRICAL WAVE—A cylindrical wave is a wave in which the surfaces
of constant phase are coaxial cylinders. A line of closely-spaced sound
sources radiating into an open space produces a free sound field of
cylindrical waves. See also cylindrical divergence.
CYCLES PER SECOND—A measure of frequency numerically equivalent
to Hertz.
DAMAGE RISK CRITERION-A statement of noise levels (including fre-
quency, duration, intermittancy, and other factors) above which
permanent hearing loss of at least a specified amount is likely to be
sustained by a person (to a given degree of probability). See also
hearing loss, criterion.
DAMPING—The dissipation of energy with time or distance. The term is
generally applied to the attenuation of sound in a structure owing
to the internal sound-dissipative properties of the structure or owing
to the addition of sound-dissipative materials.
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DECIBEL—The decibel (abbreviated "dB") is a measure, on a logarithmic
scale, of the magnitude of a particular quantity (such as sound pres-
sure, sound power, intensity) with respect to a standard reference
value (0.0002 microbars for sound pressure and 10-12 watt for sound
power).
DIFFUSE SOUND FIELD—The presence of many reflected waves (echoes)
in a room (or auditorium) having a very small amount of sound
absorption, arising from repeated reflections of sound in various
directions. In a diffuse field, the sound pressure level, averaged over
time, is everywhere the same and the flow of sound energy is equally
probable in all directions.
DIRECTIVITY INDEX—In a given direction from a sound source, the
difference in decibels between (a) the sound-pressure level produced
by the source in that direction, and (b) the space-average sound-
pressure level of that source, measured at the same distance.
DIRECTIVITY PATTERN-The directivity pattern of a source of sound
is the hypothetical surface in space over which the sound pressure
levels produced by the source are constant. See also directivity index.
DOPPLER EFFECT (DOPPLER SHIFT)-The apparent upward shift in
frequency of a sound as a noise source approaches the listener (or
vice versa), and the apparent downward shift when the noise source
recedes. The classic example is the change in pitch of a railroad
whistle as the locomotive approaches and passes by.
DUCT LINING OR WRAPPING-Usually a sheet of porous material placed
on the inner or outer wall(s) of a duct to introduce sound attenuation
and heat insulation. It is often used in air conditioning systems. Lin-
ings are more effective in attenuating sound that travels inside along
the length of a duct, while wrappings are more effective in prevent-
ing sound from being radiated from the ductsidewalls into surround-
ing spaces.
EFFECTIVE PERCEIVED NOISE LEVEL (EPNL)-A physical measure
designed to estimate the effective "noisiness" of a single noise event
usually an aircraft fly-over; it is derived from instantaneous Perceived
Noise Level (PNL) values by applying corrections for pure tones and
for the duration of the noise.
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ELECTROACOUSTICS-The science and technology of transforming sound
waves into currents in electrical circuits (and vice versa), by means
of microphones, loudspeakers, and electronic amplifiers and filters.
FAR FIELD—Consider any sound source in free space. At a sufficient
distance from the source, the sound pressure level obeys the inverse-
square law (the sound pressure decreases 6 dB with each doubling of
distance from the source). Also, the sound particle velocity is in phase
with the sound pressure. This region is called the far field of the sound
source. Regions closer to the source, where these two conditions do
not hold, constitute the near field. In an enclosure, as opposed to free
space, there can also sometimes be a far field region if there is not so
much reflected sound that the near field and the reverberant field
merge. See also reverberant field.
FILTER—A device that transmits certain frequency components of the
signal (sound or electrical) incident upon it, and rejects other fre-
quency components of the incident signal.
FLOW RESISTANCE—The flow resistance of a porous material is one of
the most important quantities determining the sound absorbing
characteristics of the material. Flow resistance is a ratio of the pres-
sure differential across a sample of the porous material to the air
velocity through it.
FOOTPRINT (NOISE)-The shape and size of the geographical pattern of
of noise impact that an aircraft makes on the areas near an airport
while landing or taking off.
FREE SOUND FIELD (FREE FIELD)-A sound field in which the effects
of obstacles or boundaries on sound propagated in that field are
negligible.
FREQUENCY-The number of times per second that the sine-wave of
sound repeats itself, or that the sine-wave of a vibrating object re-
peats itself. Now expressed in Hertz (Hz), formerly in cycles per
second (cps).
