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
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
AIRBORNE SOUND. Sound that
reaches the point of interest by propagation through
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
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,
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
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
CYCLE. The complete sequence
of values of a periodic quantity that occurs during
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
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
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
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
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
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
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,
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
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)
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:
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.
(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
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
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.