This glossary of terms has been provided with a brief description, for the most part in non technical terms in an effort to remove some of the mystery surrounding Acoustics. While the explanations may not be totally correct in their literal interpretations it is hoped that the plain language approach will provide a better understanding of the terminology frequently used in the field of acoustics.
ACOUSTICS: The science of Sound. Its production,
transmission and effects.
ACOUSTICAL: The properties of a
material to absorb or reflect Sound (adjective) Acoustically, (Adverb).
ACOUSTICAL ANALYSIS: A review of a space to determine
the level of reverberation or reflected sound in the space (in seconds)
influenced by the building materials used to construct the space. Also the amount of acoustical absorption required to reduce reverberation
ACOUSTICAL CONSULTANT: A professional usually with
an engineering degree who is experienced in providing advice on acoustical
requirements, and noise control in a variety of situations.
ACOUSTICAL ENVIRONMENT: The acoustical characteristics
of a space or room influenced by the amount of acoustical absorption,
or lack of it in the space.
AIRBORNE SOUND: Sound that reaches the point of interest by propagation through air.
ARCHITECTURAL ACOUSTICS: The control of noise in
a building space to adequately support the communications function within
the space and its effect on the occupants. The qualities of the building
materials used determine its character with respect to distinct hearing.
ARTICULATION CLASS: A single number rating used
for comparing acoustical ceilings and acoustical screens for speech
privacy purposes. AC values increase with increasing privacy and range
from approximately 100-250. This classification supercedes Speech Privacy
Noise Isolation Class (NIC) rating method.
ARTICULATION INDEX (AI): A measure
of speech intelligibility influenced by Acoustical Environment rated
from 0.01 to 1.00.The higher the number the higher the intelligibility
of words and sentences understood from 0-100%.
ABSORPTION: The properties
of a material composition to convert sound energy into heat thereby
reducing the amount of energy that can be reflected.
AREA EFFECT: Acoustical materials
spaced apart can have greater absorption than same amount of material
butted together. The increase in efficiency is due to absorption by
soft exposed edges and also to diffraction of sound energy around panel
ASSISTIVE LISTENING DEVICE: An electronic device
that provides amplification of sound to a hearing impaired person. Device
include personal hearing aids, magnetic induction loops, FM radio systems
and infrared systems. All have advantages and disadvantages and some
may be dependent on good acoustical environment for optimal performance.
ATTENUATION: The reduction of sound energy as a
function of distance traveled. (See also Inverse Square Law).
A WEIGHTING: An electronic
filtering system in a sound meter that allows meter to largely ignore
lower frequency sounds in a similar fashion to the way our ears do.
AMBIENT NOISE/SOUND: Noise level in a space from
all sources such as HVAC or extraneous sounds from outside the space.
Masking sound or low-level background music can contribute to ambient
level of sound or noise.
BACKGROUND NOISE: The sum total of all noise generated
from all direct and reflected sound sources in a space that can represent
an interface to good listening and speech intelligibility. (Hearing
impaired persons are especially victimized by background
BAFFLE: A free hanging acoustical
sound absorbing unit. Normally suspended vertically in a variety of
patterns to introduce absorption into a space to reduce reverberation
and noise levels.
BARRIER: Anything physical or an environment that
interferes with communication or listening. A poor acoustical environment
can be a barrier to good listening and especially so for persons
with a hearing impairment.
BEL: A measurement of sound intensity named in
honor of Alexander Graham Bell. First used to relate intensity to a
corresponding to hearing sensation.
BOOMINESS: Low frequency reflections. In small
rooms acoustical panels with air space behind can better help control
low frequency reflectivity.
CLOUD: In acoustical industry terms,
an acoustical panel suspended in a horizontal position from ceiling/roof
structure. Similar to a baffle but in a horizontal position.
COCKTAIL PARTY EFFECT: Sound in a noisy crowded
room generated mostly by conversation. Levels rise and fall as people
compete with one another to be heard. Perception of speech can be nearly
impossible in high levels of noise.
COCHLEA: A snail shaped mechanism
in the inner ear that contain hair cells of basilar membrane that vibrate
to aid in frequency recognition.