FUNCTION—A quantity that varies as a result of variations of another
quantity.
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FUNDAMENTAL FREQUENCY-The frequency with which a periodic
function reproduces itself, sometimes called the first harmonic
(see also harmonic).
GAUSSIAN DISTRIBUTION (or NORMAL DISTRIBUTION)-A term
used in statistics to describe the extent and frequency of deviations
or errors. The outstanding characteristics are a tendency to a maxi-
mum number of occurrences at or near the center or mean point,
the progressive decrease of frequency of occurrence with distance
from the center, and the symmetry of distribution on either side
of the center. In respect of random noise, each fluctuation of ampli-
tude is an occurrence, whether above or below the mean level; the
peak value of each fluctuation is the error and the distribution of
errors with time is Gaussian.
GRADIENT—A variation of the local speed of sound with height above
ground or other measure of distance causing refraction of sound.
It is most commonly caused by rising or falling temperature with
altitude or by differences in wind speed.
HARMONIC—A sinusoidal (pure-tone) component whose frequency is a
whole-number multiple of the fundamental frequency of the wave.
If a component has a frequency twice that of the fundamental it is
called the second harmonic.
HEARING DISABILITY-An actual or presumed inability, due to hearing
impairment, to remain employed at full wages.
HEARING HANDICAP—The disadvantage imposed by a hearing impair-
ment sufficient to affect one's efficiency in the situation of every-
day living.
HEARING IMPAIRMENT—A deviation or change for the worse in either
hearing structure or function, usually outside the normal range;
see hearing loss.
HEARING LOSS—At a specified frequency, an amount, in decibels, by
which the threshold of audibility for that ear exceeds a certain speci-
fied audiometric threshold, that is to say, the amount by which a
person's hearing is worse than some selected norm. The norm may
be the threshold established at some earlier time for that ear, or the
average threshold for some large population, or the threshold selected
by some standards body for audiometric measurements.
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HEARING LOSS FOR SPEECH-The difference in decibels between the
speech levels at which the "average normal" ear and a defective ear,
respectively, reach the same intelligibility, often arbitrarily set at
50 percent.
HERTZ—Unit of measurement of frequency, numerically equal to cycles
per second.
IMPACT—(1) An impact is a single collision of one mass in motion with
a second mass that may be either in motion or at rest. (2) Impact
is a word used to express the extent or severity of an environmental
problem; e.g., the number of persons exposed to a given noise en-
vironment.
IMPACT INSULATION CLASS (IIC)-A single-figure rating that is in-
tended to permit the comparison of the impact sound insulating
merits of floor-ceiling assemblies in terms of a reference contour.
IMPACT SOUND—The sound arising from the impact of a solid object
on an interior surface (wall, floor, or ceiling) of a building. Typical
sources are footsteps, dropped objects, etc.
INFRASONIC—Of a frequency below the audiofrequency range.
INVERSE-SQUARE LAW-The inverse-square law describes that acous-
tic situation where the mean-square pressure changes in inverse
proportion to the square of the distance from the source. Under
this condition, the sound-pressure level decreases 6 decibels with
each doubling of distance from the source. See also spherical diver-
gence.
ISOLATION-See vibration isolator.
JET NOISE—Noise produced by the exhaust of a jet into its surrounding
atmosphere. It is generally associated with the turbulence generated
along the interface between the jet stream and the atmosphere.
LI o LEVEL—The sound level exceeded 10 percent of the time. Corresponds
to peaks of noise in the time history of environmental noise in a
particular setting.
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LEVEL—The sound level exceeded 50 percent of the time. Corresponds
to the average level of noise in a particular setting, over time.
LEVEL-The sound level exceeded 90 percent of the time. Corresponds
to the residual noise level.
LEVEL—The value of a quantity in decibels. The level of an acoustical
quantity (sound pressure or sound power), in decibels, is 10 times
the logarithm (base 10) of the ratio of the quantity to a reference
quantity of the same physical kind.
LINE SPECTRUM—The spectrum of a sound whose components occur at
a number of discrete frequencies.
LIVE ROOM—One characterized by an unusually small amount of sound
absorption. See reverberation room.
LOUDNESS—The judgment of intensity of a sound by a human being.
Loudness depends primarily upon the sound pressure of the stimulus.
