"Good neighbors keep their noise to themselves."
This dictionary of terms related to noise and noise control
is a useful tool for those who are investigating how to reduce the amount
of noise pollution in their environment. The original authors did an
excellent job of defining terms associated with sound and noise, and we
have inserted information in green where we
felt there were new or useful words or terms, and where we felt a longer
definition might improve communication.
If you come across a word not defined here, or
where the definition isn't clear enough, let us know using our web form and we'll give you
further information and improve the dictionary.
You can navigate the dictionary using your browsers' "Find" function (under "Edit" usually) and you can click on the internal links.
INTRODUCTION
Trying to communicate is difficult enough even when we have clear
and sharp definitions of all the words we exchange.
Scientists, in their extreme pursuit of accuracy, are known to
burden their communications with endless qualifiers rendering
comprehension impossible.
The authors have felt, for some time, the need to compile a
glossary of technical terms commonly used in the field of noise
control. To achieve this goal we borrowed freely from many sources.
Those we consciously remember are listed in the Bibliography and to
the authors we express our deepest thanks.
We equally appreciated the help and suggestions of Lon Loken and
Sandee
Plummer with Northern Sound, and Jayne
Stilwell with the Minnesota Pollution
Control Agency.
Special thanks go to Brian Lowey, Brian
Lowey Associates, who helped us immensely in the translations
from acoustics to English, and managed to survive the task with a
minimum of damage to his intellect.
As authors we must recognize with appreciation those who helped us
along the way; however, we accept full responsibility nor any mistakes
and inaccuracies that you rind in the following pages. We don't intend
this work to be the final word, rather the beginning of a continuing
process to focus on the language we use in our profession and improve
our communications.
If you must grind your teeth while reading any of our
definitions, or you don't find what you are looking for, save the wear
and tear on your molars and send us your suggestions. We would really
appreciate your input.
ABSORPTION. A property of materials that
allows a reduction in the amount of sound energy reflected. The
introduction of an absorbent into the surfaces of a room will reduce
the sound pressure level in that room
by not reflecting all of the sound energy striking the room's
surfaces. 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 the sound-absorbing ability of a surface. It is defined as
the fraction of incident sound energy absorbed or otherwise not
reflected by a surface. Unless otherwise specified, a diffuse sound
field is assumed. The values at the sound-absorption coefficient
usually range from about 0.01 for marble slate to almost 1.0 for long
absorbing wedges often used in anechoic rooms.
ACOUSTICS. (1) The science of sound,
including the generation, transmission, 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 within the room.
ACOUSTIC TRAUMA. Damage to the
hearing mechanism caused by a sudden burst of intense noise, or by a
blast. The term usually implies a single traumatic event.
AIRBORNE SOUND. Sound that reaches
the point of interest by propagation through air.
AMBIENT NOISE. The total of all
noise in the environment, other than the noise
from the source of interest. This term is used interchangeably with
background noise.
ANECHOIC ROOM. A room in which the
boundaries absorb nearly all the incident sound, thereby, effectively
creating free field conditions.
ANSI. The American National Standards
Institute.
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 AI is less than 0.1,
speech intelligibility is generally low. If it is above 0.6, speech
intelligibility is generally high.
ATTENUATION. The reduction of sound
intensity by various means (e.g., air, humidity, porous materials...).
AUDIO FREQUENCY. The frequency of
oscillation of an audible sound wave. Any frequency between 20 and
20,000 Hz.
AUDIOGRAM. A graph showing individual
hearing acuity as a function of frequency.
AUDIOMETER. An instrument for
measuring individual hearing acuity.
A-WEIGHTED SOUND LEVEL. A
measure of sound pressure level designed to reflect the acuity of the
human ear, which does not respond equally to all frequencies. The ear
is less efficient at low and high frequencies than at medium or
speech-range frequencies. Therefore, to describe a sound 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 dBA. The
A-weighted sound level is also called the noise level. Sound level
meters have an A-weighting network for measuring A-weighted sound
level.
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 often used to mean the noise in the environment, other
than the noise from the source of interest.
BAND. Any segment of the frequency spectrum.
