Background noise: Ambient noise which remains at a given site when occasional and transient bursts of higher level ambient noise levels have subsided to typically low levels; the noise normally present for most of the time at a given site. It is usually described by the LA90 level.

Background noise level: defined in BS 4142 as the value of the A-weighted residual noise at the assessment position that is exceeded for 90% of a given time interval, T, (i.e. LA90, T) measured using time weighting, F, and quoted to the nearest whole number of decibels (also see under residual noise).

Band pass filter: a filter which provides zero attenuation to all frequencies within a certain band but which attenuates completely all other frequencies.

Band sound pressure level: the sound pressure level of the sound signal within a certain frequency band.

Bandwidth: the range of frequencies contained within a signal, passed by a filter, or transmitted by a structure or device.

Basic noise level: a term used in connection with the prediction of environmental noise such as road traffic or train noise, where, as a first stage, factors relating to the noise source are used to predict the noise level at some arbitrary point close to the source prior to using other factors relating to the propagation of the sound to the reception point(s).

Basilar membrane: a membrane inside the cochlea of the inner ear, which vibrates in response to sound, thus exciting the hair cells.

Beats: periodic variations, which are heard when two pure tones of slightly differing frequencies are superimposed.

Bel: 10 decibels; a unit of level on a logarithmic scale which is based on the ratio of two powers, or of power-related quantities such as sound intensity or the square of sound pressure.

Bending or flexural waves: elastic waves in plates, panels, beams, etc., which are a combination of compression and shear waves and which are responsible for the transmission of structure-borne sound in buildings and other structures.

Binaural: relating to hearing using both ears, e.g. binaural localisation; the use of both ears to locate the direction of sounds.

Bit: abbreviation of binary digit; the smallest possible unit of information in binary form, i.e. on or off, yes or no, 0 or 1.

Broadband: containing a wide range of frequencies.

Byte: a binary word or group of bits.