Glossary of Acoustical Terms


An electrical filter network used by sound level meters, whose output simulates the response of the human ear to low-level sounds. The A-weighting metric also has a strong statistical correlation to noise-induced hearing loss.

A-weighted sound pressure level

an overall sound pressure level reading obtained from a sound level meter by filtering the sound with the A-weighting filter. This term is interchangeable with “sound level.”

background noise

Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structureborne, and instrument noise.

decibel, dB

the term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. Thus, one decibel corresponds to a power ratio of 10, and N decibels corresponds to a power ratio of l0°.

Note: Since the decibel expresses the ratio of two like quantities, it has no dimensions. It is, however, common practice to treat “decibel” as a unit, for example, as in the sentence, “The average sound pressure level in the room is 45 decibels.”

equivalent sound pressure level, Le4

the level of constant sound pressure level that has the same overall energy content (integrated over the measurement period) as the fluctuating measured signal.

frequency, f[Hz]

the number of sound pressure fluctuations occurring per second due to the presence of a sound wave.

Note: A sound may be composed of many frequencies. See spectrum.

insertion loss, IL

of a sound-reducing element, in a specified frequency band. The decrease in sound pressure level, measured at the location of the receiver, where a sound. insulator or a sound attenuator is inserted in the transmission path between the source and the receiver. The quantity so obtained is expressed in decibels. Insertion Loss is a meaningful descriptor only when measured in the absence of significant background noise.

maximum permissible sound level (MPSL)

the sound level that may not be exceeded at a distance of 1 meter from equipment specified according to this guide.

octave band

a frequency band which covers a complete musical octave. The ratio of the frequency of the higher band edge to the lower band edge is 2.1. Octave bands are named by their center frequency. Preferred octave bands of interest in this document are 63 Hz 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 8000 Hz.

reverberant sound

the sound in an enclosed or partially enclosed space that has been reflected repeatedly from the boundaries.

reverberant environment

for the purposes of this guide, an environment is reverberant if it is enclosed by at least two man-made surfaces and has a Room Constant of 100 square meters or less. Calculation of the room constant is beyond the scope of this guide. A simplified graphical method for comparing sound-absorptive and sound-reflective surface areas is presented in Section 3.2f and in Schedule A of the Noise Emission Worksheet for New Equipment.


(1) the process and property of the persistence of sound in an enclosed or partially enclosed space after the source of sound has stopped. (2) the buildup of the reflected sound energy within the space, manifested as increased sound pressure levels.

sound absorption

(1) the process of dissipating sound energy. (2) the property possessed by materials, objects and structures such as rooms of absorbing sound energy, expressed as sabins for objects or unit absorbers, and sound absorption coefficient for extended surfaces.

Note: Sound energy passing through a wall or opening is regarded as being absorbed in calculations accordingg to this guide.

sound absorption coefficient, α; [dimensionless]

of a surface, in a specified frequency band, the magnitude of the fraction of sound energy striking a material or an object which is absorbed or otherwise not reflected (e.g., sound traveling out through an open window). The sound absorption coefficient is calculated by dividing the sound absorption (in sabins) by the plan area (in square meters) of the test article according to ASTM Test Method C423.

sound-isolating enclosure

any structure that houses the equipment under consideration, and provides significant acoustical isolation.

sound level

A-weighted sound pressure level.

sound power level (PWL)

of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration to the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.

sound pressure

a fluctuating pressure superimposed on atmospheric pressure by the presence of sound. In analogy with alternating voltage its magnitude can be expressed in several ways, such as instantaneous sound pressure or peak sound pressure, but the unqualified term means root-mean-square sound pressure. In air, the static pressure is barometric pressure.

sound pressure level (SPL)

of airborne sound, ten times the common logarithm of the ratio of the square of the sound pressure under consideration to the square of the standard reference pressure of 20 µPa. The quantity so obtained is expressed in decibels.

Note: The pressures are squared because pressure squared, rather than pressure, is proportional to power or energy.

sound transmission class (STC)

a rank order classification assigned to partitions, doors, windows and other sound isolating elements to describe their ability to isolate certain sounds. Higher values indicate more effective sound isolation. The classification is effective for sound spectra dominated by high frequencies (500 to 8000 Hz), but may be misleading for sound spectra dominated by low frequencies (31.5 Hz to 250 Hz).


the distribution of sound energy across octave bands.


for the purposes of this guide, SPL refers to octave band sound pressure levels.

test code

a document, issued by a code organization, an engineering organization, or manufacturers’ group, which specifies the method of test for a definable class of equipment. Specialized guidance for reporting sound pressure level or sound power level, reference to a basic test standard, and required operating conditions for the equipment under test are typically provided.

test standard

the general method by which tests are conducted to measure fundamental quantities. Topics usually include requirements for equipment, test environment, measurement locations, and expected accuracy.

vibration isolation

reduction, attained by the use of a resilient coupling or mount, in the capacity of a system to vibrate in response to mechanical excitation or to transmit vibration.

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