Acoustical Terms



The science of sound: its production, transmission and effects.


The properties of a material that absorb or reflect sound.

Acoustical Analysis

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 construct the space. Also, a study of the amount of acoustical absorption required to reduce reverberation and noise.

Acoustical Consultant

A professional, usually with an engineering degree, or certification (s) whom belongs to professional organizations, whose primary role is to provide 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.

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 to determine its character with respect to distinct hearing.


The properties of a material composition to convert sound energy into heat, thereby reducing the amount of energy that can be reflected.


The reduction of sound energy as a function of distance traveled.

A-Weighted Sound Level (Noise level)

A measure of sound pressure designed to reflect the response 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. 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. Most levels of occupational, industrial and environmental noise are measured using A-weighting.


A chart or table relating hearing level for pure tones to frequency.


An instrument for measuring hearing acuity.


A free hanging acoustical sound-absorbing unit. Normally suspended vertically in a variety of patterns to absorb and therefore reduce reverberation and noise levels. Or an arrangement within a silencer or muffler.


A material that when placed around a source of noise inhibits the transmission of that noise beyond the barrier. Also, an environment or any physical thing that interferes with communication or listening.


A measurement of sound intensity named in honor of Alexander Graham Bell. Initially used to relate intensity to a level corresponding to hearing sensation.


Low frequency reflections. In small rooms, acoustical panels with air space behind can better help control low frequency reflectivity.


A snail shaped mechanism in the inner ear that contains hair cells of basilar membrane that vibrate to aid in frequency recognition.


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.


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.

Decibel (dB)

Sound level in Bels as a logarithmic ration. Sound intensity described in decibels. For example: breathing, 5 dB; office activity 50 dB; jet aircraft during takeoff at a distance of 300 feet, 130 dB.


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. In mountains, echo is distinct by reason of sound wave travel after original signal has ceased.

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’ ).


Free Field

Sound waves from an outdoor source 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.

Frequency Analysis

An analysis to determine the character of a sound (i.e., high vs. low frequency) by measuring the amount of resonance at various frequencies that compose the overall sound spectrum.

Hearing Impairment

A degree of hearing loss, temporary or permanent, due to numerous causes. It may be caused by illness, disease, or exposure to excessively high noise levels. Affects 25 – 50 million people of all ages in the U.S. In general, hearing impairment means a hearing loss from a mild to severe degree, as opposed to “deafness” which is generally described as little or no residual hearing with or without the aid of a listening device. Hearing-impaired persons are particularly adversely affected 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’).


See ‘Loudness’.

Inverse Square Law

Newton’s mathematical equation, proving for every given distance traveled from the source, sound levels drop 6 dB.


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 one sound is raised due to the presence of another sound.


Standards established by ASTM to test the acoustics of materials by representing a typical installation. For example, a mounting test specimen is attached directly to the test room surface or furred out to produce an air space behind.


Unwanted sound that is obtrusive or interferes with listening. To qualify as interference, noise does not have to be excessively loud.

Noise Criteria (NC)

Noise criteria curves evaluate existing listening conditions by measuring sound levels (preferably at ear level) at the loudest locations in a room. Noise criteria may also be referred to 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 decreased through the introduction of sound absorbing materials. The level (in decibels) of sound reduced on a logarithmic basis.

Noise Reduction Coefficient (NRC)

The NRC of an acoustical material is the mathematical average, to the nearest multiple of 0.05, of its absorption coefficients at center frequencies of 250,500,1000, 2000 Hertz Octave. A pitch interval of 2 to 1. A tone whose frequency is twice that of the given tone.

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, 250, 500, 1000, 2000, and 4000 Hz.


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.


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 rebounded from a surface. Hard non-porous surfaces reflect more sound than 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.


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.

Reverberation Time

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 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 amount of sabins developed by other acoustical materials is 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 (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, such as foil, lead, steel, etc., between 2 layers of absorptive material, that prevents sound waves from passing through the absorptive material.

Signal to Noise Ratio

The sound level of a speaker above background noise, at the listener’s ear level. The inverse square law impacts the S/N ratio.


Sound is an oscillation of pressure, stress particle displacement, and particle velocity in a medium. Sound produces an auditory sensation caused by the oscillation.

Sound Absorption

The property possessed by materials, objects and air to convert sound energy into heat. Sound waves reflected by a surface create a loss of energy. That energy not reflected is referred to as the 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. SAC=Absorption/Area in sabins per sq. ft.

Sound Level

A subjective measure of sound expressed in decibels as a comparison corresponding to familiar sounds experienced in a variety of situations.

Sound Pressure Level (SPL)

An important measure of sound loudness, the level is calculated in decibels by 20 times the logarithm to the base 10 of the ratio of the measured sound pressure level and the reference point.

Sound Level Meter

A device that converts sound pressure variations in air into corresponding electronic signals. The signals are filtered to exclude sound waves outside the desired frequencies.

Sound Transmission Class (STC)

A single-number system used to rate the sound transmission performance of a wall, panel, ceiling, etc. The higher the ranking, the better the ability to obstruct sound transmission.

Speech Intelligibility

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.


The description of a sound wave’s components of frequency and amplitude.

Time Weighted Average (TWA)

The measure 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.


The cubic space of a room bounded by walls, floors, and ceilings determined by the mathematical equation Volume=Length x Width x Height of space. Volume influences reverberation time.


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 (waves) produced by dropping a stone in water. Length of sound wave varies with frequency: low frequency is created by longer wavelengths, whereas high frequency is produced by shorter wavelengths.