Acoustic Glossary

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

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

We have a full page on background noise

Other noise descriptors, ambient noise • broadband noise • gaussian noise • narrowband noise • periodic • pink noise • pseudo random noise • random noise • residual sound • specific sound • white noise • wideband noise

Backward Curve Integration Method for calculating reverberation time from the impulse decay curve, also known as the Schroeder method.

Early Decay Time (EDT), based on the impulse decay curve, 0 dB to -10 dB
T10 decay time, based on the impulse decay curve, -5 dB to -15 dB
T20 decay time, based on the impulse decay curve, -5 dB to -25 dB
T30 decay time, based on the impulse decay curve, -5 dB to -35 dB

Balanced Noise Criteria (NCB) 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.
Balance noise criteria also includes the 16 and 31.5 Hz octave bands
See also noise rating curves and room criteria

Bandwidth 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

Bar the 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 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.

Bel 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.

Binaural 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)

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, so the bow tie correction compensates for this.

British Standards are published by the British Standards Institution (BSI), incorporated under a royal charter. Standards Organisations publications are copyright so we cannot reproduce them here. However they are readily available on the internet to purchase and download.

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

Other noise descriptors ambient noise • background noise • gaussian noise • periodic • pink noise • pseudo random noise • random noise • residual sound • specific sound • white noise • wideband 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.

Building Acoustics • absorption • flanking transmission • insulation • reverberation