Over much of the loudness range it takes about a threefold increase
in sound pressure (approximately 10 dB) to produce a doubling of
loudness.
LOUDNESS LEVEL-The loudness level of a sound, in phons, is numeri-
cally equal to the median sound pressure level, in decibels, relative
to 0.0002 microbar, of a free progressive wave of frequency 1000
Hz presented to listeners facing the source, which in a number of
trials is judged by the listeners to be equally loud.
MACH NUMBER-The ratio of a speed of a moving element to the speed
of sound in the surrounding medium.
MASKING-The action of bringing one sound (audible when heard alone)
to inaudibility or to unintelligibility by the introduction of another
sound. It is most marked when the masked sound is of higher fre-
quency than the masking sound.
MASKING NOISE-A noise that is intense enough to render inaudible or
unintelligible another sound that is simultaneously present.
MEAN FREE PATH-The average distance sound travels between suc-
cessive reflections in a room.
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MEDIUM—A substance carrying a sound wave.
MICROBAR—A microbar is a unit of pressure, equal to 1 dyne per square
centimeter.
MICROPHONE—An electroacoustic transducer that responds to sound
waves and delivers essentially equivalent electric waves.
NEAR FIELD-See far field.
NODE—A point, line, or surface where a wave has zero amplitude.
NOISE—Any sound that is undesirable because it interferes with speech
and hearing, or is intense enough to damage hearing, or is otherwise
annoying.
NOISE CRITERION (NC) CURVES-Any of several versions (SC, NC,
NCA, PNC) of criteria used for rating the acceptability of continuous
indoor noise levels, such as produced by air-handling systems.
NOISE EXPOSURE FORECAST-Noise exposure forecast (NEF) is a scale
(analogous to CNEL and CNR) that has been used by the federal
government in land use planning guides for use in connection with
airports.
In the NEF scale, the basic measure of magnitude for individual
noise events is the effective perceived noise level (EPNL), in units
of EPNdB. This magnitude measure includes the effect of duration
per event. The terms accounting for number of flights and for weight-
ing by time period are the same as in the CNR scale. Very approxi-
mately, the noise exposure level at a point expressed in the NEF
scale will be numerically about 33 dB lower than if expressed in the
CNEL scale.
NOISE INSULATION-See sound insulation.
NOISE ISOLATION CLASS (NIC)-A single number rating derived in a
prescribed manner from the measured values of noise reduction.
It provides an evaluation of the sound isolation between two enclosed
spaces that are acoustically connected by one or more paths.
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NOISE LEVEL-See sound level.
NOISE AND NUMBER INDEX (NNI)-A measure based on Perceived
Noise Level, and with weighting factors added to account for the
number of noise events, and used (in some European countries) for
rating the noise environment near airports.
NOISE POLLUTION LEVEL (LNP or NPL)-A measure of the total com-
munity noise, postulated to be applicable to both traffic noise and
aircraft noise. It is computed from the "energy average" of the noise
level and the standard deviation of the time-varying noise level.
NOISE REDUCTION (NR)-The noise reduction between two areas or
rooms is the numerical difference, in decibels, of the average sound
pressure levels in those areas or rooms. A measurement of "noise re-
duction" combines the effect of the transmission loss performance
of structures separating the two areas or rooms, plus the effect of
acoustic absorption present in the receiving room.
NOISE REDUCTION COEFFICIENT (NRC)-A measure of the acous-
tical absorption performance of a material, calculated by averaging
its sound absorption coefficients at 250, 500, 1000, and 2000 Hz,
expressed to the nearest integral multiple of 0.05.
NORMAL DISTRIBUTION-See Gaussian distribution.
NOYS—A unit used in the calculation of Perceived Noise Level.
OCTAVE—An octave is the interval between two sounds having a basic
frequency ratio of two. For example, there are 8 octaves on the
keyboard of a standard piano.
OCTAVE BAND-AM of the components, in a sound spectrum, whose
frequencies are between two sine wave components separated by an
octave.
OCTAVE-BAND SOUND PRESSURE LEVEL-The integrated sound
pressure level of only those sine-wave components in a specified
octave band, for a noise or sound having a wide spectrum.
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OSCILLATION—The variation with time, alternately increasing and decreas-
ing, (a) of some feature of an audible sound, such as the sound pres-
sure, or (b) of some feature of a vibrating >olid object, such as the
displacement of its surface.
