Acoustic Glossary of Terms and Definitions

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Acoustic Glossary of Terms and Definitions for soundproofing and noise control research, products and materials

All Noise Control Glossary


Graph of hearing threshold level as a function of frequency (ANSI S3.20-1995: audiogram).

Baseline audiogram

The audiogram obtained from an audiometric examination administered before employment or within the first 30 days of employment that is preceded by a period of at least 12 hr of quiet. The baseline audiogram is the audiogram against which subsequent audiograms will be compared for the calculation of significant threshold shift.

Continuous noise

Noise with negligibly small fluctuations of level within the period of observation (ANSI S3.20-1995: stationary noise; steady noise).

Crest factor

Ten times the logarithm to the base ten of the square of the wideband peak amplitude of a signal to the time-mean-square amplitude over a stated time period. Unit, dB (ANSI S3.20-1995: crest factor).

Decibel (dB)

Unit of level when the base of the logarithm is the 10th root of 10 and the quantities concerned are proportional to power (ANSI S1.1-1994: decibel).

Decibel, A-weighted (dBA)

Unit representing the sound level measured with the A-weighting network on a sound level meter. (Refer to Table 4-1 for the characteristics of the weighting networks.)

Decibel, C-weighted (dBC)

Unit representing the sound level measured with the C-weighting network on a sound level meter. (Refer to Table 4-1 for the characteristics of the weighting networks.)


To use a fraction of a hearing protectors noise reduction rating (NRR) to calculate the noise exposure of a worker wearing that hearing protector. (See NRR below.)


The amount of actual exposure relative to the amount of allowable exposure, and for which 100% and above represents exposures that are hazardous. The noise dose is calculated according to the following formula: D = [C1/T1 + C2/T2 + … + Cn/Tn] H 100 Where Cn = total time of exposure at a specified noise level Tn = exposure time at which noise for this level becomes hazardous

Effective noise level

The estimated A-weighted noise level at the ear when wearing hearing protectors. Effective noise level is computed by (1) subtracting derated NRRs from C-weighted noise exposure levels, or (2) subtracting derated NRRs minus 7 dB from A-weighted noise exposure levels. Unit, dB. (See Appendix.)

Equal-energy hypothesis

A hypothesis stating that equal amounts of sound energy will produce equal amounts of hearing impairment, regardless of how the sound energy is distributed in time.

Equivalent continuous sound level

Ten times the logarithm to the base ten of the ratio of time-mean-square instantaneous A-weighted sound pressure, during a stated time interval T, to the square of the standard reference sound pressure. Unit, dB; respective abbreviations, TAV and TEQ; respective letter symbols, LAT and LAeqT (ANSI S1.1-1994: time-average sound level; time-interval equivalent continuous sound level; time-interval equivalent continuous A-weighted sound pressure level; equivalent continuous sound level).

Excess risk

Percentage with material impairment of hearing in an occupational-noise-exposed population after subtracting the percentage who would normally incur such impairment from other causes in a population not exposed to occupational noise.

Exchange rate

An increment of decibels that requires the halving of exposure time, or a decrement of decibels that requires the doubling of exposure time. For example, a 3-dB exchange rate requires that noise exposure time be halved for each 3-dB increase in noise level; likewise, a 5-dB exchange rate requires that exposure time be halved for each 5-dB increase.


The hearing threshold level above which a material impairment of hearing is considered to have occurred.


For a function periodic in time, the reciprocal of the period. Unit, hertz (Hz) (ANSI S1.1-1994: frequency).

Hearing threshold level (HTL)

For a specified signal, amount in decibels by which the hearing threshold for a listener, for one or both ears, exceeds a specified reference equivalent threshold level. Unit, dB (ANSI S1.1-1994: hearing level; hearing threshold level).

Immission level

A descriptor for noise exposure, in decibels, representing the total sound energy incident on the ear over a specified period of time (e.g., months, years).


Single collision of one mass in motion with a second mass that may be in motion or at rest (ANSI S1.1-1994: impact).


Product of a force and the time during which the force is applied; more specifically, impulse is the time integral of force from an initial time to a final time, the force being time-dependent and equal to zero before the initial time and after the final time (ANSI S1.1-1994: impulse).

Impulsive noise

Impulsive noise is characterized by a sharp rise and rapid decay in sound levels and is less than 1 sec in duration. For the purposes of this document, it refers to impact or impulse noise.

Intermittent noise

Noise levels that are interrupted by intervals of relatively low sound levels.


(1) Undesired sound. By extension, noise is any unwarranted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device. (2) Erratic, intermittent, or statistically random oscillation (ANSI S1.1-1994: noise).

Noise reduction rating (NRR)

The NRR, which indicates a hearing protectors noise reduction capabilities, is a single-number rating that is required by law to be shown on the label of each hearing protector sold in the United States. Unit, dB.

