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.
