Glossary of Acoustical Terms A-Z
The acoustical terms below will help you understand acoustics terminology and acoustical design.
The properties of a material composition to convert sound energy into heat thereby reducing the amount of sound energy that can be reflected.
The properties of a material to absorb or reflect sound (adjective) acoustically, (adverb).
A review of a space to determine the level or reverberation or reflected sound in the space (in seconds) as influenced by the building materials used
to construction the space. Also, a study of the amount of acoustical absorption required to reduce reverberation and noise.
The acoustical characteristics of a space or room influenced by the amount of acoustical absorption, or lack of it, in the space.
Acoustics is the science of sound, including its production, transmission and effects. The acoustics of a room are those qualities that together
determine its character with respect to the perception of sound.
Ambient noise encompasses all sound present in a given environment, being usually a composite of sounds from many sources near and far.
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 to determine its character with respect to distinct hearing.
Acoustical materials spaced apart can have greater absorption than the 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 perimeters.
Acronym for American Society of Testing and Materials
Test Method for Surface Burning Characteristics of Building Materials. This test evaluates the spread of fire along the surface materials. It is not a fire
The reduction of sound energy as a function of distance traveled.
A-Weighted Sound Level
A measure of sound pressure level designed to reflect the response of the human ear, which does not respond equally to all frequencies. To
describe sound in a manner representative of the human 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 levels.
An instrument for measuring hearing acuity.
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.
Low frequency reflections. In small rooms, acoustical panels with air space behind can better help control low frequency reflectivity.
In acoustical industry terms, an acoustical panel suspended in a horizontal position from ceiling/roof structure. Similar to a baffle but in a horizontal
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.
In acoustics, the cycle is the complete oscillation of pressure above and below the atmospheric static pressure. The number of oscillations that
occur in the time frame on one second. (See Frequency). Low frequency sounds have fewer and longer oscillations.
Cycles Per Second
Damping is the dissipation of vibratory energy in solid media and structures with time or distance. It is analogous to the absorption of sound in air.
A dimensionless unit which denotes the ratio between two quantities that are proportional to power, energy or intensity. One of these quantities is a
designated reference by which all other quantities of identical units are divided. The sound pressure level in decibels is equal to 10 times the
logarithm (to the base 10) of the ratio between the pressure squared divided by the reference pressure squared. The reference pressure used in
acoustics is 20 microPascals. Please visit our Sound Bytes page for more information.
Deflection is the distance an elastic body or spring moves when subjected to a static or dynamic force. Typical units are inches or mm.
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.
Reflected sound producing a distinct repetition of the original sound. Echo in mountains is distinct by reason of travel after original signal has
Flexible, low modulus material capable of expanding and contracting and returning to original dimensions without fatigue.
An assembly consisting of 1) the penetrant or penetrants 2) the opening through a particular type of wall or floor, and 3) the firestop materials and
design that are used to seal the opening or protect the penetrants.
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.
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.
Short echoes in 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.
Sound waves from a source outdoors where there are no obstructions.
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.
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. For example, higher frequency sound or pitch vs. low frequency.
A degree of hearing loss, temporary or permanent, due to many causes. Hearing loss can be caused by illness, disease, or 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 mild, moderate
or severe degree as opposed 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 adversely affected by long reverberation times.
16 – 2000 Hz (Speech Intelligibility)
600 – 4800 Hz (Speech Privacy)
250 – 2500 Hz (Typical small table radio)
Frequency of sound expressed by cycles per second. (See Cycle).
Inverse Square Law
Sound levels fall off with distance traveled. Sound level drops off 6 dB from the 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.
A listener’s auditory impression of the strength of a sound. The average deviation above and below the static value due to a 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.
The process by which the threshold of hearing of one sound is raised due to the presence of another.
Mass is 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.
Standards established by ASTM to represent typical installation for purpose of testing materials. For example, a mounting test specimen is mounted
directly the test room surface or furred out to produce an air space behind.
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 the loudest locations in a room. NC
criteria can be referred to equivalent dBA levels.
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 method.
Noise Reduction (NR)
The amount of noise that is reduced through the introduction of sound absorbing materials. The level (in decibels) of sound reduced on a
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. The NRC rating can be viewed as a percentage (example: .80 = 80%) of what soundwaves
that come in contact with the acoustical material are absorbed by the material and NOT reflected back within the room.
A pitch interval of 2 to 1. The tone whose frequency is twice that of the given tone.
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, 250, 500, 1000, 2000, and 4000 Hz
The perceived auditory sensation of sounds expressed in terms of high or low frequency stimulus of the sound.
The amount of sound wave energy (sound) that is reflected off a surface. Hard non-porous surfaces reflect more sound that soft porous surfaces.
Some sound reflection can enhance the quality of the signal of speech and music.
The emphasis of sound at a particular frequency.
Resonant Frequency is a frequency at which resonance exists.
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. Please
visit our Sound Bytes page for more information.
The reverberation time of a room is the time it takes for sound to decay by 60 dB once the source of sound has stopped. 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 intelligibility.
A unit of sound absorption based on 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 125 – 4000 Hz. The number of sabins developed by other acoustical materials
are determined by the amount of material used and its absorption coefficients.
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 (time required for sound to decay 60
dB after source has stopped) in seconds. V=Volume of room in cubic feet. A=total square footage of absorption in sabins.
A thin layer of material between 2 layers of absorptive material, such as foil, lead, steel, etc. that prevents sound wave from passing through
Signal to Noise Ratio
The sound level at the listeners ear of a speaker above the background noise level. The inverse square law impacts the S/N ratio.
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 is an oscillation in pressure, stress particle displacement, particle velocity in a medium. Sound produces an auditory sensation caused by
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 call is absorption coefficient.
Sound Absorption Coefficient
The fraction of energy striking a material or object that is not reflected. For instance, if a material reflects70% of the sound energy incident upon its
surface, then its Sound Absorption Coefficient would be 0.30. SAC=absorption/area in sabins per sq. ft.
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.
A subjective measure of sound expressed in decibels as a comparison corresponding to familiar sounds experienced in a variety of situations.
The sound pressure is 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 time the logarithm to the base of 10 of the ratio of the sound pressure to the reference
pressure. The reference pressure shall be explicitly stated and is defined by standard.
Sound Level Meter
A device that converts sound pressure variations in air into corresponding electronic signals. The signals are filtered to excluded signals outside
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.
The ability of a listener to hear and correctly interpret verbal messages. In a classroom with high ceilings and hard parallel surfaces such as glass
and tile, speech intelligibility is a particular problem. Sound bounces off walls, ceilings and floors, distorting the teacher’s instructions and interfering
with students’ ability to comprehend. Centers between 250 and 4000k.
The degree to which speech is unintelligible between offices. Three ratings are used: Confidential, Normal (Non Obtrusive) and Minimal.
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.
Sounds of a frequency higher than 20,000 Hz. The frequency region containing these frequencies is called the ultrasonic region.
Vibration is 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.
A resilient support that tends to isolate a mechanical system from steady state excitation.
The cubic space of a room bounded by walls, floors, and ceilings determined by the Volume=Length x Width x Height of space. Volume influences
Sound that passes through air produces a wavelike motion of compression and refraction. Wavelength is the distance between two identical
positions in the cycle or wave. Similar to ripples or waves produces by dropping a stone in water. Length of sound wave varies with frequency. Low
frequency equals longer wavelengths.
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