CYCLE: In acoustics, the cycle is the complete
oscillation of pressure above and below the atmospheric static pressure.
CYCLES PER SECOND: The number of oscillations that
occur in the time frame of one second. (See FREQUENCY.) Low frequency
sounds have fewer and longer oscillations.
DAMPING: The dissipation of vibratory energy in solid media and structures with time or distance. It is analogous to the absorption of sound in air.
DECIBEL (dB): Sound level in decibels as a logarithmic
ratio. Sound intensity described in decibels. i.e.: Breathing 5 dB,
activity 50 dB, Jet Aircraft during takeoff at 300' distance 130 dB. (See submenu TABLES under Acoustics for a table on Sound Source of
DEFLECTION: The distance an elastic body or spring moves when subjected to a static or dynamic force. Typical units are inches or mm.
DEAF: Loss of auditory sensation with or without
use of assistive listening device. Loss of hearing more severe than
characterized as "Hearing Impaired".
DIFFUSION: is the scattering or random reflection
of a sound wave from a surface. The directions of reflected sound is
changed so that listeners may have sensation of sound coming from
all directions at equal levels.
EAR: An incredible hearing mechanism consisting
of outer, middle and inner ear segments that cause sound pressures to
be picked up by the ear that are transmitted through auditory nerves
where signals are interpreted by brain as sound.
ECHO: Reflected sound producing
a distinct repetition of the original sound. Echo in mountains is distinct
by reason of distance of travel after original signal has ceased.
ECHO FLUTTER: Short echoes in a small reverberative
spaces that produce a clicking, ringing or hissing sound after the original
sound signal has ceased. Flutter echoes may be present in long narrow
spaces with parallel walls.
EQUAL LOUDNESS CONTOURS: Curves represented in
graph form as a function of sound level and frequency which listeners
perceive as being equally loud. High frequency sounds above 2000 Hz
are more annoying. Human hearing is less sensitive to low frequency
sound. (See also PHON.)
FLAME SPREAD: Classification indicating propagation of flame across a sample compared to flame propagation across concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.
FLANKING: The transmission of sound around the perimeter or through holes within partitions (or barriers) that reduces the otherwise obtainable sound transmission loss of a partition. Examples of flanking paths within buildings are ceiling plena above partitions; ductwork, piping, and electrical conduit penetrations through partitions; back-to-back electrical boxes within partitions, window mullions, etc.
FREE FIELD: Sound waves from a source outdoors
where there are no obstructions.
FREQUENCY: The number of oscillations or cycles
per unit of time. Acoustical frequency is usually expressed in
units of Hertz (Hz) where one Hz is equal to one cycle per second.
FREQUENCY ANALYSIS: An analysis
of sound to determine the character of the sound by determining the
amount of sounds at various frequencies that make up the overall sound
spectrum. i.e.: higher frequency sound or pitch vs. low frequency.
HEARING IMPAIRMENT: A degree of hearing loss, temporary
or permanent due to many causes. Hearing loss can be caused by illness,
disease, or by exposure to excessively high noise levels. Affects
25-50 million people in USA of all ages. Hearing impairment as
generally used means a hearing loss of a mild, moderate, or severe degree
as apposed to "Deafness" which is generally described as little
or no residual hearing with or without the aid of an assistive listening
device. Hearing Impaired persons are particularly victimized by
long reverberation times.
HEARING RANGE: 16-20000 Hz (Speech Intelligibility)
600-4800 Hz (Speech Privacy)
250-2500 Hz (Typical small table radio)
HERTZ (Hz): Frequency of sound expressed by cycles
per second. (See CYCLE).
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.
INTENSITY: (See LOUDNESS).
INVERSE SQUARE LAW: Sound levels fall off with
distance traveled. Sound level drops off 6 dB from
source point for every doubling of distance.
LIVE END/DEAD END: An acoustical treatment plan for rooms in which one end is highly absorbent and the other end is reflective and diffusive.
LOUDNESS: The average deviation above and below
the static value due to sound wave is called sound pressure. The energy
expended during the sound wave vibration is called intensity and is
measured in intensity units. Loudness is the physical resonance to sound
pressure and intensity.