BAND PASS FILTER. A wave filter
that has a single transmission band extending from a lower cutoff
frequency greater than zero to a finite upper cutoff frequency.
BROADBAND NOISE. Noise with
components over a wide range of frequencies.
CALIBRATOR (ACOUSTICAL). A device which
produces a known sound pressure on the microphone of a sound level
measurement system, and is used to adjust the system to Standard
specifications.
COCHLEA. A spirally coiled organ located
within the inner ear which contains the receptor organs essential to
hearing.
CUTOFF FREQUENCIES. The
frequencies that mark the ends of a band, or the points at Which the
characteristics of a filter change from pass to no-pass.
CYCLE. The complete sequence of values of a
periodic quantity that occurs during one period.
CYCLES PER SECOND. A measure of
frequency numerically equivalent to hertz.
CYLINDRICAL WAVE. 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.
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 to the addition of sound-dissipative
materials.
dBA. Unit of sound level. The weighted sound
pressure level by the use of the A metering characteristic and
weighting specified in ANSI Specifications for Sound Level Metere,
S1.4-1983. dBA is used as a measure of human response to sound.
DECIBEL. A unit of sound pressure level,
abbreviated dB.
DIFFRACTION. A modification which
soundwaves undergo in passing by the edges of solid bodies.
DIRECTIVITY INDEX. In a given
direction froma 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.
DOPPLER EFFECT (DOPPLER SHIFT). The
apparent upward shift in frequency of a sound as a noise source
approaches the listener or 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.
DOSIMETER. A device worn by a worker
for determining the worker's accumulated noise exposure with regard to
level and time according to a pre-determined integration formula.
ECHO. A wave that has been reflected or
otherwise returned with sufficient magnitude and delay, so as to be
detected as a wave distinct from that directly transmitted.
EQUIVALENT A-WEIGHTED
SOUND LEVEL (Leq). The constant sound level that, in a given time
period, would convey the same sound energy as
the actual time-varying A-weighted sound level.
FAR FIELD. Describes a sound source
region in free space where the sound pressure level obeys the
inverse-square law (the sound pressure level decreases 6 dB with each
doubling of distance from the source). Also, in this region the sound
particle velocity is in phase with the sound pressure. Closer to the
source where these two conditions do not hold constitutes the "near
field" region.
FILTER. A device for separating components
of a signal on the basis of their frequency. It allows components in
one or more frequency bands to pass relatively unattenuated, and it
attenuates components in other frequency bands.
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 repeats itself. Now expressed in
hertz (Hz), formerly in cycles per second (cps).
HAIR CELL. Sensory cells in the cochlea
which transform the mechanical energy of sound into nerve impulses.
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, etc...
HEARING. The subjective human response to
sound.
HEARING LEVEL. A measured threshold
of hearing at a specified frequency,
expressed in decibels relative to a specified
standard of normal hearing. The deviation in decibels of an
individual's threshold from the zero reference of the audiometer.
HEARING LOSS. A term denoting an
impairment of auditory acuity. The amount of hearing impairment, in
decibels, measured as a set of hearing threshold levels at specified
frequencies. Types of hearing loss are: 1. Conductive: A loss
originating in the conductive mechanism of the ear; 2. Sensor-neural:
A loss originating in the cochlea or the fibers of the auditory nerve;
3. Noise induced: A sensor-neural loss attributed to the effects of
noise.
HEARING THRESHOLD LEVEL
(HTL). Amount (in decibels) by which an individual's threshold of
audibility differs from a standard audiometric threshold.
HERTZ (Hz). Unit of measurement of
frequency, numerically equal to cycles per
second
IMPACT INSULATION CLASS
(IC). A single-figure rating that compares the impact sound insulating
capabilities of floor-ceiling assemblies to a reference contour.
IMPACT SOUND. The sound produced by the
collision of two solid objects. Typical sources are footsteps, dropped
objects, etc., on an interior surface (wall, floor, or ceiling)
of a building.
IMPULSIVE NOISE, a) Either a
single sound pressure peak (with either a rise time less than 200
milliseconds or total duration less than 200 milliseconds) or multiple
sound pressure peaks (with either rise time less than 200 milliseconds
or total duration less than 200 milliseconds) spaced at least by 200
millisecond pauses, b) A sharp sound pressure peak occurring in a
short interval of time.
