Acoustic Glossary


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


Background Noise IEC Definition,

total of interference from all sources in a system used for the production, transmission, detection, measurement, or recording of a signal


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

We have a separate page on Background Noise

LA90, the level just exceeded for 90% of the measured time, is commonly used to quantify background noise. See also : Statistical Noise Levels

Backward Curve Integration method for calculating reverberation time from the impulse noise decay curve. Also known as the Schroeder method.

Balanced Noise Criteria

Band a continuous range of frequencies between two limiting frequencies.

See also : octave and one-third octave

Band Pass Filter a 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.

Band Sound Pressure Level

Bandwidth 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

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

Bark 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 Frequency if two vibration components are quite close together in frequency and if they are present at the same time at the same place, they will combine in such a way that their sum will vary in level up and down 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.

Bel IEC Definition,

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 : examples of power-like quantities are sound power and sound energy. Examples of field quantities are sound pressure and voltage.


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

The decibel (dB) is used extensively to 'describe' acoustic levels.

Bending Wave

Binaural hearing involving both ears

Binaural Recording sound recorded using 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 - noise whose energy is distributed over a wide section of the audible range as opposed to narrowband noise.

Related Terms : Narrowband NoisePink NoiseRandom NoiseWhite Noise
See also : Constant BandwidthConstant Percentage BandwidthsOctave BandsFFT- Fast Fourier Transform

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.

Building Acoustics see - FlankingReverberationSound Insulation


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