PARTIAL NODE—A partial node is the point, line, or surface in a standing
wave system where there is a minimum amplitude differing from
zero.
PEAK SOUND PRESSURE-The maximum instantaneous sound pressure
(a) for a transient or impulsive sound of short duration, or (b) in a
specified time interval for a sound of long duration.
PERCEIVED NOISE LEVEL (PNL)-A quantity expressed in decibels
that provides a subjective assessment of the perceived "noisiness"
of aircraft noise. The units of Perceived Noise Level are Perceived
Noise Decibels, PNdB.
PERIOD—The duration of time it takes for a periodic wave form (like a
sine wave) to repeat itself.
PERMANENT THRESHOLD SHIFT (PTS)-See temporary threshold shift.
PHASE—For a particular value of the independent variable, the fractional
part of a period through which the independent variable has advanced,
measured from an arbitrary reference.
PHON—The unit of measurement for loudness level.
Phons = 40 + Iog2 sone.
PINK NOISE—Noise where level decreases with increasing frequency to
yield constant energy per octave of band width.
PITCH—A listener's perception of the frequency of a pure tone; the higher
the frequency, the higher the pitch.
PLANE WAVE—A wave whose wave fronts are parallel and perpendicular
to the direction in which the wave is travelling.
PNdB—See perceived noise level.
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PRESBYCUSIS—The decline in hearing acuity that normally occurs as a
person grows older.
PURE TONE—A sound wave whose waveform is that of a sine-wave.
RANDOM INCIDENCE-lf an object is in a diffuse sound field, the sound
waves that comprise the sound field are said to strike the object
from all angles of incidence at random. See also Gaussian distri-
bution.
RANDOM NOISE—An oscillation whose instantaneous magnitude is not
specified for any given instant of time. It can be described in a statis-
tical sense by probability distribution functions giving the fraction
of the total time that the magnitude of the noise lies within a speci-
fied range.
RATE OF DECAY-Rate of decay is the time rate at which the sound-
pressure level (or other stated characteristic, such as a vibration
level) decreases at a given point and at a given time after the source
is turned off. The commonly used unit is decibels per second.
REFRACTION—The bending of a sound wave from its original path,
either because it is passing from one medium to another or because
(in air) of a temperature or wind gradient in the medium.
RESIDUAL NOISE LEVEL-The term "residual noise" has been adopted
to mean the noise that exists at a point as a result of the combina-
tion of many distant sources, individually indistinguishable. In statis-
tical terms, it is the level that exists 90 percent of the time. (Acous-
ticians should note it means the same level to which they have custo-
marily applied that term "ambient.") See also background noise.
RESONANCE—The relatively large amplitude of vibration produced when
the frequency of some source of sound or vibration "matches" or
synchronizes with the natural frequency of vibration of some object,
component, or system.
RESONATOR—A resonator is a device that resounds or vibrates in sympa-
thy with some source of sound or vibration.
RETROFIT—The retroactive modification of an existing building or ma-
chine. In current usage, the most common application of the word
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"retrofit" is to the question of modification of existing jet aircraft
engines for noise abatement purposes.
REVERBERANT FIELD-The region in a room where the reflected sound
dominates, as opposed to the region close to the noise source where
the direct sound dominates.
REVERBERATION—The persistence of sound in an enclosed space, as a
result of multiple reflections, after the sound source has stopped.
REVERBERATION ROOM-A room having a long reverberation time,
especially designed to make the sound field inside it as diffuse (homo-
geneous) as possible. Also called a live room.
REVERBERATION TIME (RT)-The reverberation time of a room is the
time taken for the sound pressure level (or sound intensity) to de-
crease to one-millionth (60 dB) of its steady-state value when the
source of sound energy is suddenly interrupted. It is a measure of the
persistence of an impulsive sound in a room and of the amount of
acoustical absorption present inside the room.
ROOM CONSTANT—The room constant is equal to (a) the product of the
average absorption coefficient of the room and the total internal
area of the room, divided by (b) the quantity one minus the average
absorption coefficient.
ROOT-MEAN-SQUARE (RMS)-The root-mean square value of a quantity
that is varying as a function of time is obtained by squaring the func-
tion at each instant, obtaining the average of the squared values over
the interval of interest, and taking the square root of this average.