INFRASONIC. Sounds of a frequency
lower than 20 hertz.
INTENSITY. The sound energy flow
through a unit area in a unit time.
INVERSE SQUARE LAW. A
description of the acoustic wave behavior in which the mean-square
pressure varies inversely with the square of the distance from the
source. This behavior occurs in free
field situations, where the sound pressure level decreases 6 dB
with each doubling of distance from the source.
.
ISO. The International Organization for
Standardization.
LEVEL. The logarithm of the ratio of a
quantity to a reference quantity of the same kind. The base of the
logarithm, the
reference quantity, and the kind of level must be specified.
LOGARITHM. The exponent that indicates
the power to which a number must be raised to produce a given number.
For example, for the base 10 logarithm, used in acoustics, 2 is the
logarithm of 100.
LOUDNESS. The subjective judgment of
intensity of a sound by humans. Loudness depends upon the sound
pressure and frequency of the stimulus. Over much of the frequency
range it takes about a threefold increase in sound pressure (a tenfold
increase in acoustical energy, or, 10 dB) to produce a doubling of
loudness.
.
LOUDNESS LEVEL. Measured in
phons it is numerically equal to the median sound
pressure level (dB) of a free progressive 1000 Hz wave presented to
listeners facing the source, which in a number of trials is judged by
the listeners to be equally loud.
MASKING. 1. The process by which the
threshold of audibility for a
sound is raised by the presence of another (masking) sound. 2. The
amount by which the threshold of audibility of a sound is raised by
the presence of another (masking) sound.
MASKING NOISE. A noise that is intense enough to render
inaudible or unintelligible another sound that is also present.
MEDIUM. A substance carrying a sound wave.
NEAR FIELD. The sound field very near to a
source, where the sound pressure does
not obey the inverse-square law and
the particle velocity is not in phase with the sound pressure.
NIOSH. The National Institute for
occupational Safety and Health.
NOISE, 1. Unwanted sound. 2. Any sound not
occurring in the natural environment, such as sounds emanating from
aircraft, highways, industrial, commercial and residential sources. 3.
An erratic, intermittent, or statistically random oscillation.
NOISE ISOLATION CLASS.
(NIC). A single number rating derived in a prescribed manner from the
measured values of noise reduction between two areas or rooms. It
provides an evaluation of the sound isolation between two enclosed
spaces that are acoustically connected by one or more paths.
NOISE LEVEL. For airborne sound ,
unless specified to the contrary, it is the A-weighted sound level.
NOISE REDUCTION (NR). The
numerical difference, in decibels, of the average sound pressure
levels in two areas or rooms. A measurement of "noise reduction"
combines the effect of the sound 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 acoustical absorption performance
of a material, calculated by averaging its sound absorption
coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest
multiple of 0.05.
NON-IMPULSIVE NOISE. all
noise not included in the definition of impulsive noise.
OCTAVE. The interval between two sounds
having a frequency ratio of two.- There are 8 octaves on the keyboard
of a standard piano.
OCTAVE BAND. A segment of the
frequency spectrum separated by an octave.
OCTAVE BAND LEVEL. The
integrated sound pressure level of only those sine-wave components in
a specified octave band.
OSCILLATION. The variation with time,
alternately increasing and decreasing, of (a) some feature of an
audible sound, such as the sound pressure;
or (b) some feature of a vibrating solid object, such as the
displacement of its surface.
OSHA The Occupational Safety and Health
Administration.
PEAK SOUND PRESSURE. The
maximum absolute value of the instantaneous sound pressure in a
specific time interval. Note: in the case of a periodic wave, if the
time interval considered is a complete period, the peak sound pressure
becomes identical with the maximum sound pressure.
PERIOD. The duration of time it takes for a
periodic wave form (like a sine wave) to repeat itself.
PERMANENT THRESHOLD
SHIFT (PTS). A permanent decrease of the acuity of the ear at a
specified frequency as compared to a previously established reference
level. The amount of permanent threshold shift is customarily
expressed in decibels.