MASKING: The process by which the threshold of hearing of one sound is raised due to the presence of another.
MASS: The fundamental property of a material relevant to sound transmission loss through that material. Generally, the more massive the material, the greater the sound transmission loss.
MOUNTING: Standards established
by ASTM to represent typical installation for purpose of testing materials.
i.e.: a mounting test specimen mounted directly to test room surface.
D mounting furred out to produce air space behind.
NOISE: Unwanted sound that is annoying or interferes
with listening. Not all noise needs to be excessively loud to
represent an annoyance or interference.
NOISE CRITERIA (NC): Noise criteria curves used
to evaluate existing listening conditions at ear level by measuring
sound levels at loudest locations in a room. NC criteria can be referred
to equivalent dBA levels. NC curves are critical to persons with hearing
NOISE ISOLATION CLASS (NIC): A
Single number rating of the degree of speech privacy achieved through
the use of an Acoustical Ceiling and sound absorbing screens in an open
office. NIC has been replaced by the Articulation Class (AC) rating
NOISE REDUCTION (NR): The amount of noise that
is reduced through the introduction of sound absorbing materials. The
(in decibels) of sound reduced on a logarithmic basis. (See TABLES
submenu under Acoustics for Sound Pressure Level Changes).
NOISE REDUCTION COEFFICIENT (NRC): The NRC of an
acoustical material is the arithmetic average to the nearest multiple
of 0.05 of its absorption coefficients at 4 one third octave bands with
center frequencies of 250, 500, 1000, 2000 Hertz.
OCTAVE BANDS: Sounds that contain energy over a
wide range of frequencies are divided into sections called bands. A
common standard division is in 10 octave bands identified by their
center frequencies 31.5, 63, 125, 250, 500, 1000, 2000, 4000 Hz.
OTO: Pertaining to the ear.
OTOLOGIST: A doctor specializing in the structor,
disorders and treratment of the ear.
OTOLARYNGOLIST: A doctor specializing in disorders and treatment of the ear nose and
PHON: Loudness contours. A subjective impression of equal loudness
by listeners as a function of frequency and sound level (dB). An increase
in low frequency sound will be perceived as being much louder than an
equivalent high frequency increase.
PITCH: The perceived auditory sensation of sounds
expressed in terms of high or low frequency stimulus of the sound.
PRESBYCUSIS: The loss of hearing due primarily
to the aging process High frequency loss is frequently a result of early
REFLECTION: The amount of sound wave energy (sound)
that is reflected off a surface. Hard non porous surfaces reflect more
sound than soft porous surfaces. Some sound reflection can enhance quality
of signal of speech and music. (See Echo).
RESONANCE: The emphasis of sound at a particular
RESONANT FREQUENCY: A frequency at which resonance exists.
REVERBERATION: The time taken for sound to decay
60 dB to 1/1,000,000 of its original sound level after the sound source
has stopped. Sound after it has ended will continue to reflect off surfaces
until the wave loses enough energy by absorption to eventually die out.
Reverberation time is the basic acoustical property of a room which
depends only on its dimensions and the absorptive properties of its
surfaces and contents. Reverberation has an important impact on speech
REVERBERATION TIME: Sound after
it is ended at the source will continue to reflect off surfaces until
the sound wave loses energy by absorption to eventually die out.
SABIN: A unit of sound absorption based of one
square foot of material. Baffles are frequently described as providing
X number of sabins of absorption based on the size of the panel tested,
through the standard range of frequencies 125 - 4000 Hz. The number
of sabins developed by other acoustical materials are determined by
the amount of material used and its absorption coefficients.
SABINE FORMULA: A formula developed
by Wallace Clement Sabine that allows designers to plan reverberation
time in a room in advance of construction and occupancy. Defined and
improved empirically the Sabine Formula is T=0.049(V/A) where T = reverberation
time or time required (for sound to decay 60 dB after source has stopped)
V = Volume of room in cubic feet.
A = Total square footage of absorption in sabins.
SEPTUM: A thin layer of material between 2 layers
of absorptive material. i.e.: foil, lead, steel, etc. that prevents
from piercing through absorptive material.