For a sine wave, multiply the RMS value by \/~2, or about 1.43,
to get the peak value of the wave. The RMS value, also called the
effective value of the sound pressure, is the best measure of ordinary
continuous sound, but the peak value is necessary for assessment of
impulse noises.
SHIELDING—The attenuation of a sound by placing walls, buildings, or
other barriers between a sound source and the receiver.
SINE-WAVE-A sound wave, audible as a pure tone, in which the sound
pressure is a sinusoidal function of time.
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SOME—The unit of measurement for loudness. One sone is the loudness
of a sound whose level is 40 phons.
SONIC BOOM—The pressure transient produced at an observing point by a
vehicle that is moving past (or over) it faster than the speed of sound.
SOUND-See acoustics (1).
SOUND ABSORPTION COEFFICIENT-See absorption coefficient.
SOUND ANALYZER—A sound analyzer is a device for measuring the
band-pressure level or pressure-spectrum level of a sound as a function
of frequency.
SOUND INSULATION-(I) The use of structures and materials designed
to reduce the transmission of sound from one room or area to another
or from the exterior to the interior of a building. (2) The degree by
which sound transmission is reduced by means of sound insulating
structures and materials.
SOUND LEVEL (NOISE LEVEL)-The weighted sound pressure level
obtained by use of a sound level meter having a standard frequency-
filter for attenuating part of the sound spectrum.
SOUND LEVEL METER—An instrument, comprising a microphone, an
amplifier, an output meter, and frequency-weigh ting networks, that
is used for the measurement of noise and sound levels in a specified
manner.
SOUND POWER—The total amount of energy radiated into the atmospheric
air per unit time by a source of sound.
SOUND POWER LEVEL-The level of sound power, averaged over a
period of time, the reference being 1CH2 watts.
SOUND PRESSURE-(I) The minute fluctuations in atmospheric pres-
sure that accompany the passage of a sound wave; the pressure
fluctuations on the tympanic membrane are transmitted to the inner
ear and give rise to the sensation of audible sound. (2) For a steady
sound, the value of the sound pressure averaged over a period of time.
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(3) Sound pressure is usually measured (a) in dynes per square centi-
meter (dyn/cm2), or (b) in newtons per square meter (N/m2). 1
N/m2 = 10 dyn/cm2 10-5 times the atmospheric pressure.
SOUND PRESSURE LEVEL-The root-mean-square value of the pressure
fluctuations above and below atmospheric pressure due to a sound
wave; expressed in decibels re a reference pressure of 0.0002 micro-
bars (2 x 10-5 newtons per square meter).
SOUND SHADOW-The acoustical equivalent of a light shadow. A sound
shadow is often partial because of diffraction effects.
SOUND TRANSMISSION CLASS (STC)-The preferred single figure
rating system designed to give an estimate of the sound insulation
properties of a partition or a rank ordering of a series of partitions.
It is intended for use primarily when speech and office noise con-
stitute the principal noise problem.
SOUND TRANSMISSION COEFFICIENT-The fraction of incident sound
energy transmitted through a structural configuration.
SOUND TRANSMISSION LOSS (TRANSMISSION LOSS (TL))-A mea-
sure of sound insulation provided by a structural configuration. Ex-
pressed in decibels, it is 10 times the logarithm to the base 10 of the
reciprocal of the sound transmission coefficient of the configuration.
SPACE-AVERAGE SOUND-PRESSURE LEVEL-The space-average sound
pressure level is the sound-pressure level averaged over all directions
at a constant distance from the source.
SPECTRUM—The description a sound wave's resolution into components,
each of different frequency and (usually) different amplitude and
phase.
SPEECH-INTERFERENCE LEVEL (SIL)-A calculated quantity provid-
ing a guide to the interfering effect of a noise on reception of speech
communication. The speech-interference level is the arithmetic
average of the octave-band sound-pressure levels of the interfering
noise in the most important part of the speech frequency range.
The levels in the three octave-frequency bands centered at 500,
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1000, and 2000 Hz are commonly averaged to determine the speech-
interference level. Numerically, the magnitudes of aircraft sounds
in the speech-interference level scale are approximately 18 to 22 dB
less than the same sound in the perceived noise level scale in PNdB,
SPEED (VELOCITY) OF SOUND IN AIR-The speed of sound in air is
344 m/sec or 1128 ft/sec at 78°F.