PHON. The unit of measurement for loudnes
level.
PINK NOISE. Noise with constant energy
per octave band width.
PITCH. The attribute of auditory sensation
that orders sounds on a scale extending from low to high. Pitch
depends primarily upon the frequency of the sound stimulus, but it
also depends upon the sound pressure and wave form of the stimulus.
PLANE WAVE. A wave whose wave fronts
are parallel and perpendicular to the direction in which the wave is
traveling.
PRESBYCUSIS. The decline in hearing
acuity that is attributed to the aging process.
PURE TONE. A sound for which the sound
pressure is a simple sinusoidal function of the time, and
characterized by its singleness of pitch.
RANDOM NOISE. An oscillation whose
instantaneous magnitude is not specified for any given instant of
time. It can be
described statistically by probability distribution functions giving
the traction of the total time that the magnitude of the noise lies
within a specified range.
REFLECTION. The return of a sound wave
from a surface.
REFRACTION. The bending of a sound
wave from its original path, either because it is passing from one
medium to another or by changes in the physical properties of the
medium, e.g., a temperature or wind gradient in the air.
RESONANCE. The relatively large
amplitude of vibration produced when the frequency of some source
of sound or vibration "matches" the natural frequency of
vibration of some object, component, or system.
RESONATOR. A device that resounds or
vibrates in sympathy with a source of sound or vibration.
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 (homogeneous) as possible.
REVERBERATION TIME (RT). The
reverberation time of a room is the time taken for the sound pressure
level to decrease 60 dB from 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 as well as of the amount
of acoustical absorption present inside the room. Rooms with long
reverberation times are called live rooms.
RMS SOUND PRESSURE. The square
root of the time averaged square of the sound pressure.
ROOT-MEAN-SQUARE (RMS). 1. The
root-mean-square value of a time-varying quantity is obtained by
squaring the function at each instant, obtaining the average of the
squared values over the interval of interest, and then taking the
square root of this average. For a sine wave, if you multiply the RMS
value by the square root of 2, or about l.41, you 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 impulsive noises. 2. A
term' describing the mathematical process of determining an 'average'
value of a complex signal.
SABIN. A measure of the sound absorption of
a surface; it is the equivalent of one square foot of a perfectly
absorptive surface.
SHIELDING. The attenuation of a sound,
achieved by placing barriers between a sound source and the receiver
SONE. The unit of measurement for loudness. One
sone is the loudness of a sound whose loudness level is 40
phons. Loudness is proportional to the sound's
loudness rating, e.g., two sones are twice as loud as one sone.
SOCIOCUSIS. Loss of hearing caused by
noise exposures that are part of the social environment, exclusive of
occupational-noise exposure, physiological changes with age, and
disease.
SOUND. 1. An oscillation
in pressure, stress, particle displacement, particle velocity, etc.,
in an elastic or partially elastic medium, or the superposition of
such propagated alterations. 2. An auditory sensation evoked by the
oscillation described above. Not all sound waves can evoke an auditory
sensation: e.g. ultrasound.
SOUND LEVEL. The weighted sound
pressure level obtained by the use of a sound level meter and
frequency weighting network, such as A, B, or C as specified in ANSI
specifications for sound level meters (ANSI Sl.4-1971, or the latest
approved revision). If the frequency weighting employed is not
indicated, the A-weighting is implied.
SOUND LEVEL METER. An
instrument comprised of a microphone, amplifier, output meter, and
frequency-weighting networks which is used for the measurement of
noise and sound levels.
SOUND POWER. The total sound energy
radiated by a source per unit time. The unit of measurement is the
watt.
SOUND PRESSURE. The instantaneous
difference between the actual pressure produced by a sound wave and
the average or barometric pressure at a given point in space.
SOUND PRESSURE LEVEL (SPL).
20 times the logarithm, to the base 10, of the ratio of the pressure
of the sound measured to the reference pressure, which is 20
micronewtons per square meter. In equation form, sound pressure level
in units of decibels is expressed as SPL (dB) = 20 log p/pr.
SOUND TRANSMISSION CLASS
(STC). The preferred single figure rating system designed to give an
estimate of the sound insulation properties of a structure or a rank
ordering of a series of structures.