SIGNAL TO NOISE RATIO: Is the sound level at the
listeners ear of a speaker above the background noise level. The inverse
square law impacts on the S/N ratio. Signal to Noise Ratios are important
in classrooms and should be in range of 15 to 20 dB.
SMOKE DEVELOPED INDEX: Classification that relates to a comparison of smoke development of a particular material compared to concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.
SOUND: Sound is an oscillation in pressure, stress
particle displacement, particle velocity in a medium - (in room temperature.
In air speed of sound is 1125'/second or one mile in 5 seconds.) Sound
produces an auditory sensation caused by the oscillation.
SOUND ABSORPTION: is the property possessed by
materials, objects and air to convert sound energy into heat. Sound
waves reflected by a surface causes a loss of energy. That energy not
reflected is called its absorption coefficient.
SOUND ABSORPTION COEFFICIENT: The fraction of energy
striking a material or object that is not reflected. For instance if
a material reflects 70% of the sound energy incident upon its surface,
then its Sound Absorption Coefficient would be 0.30.
SOUND BARRIER: A material that when placed around a source of noise inhibits the transmission of that noise beyond the barrier. Also, anything physical or an environment that interferes with communication or listening. For example, a poor acoustical environment can be a barrier to good listening and especially so for persons with a hearing impairment.
SOUND LEVEL: A subjective measure of sound expressed
in decibels as a comparison corresponding to familiar sounds experienced
in a variety of situations. (See TABLES submenu under Acoustics for
Sound Source of Environment table.)
SOUND PRESSURE: The total instantaneous pressure at a point in space, in the presence of a sound wave, minus the static pressure at that point.
SOUND PRESSURE LEVEL: The sound pressure level,
in decibels, of a sound is 20 times the logarithm to the base 10 of
the ratio of the sound pressure to the reference pressure. The reference
pressure shall be explicitly stated and is defined by standards.
SOUNDPROOFING: Building materials that makes structures impervious to sound or insulates
SOUND LEVEL METER: A device that converts sound
pressure variations in air into corresponding electronic signals. The
signals are filtered to exclude signals outside frequencies desired.
SPEECH: The act of speaking. Communication of thoughts and feelings by spoken
SPEECH PRIVACY: The degree to which speech is unintelligible between offices. Three
ratings are used, Confidential, Normal
(Non obtrusive), Minimal.
SOUND TRANSMISSION CLASS (STC): This is a rating for doors, windows, enclosures, noise barriers, partitions and other acoustical products. The rating is in terms of their relative ability to provide privacy against intrusion of speech sounds. This is a one number rating system, heavily weighted in the 500Hz to 2000Hz frequency range where speech intelligitibility largely occurs.
SPECTRUM: The description of a sound wave´s components of frequency and amplitude.
TIME WEIGHTED AVERAGE (TWA): The yardstick used by the Occupational Safety and Health Administration (OSHA) to measure noise levels in the workplace. It is equal to a constant sound level lasting eight hours that would cause the same hearing damage as the variable noises that a worker is actually exposed to. (This hearing loss, of course, occurs over long-term exposures.) Same as LOSHA.
ULTRASOUNDS: Sounds of a frequency higher than 20,000 Hz. The frequency region containing these frequencies is called the ultrasonic region.
VIBRATION: A force which oscillates about some specified reference point. Vibration is commonly expressed in terms of frequency such as cycles per second (cps), Hertz (Hz), cycles per minute (cpm) or (rpm) and strokes per minute (spm). This is the number of oscillations which occurs in that time period. The amplitude is the magnitude or distance of travel of the force.
VIBRATION ISOLATOR: A resilient support that tends to isolate a mechanical system from steady state excitation.
VOLUME: The Cubic space of a room
bounded by walls, floors, and ceilings determined by Volume = Length
x Width x Height of space. Volume influences reverberation time.
(See How to Compute Cubic Volume under ACOUSTICS in menu for formulas
of more complex volumes.)
WAVELENGTH: Sound that passes through air it produces
a wavelike motion of compression and Parefaction. Wavelength is the
distance between two identical positions in the cycle or wave. Similar
to ripples or waves produced by dropping two stones in water. Length
of sound wave varies with frequency. Low frequency equals longer