Acoustic Glossary


B : Sound and Vibration Definitions, Terms, Units, Measurements etc., ..


Background Noise at a given location and time, is measured in the absence of any alleged noise nuisance sources. Also known as residual noise.

Background Noise Definition IEC 801-21-13, total of interference from all sources in a system used for the production, transmission, detection, measurement, or recording of a signal

LA90 is the noise level just exceeded for 90% of the time. Often used to measure background noise as recommended in BS 4142 - see also statistical noise levels.

We have a full page on background noise

Other noise descriptors • ambient noisebroadband noisegaussian noisenarrowband noiseperiodicpink noisepseudo random noiserandom noiseresidual noisespecific noisewhite noisewideband noise

Backward Curve Integration

, a method for calculating reverberation time from the impulse noise decay curve, also known as the Schroeder method.

Balanced Noise Criteria

, a refinement of the noise criteria rating system. The rating is determined by the speech interference level, resulting in the following classifications
R is for Rumble, if the spectrum is rich in low frequency sound.
H is for Hiss, if the spectrum is rich in high frequency sound.
RV is for Rattle and Vibration, if the low frequency spectra is likely to produce audible rattling in lightweight building elements.


a continuous range of frequencies between two limiting frequencies.

Band Pass Filter

, covering a band of frequencies from a lower cut-off frequency to an upper cut-off frequency. Outside the filter bandwidth, the signal is attenuated.


, a range of frequencies, usually of standard size in acoustics, for example octave or one-third octave bands. The lower and upper frequencies are also known as the -3 dB or half-power points.

See also • constant percentage bandwidth


, the unit of atmospheric pressure, equal to 1000 millibars, one million dynes per square centimetre, 100 kilopascal or 29.53 inches of mercury.


, the human ear combines sounds of similar frequency into frequency bands, called critical bands. Dr Zwicker divided the audio spectrum into 24 critical bands and named the units 'barks'.

See also • loudness.

Beat Definition

IEC 801-23-14, phenomenon that results from the linear or non-linear superposition of two or more waves of the same kind but of different frequencies.

Beat Frequency

, if two sound or vibration components are quite close together in frequency and are present at the same time at the same place, they will combine in such a way that their sum will vary in level at a rate equal to the difference in frequency between the two components. This phenomenon is known as beating and its frequency is the beat frequency.


, a bel is equal to 10 decibels (dB) and because it is a ratio of two quantities it is dimensionless.

Bel Definition

IEC 801-22-02, unit of level of a quantity proportional to power when the base of the logarithm is ten. Also, the unit of level of a field quantity when the base of the logarithm is the square root of ten
Note 1 examples of power-like quantities are sound power and sound energy.
Note 2 examples of field quantities are sound pressure and voltage.

The decibel is used extensively in acoustics - so we have a more detailed description.

Bending Wave


, hearing involving both ears

Binaural Recording

, uses two microphones spaced to simulate the listeners ears and usually transmitted separately to each ear of the listener. The results, using good quality headphones, can be very realistic.

See also • localisation and our head and torso simulator - HATS

Boundary Effect

Bow-tie Correction

, multispectral measurements for correlation functions are performed using spectrum averaging. Half of the normal time record is set to zero. Accordingly, correlation functions for continuous signals will decrease with the delay. The bow-tie correction compensates for this.

Broadband Noise, also called wideband noise is noise whose energy is distributed over a wide section of the audible range as opposed to narrowband noise.

Other noise descriptors • ambient noisebackground noisegaussian noiseperiodicpink noisepseudo random noiserandom noiseresidual noisespecific noisewhite noisewideband noise

Buffer, a memory location in a computer or digital instrument that is set aside for temporary storage of digital information while it is waiting to be processed.

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