SOUND TRANSMISSION LOSS
(STL). A measure of sound insulation provided by a structural
configuration. Expressed in decibels, it is 10 times the logarithm to
the base 10 of the reciprocal of the sound transmission coefficient of
the configuration.
SPECTRUM. The description of a sound
wave's resolution into its components of frequency
and amplitude.
SPEECH-INTERFERENCE
LEVEL (SIL). A calculated quantity providing a guide to the
interference of a noise with the reception of speech. The
speech-interference level is the arithmetic average of the octave band
levels
of the interfering noise in the most important part of the speech
frequency range. The levels in octave bands centered at 500, 1000, and
2000 Hz are commonly averaged to determine the speech-interference
level.
SPEED (VELOCITY) OF SOUND IN AIR.
344 m/sec (l128 ft/sec) at 70 degrees F in air at
sea level.
SPHERICAL DIVERGENCE. The
condition of propagation of spherical waves that relates to the
regular decrease in intensity of a spherical sound wave at
progressively greater distances from the source. Under this condition
the sound pressure level decreases 6 decibels with each doubling of
distance from the source.
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.
.
STEADY-STATE SOUNDS. Sounds
whose average characteristics remain relatively constant in time. A
practical example of a steady-state sound source is an air
conditioning unit.
TEMPORARY THRESHOLD
SHIFT (TTS). A temporary impairment of hearing acuity as indicated by
a change in the 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 the 1000 Hz third-octave band in common use.
THRESHOLD OF AUDIBILITY
(THRESHOLD OF DETECTABILITY). The minimum sound pressure level at
which a person can hear a specified frequency of sound over a
specified number of trials.
THRESHOLD OF PAIN. The minimum
sound pressure level of a sound outside the ear that will produce a
transition from discomfort to definite pain.
THRESHOLD SHIFT. A change in the
threshold of audibility at a specified frequency from a threshold
previously established. The amount of threshold shift is customarily
expressed in decibels.
TIMBRE. An attribute of auditory sensation
allowing a subject to judge that two sounds similarly presented and
having the same loudness and pitch
are dissimilar, e.g., trumpet vs. violin.
TINNITUS. Ringing in the ear or noise
sensed in the head. Onset may be due to an acoustic
trauma and persist in the absence of acoustical stimulation (in
which case it may indicate a lesion of the auditory system).
TONE. A sound of definite pitch. A pure tone
has a sinusoidal wave form.
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.
ULTRASONIC. Sounds or a frequency higher
than 20,000 hertz.
VIBRATION. An oscillatory motion of
solid bodies described by displacement, velocity, or acceleration with
respect to a given reference point.
VIBRATION ISOLATOR. A
resilient support for vibrating equipment designed to reduce the
amount of vibration transmitted to the other structures.
WAVE. A disturbance that travels through a
medium by virtue of the elastic properties of that medium.
WAVELENGTH. For a periodic wave (such
as sound in air), the 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.
WEIGHTING. Prescribed frequency
filtering provided in a sound level meter.
WHITE NOISE. Noise whose energy is
uniform over wide range of frequencies, being analogous in
spectrum characteristics to white light.
WINDSCREEN. A porous device used to cover
the microphone of a sound level measurement system which is designed
to minimize the effects of winds and wind gusts on the sound levels
being measured. Typically made of open cell polyurethane foam and
spherically shaped.
"About Sound," U.S. Environmental Protection Agency,
Washington, D.C., May 1976.
"Background for Loss of Hearing Claims," Alliance of
American Insurers, Chicago, IL, 1981.
"Guidance Manual for State and Local Projectors," U.S.
Environmental Protection Agency, Washington, D.C., 1980.
"NPC 1--Definition, Severability and Variances for Noise
Pollution Control Regulations" Minnesota Pollution Control
Agency, St. Paul, MN, 1974.
Peterson, A.P.G., and Gross, E.E., "Handbook of Noise
Measurement," General Radio, Concord, MA, Seventh Edition.
"USA Standard Acoustical Terminology, S1.1," American
National Standards Institute, New York, NY, 1976.