SPHERICAL DIVERGENCE-Spherical divergence is the condition of
propagation of spherical waves that relates to the regular decrease
in intensity of a spherical sound wave at progressively greater dis-
tances from the source. Under this condition the sound-pressure
level decreases 6 decibels with each doubling of distance from the
source. See also cylindrical divergence.
SPHERICAL WAVE—A sound wave in which the surfaces of constant
phase are concentric spheres. A small (point) source radiating into
an open space produces a free sound field of spherical waves.
SPL—See sound pressure level.
STANDARD—(1) A prescribed method of measuring acoustical quan-
tities. Standards in this sense are promulgated by professional and
scientific societies like ANSI, SAE, ISO, etc., as well as by other
groups. (2) In the sense used in Federal environmental statutes,
a standard is a specific statement of permitted environmental con-
ditions.
STANDING WAVE—A periodic sound wave having a fixed distribution
in space, the result of interference of traveling sound waves of the
same frequency and kind. Such sound waves are characterized by the
existence of nodes, or partial nodes, and antinodes that are fixed
in space.
STEADY-STATE SOUNDS-Sounds whose average characteristics remain
constant in time. An example of a steady-state sound is an air con-
ditioning unit.
STRUCTUREBORNE SOUND-Sound that reaches the point of interest,
over at least part of its path, by vibration of a solid structure.
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SUBHARMONIC—A sound component of frequency a whole number of
times less than the fundamental frequency of the sounds' complex
wave.
TAPPING MACHINE—A device that produces a standard impulsive noise
by letting weights drop a fixed distance onto the floor. Used in tests
measuring the isolation from impact noise provided by various floor-
ceiling constructions.
TEMPORARY THRESHOLD SHIFT (TTS)-A temporary impairment of
hearing capability as indicated by an increase in the threshold of
audibility. By definition, the ear recovers after a given period of
time. Sufficient exposures to noise of sufficient intensity, from which
the ear never completely recovers, will lead to a permanent threshold
shift (PTS), which constitutes hearing loss. See hearing loss, threshold
shift, threshold of audibility.
THIRD-OCTAVE BAND-A frequency band whose cutoff frequencies
have a ratio of 2 to the one-third power, which is approximately
1.26. The cutoff frequencies of 891 Hz and 1112 Hz define a third-
octave band in common use. See also band center frequency.
THRESHOLD OF AUDIBILITY (THRESHOLD OF DETECTABILITY)-
The minimum sound-pressure level at which a person can hear a
specified sound for a specified fraction of trials.
THRESHOLD SHIFT—An increase in a hearing threshold level that results
from exposure to noise.
TONE—A sound of definite pitch. A pure tone has a sinusoidal wave form.
TRAFFIC NOISE INDEX (TNI)-A measure of the noise environment
created by vehicular traffic on highways; it is computed from mea-
sured values of the noise levels exceeded 10 percent and 90 percent
of the time.
TRANSMISSION LOSS-See sound transmission loss.
TRANSDUCER—A device capable of being actuated by waves from one or
more transmission systems or media and supplying related waves
to one or more other transmission systems or media. Examples are
microphones, accelerometers, and loudspeakers.
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ITS—See temporary threshold shift.
ULTRASONIC—Pertaining to sound frequencies above the audible sound
spectrum (in general, higher than 20,000 Hz).;
VIBRATION ISOLATOR-A resilient support for machinery and other
equipment that might be a source of vibration, designed to reduce
the amount of vibration transmitted to the building structure.
WAVEFORM—A presentation of some feature of a sound wave, e.g., the
sound pressure, as a graph showing the moment-by-moment variation
of sound pressure with time.
WAVEFRONT—An imaginary surface of a sound wave on its way through
the atmosphere. At all points on the wavefront, the wave is of equal
amplitude and phase.
WAVELENGTH—For a periodic wave (such as sound in air), the perpendic-
ular distance between analogous points on any two successive waves.
The wavelength of sound in air or in water is inversely proportional
to the frequency of the sound. Thus, the lower the frequency, the
longer the wavelength.
WHITE NOISE—Noise whose energy is uniform over a wide range of fre-
quencies, being analogous in spectrum characteristics to white light.
White noise has a "hissing" sound. See also broadband noise.
WRAPPING-See duct lining or wrapping.
« U. S. GOVERNMENT PRINTING OFFICE : 1980 311-726/3878
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