Permanent threshold shift (PTS):

Permanent increase in the threshold of audibility for an ear. Unit, dB (ANSI S3.20-1995: permanent threshold shift; permanent hearing loss; PTS).

Pulse range

Difference in decibels between the peak level of an impulsive signal and the root-mean-square level of a continuous noise.

Significant threshold shift

A shift in hearing threshold, outside the range of audiometric testing variability (5 dB), that warrants followup action to prevent further hearing loss. NIOSH defines significant threshold shift as an increase in the HTL of 15 dB or more at any frequency (500, 1000, 2000, 3000, 4000, or 6000 Hz) in either ear that is confirmed for the same ear and frequency by a second test within 30 days of the first test.


(1) Oscillation in pressure, stress, particle displacement, particle velocity, etc. in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillations. (2) Auditory sensation evoked by the oscillation described above (ANSI S1.1-1994: sound).

Sound intensity

Average rate of sound energy transmitted in a specified direction at a point through a unit area normal to this direction at the point considered. Unit, watt per square meter (W/m2); symbol, I (ANSI S1.1-1994: sound intensity; sound-energy flux density; sound-power density).

Sound intensity level

Ten times the logarithm to the base ten of the ratio of the intensity of a given sound in a stated direction to the reference sound intensity of 1 picoWatt per square meter (pW/m2). Unit, dB; symbol, L (ANSI S1.1-1994: sound intensity level).

Sound pressure

Root-mean-square instantaneous sound pressure at a point during a given time interval. Unit, Pascal (Pa) (ANSI S1.1-1994: sound pressure; effective sound pressure).

Sound pressure level

(1) Ten times the logarithm to the base ten of the ratio of the time-mean-square pressure of a sound, in a stated frequency band, to the square of the reference sound pressure in gases of 20 micropascals (µPa). Unit, dB; symbol, Lp. (2) For sound in media other than gases, unless otherwise specified, reference sound pressure in 1 µPa (ANSI S1.1-1994: sound pressure level).

Temporary threshold shift

Temporary increase in the threshold of audibility for an ear caused by exposure to high-intensity acoustic stimuli. Such a shift may be caused by other means such as use of aspirin or other drugs. Unit, dB. (ANSI S3.20-1995: temporary threshold shift; temporary hearing loss).

Time-weighted average (TWA)

The averaging of different exposure levels during an exposure period. For noise, given an 85-dBA exposure limit and a 3-dB exchange rate, the TWA is calculated according to the following formula: TWA = 10.0 H Log(D/100) + 85 Where D = dose

TWA = 10.0 H Log(D/100) + 85

Varying noise

Noise, with or without audible tones, for which the level varies substantially during the period of observation (ANSI S3.20-1995: nonstationary noise; nonsteady noise; time-varying noise).


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).

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

Acoustical Environment

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

Ambient noise encompasses all sound present in a given environment, being usually a composite of sounds from many sources near and far.

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.

Area Effect

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


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


An instrument for measuring hearing acuity.

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.


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 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.


In acoustics, the cycle is the complete oscillation of pressure above and below the atmospheric static pressure


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.

Decibel (dB)

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.


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 ceased.

Field Impact Insulation Class (FIIC)

When measurements are conducted in buildings according to the appropriate standards for field measurements, impact insulation class is called field impact insulation class (FIIC). The field test evaluates the dwelling’s actual construction and includes all sound paths.

Field Sound Transmission Class (FSTC)

When measurements are conducted in buildings according to the appropriate standards for field measurements, sound transmission class is called field sound transmission class (FSTC). The field test evaluates the dwelling’s actual construction and includes all sound paths.


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.

Flutter Echo

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.

Free Field

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.

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. For example, 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 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.

Hearing Range

16 – 2000 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 Insulation Class (IIC)

Rates a floor/ceiling assembly’s ability to block impact sound. Impact Insulation Class (sometimes referred to as Impact Isolation Class) measures a floor/ceiling assembly’s resistance to the transmission of structure-borne or impact noise, like human footfall. The tapping machine frequently used for this test is not designed to simulate any one type of impact, such as male or female footsteps, or to simulate the weight of a human walker. Thus the subjectively annoying creak or boom generated by human footfalls on a limber floor assembly may not be adequately evaluated by this method (American Society for Testing and Materials – ASTM, E 1007, 5.2).


See Loudness

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 logarithmic basis.

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.

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


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.


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 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.

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 between 2 layers of absorptive material, such as foil, lead, steel, etc. that prevents sound wave from passing through absorptive material.

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.


Sound is an oscillation in pressure, stress particle displacement, 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 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.

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 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.

Sound Level Meter

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

Sound Pressure

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

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. Centers between 250 and 4000k.

Speech Privacy

The degree to which speech is unintelligible between offices. Three ratings are used: Confidential, Normal (Non Obtrusive) and Minimal.

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.


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 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 or waves produces by dropping a stone in water. Length of sound wave varies with frequency. Low frequency equals longer wavelengths.