Acoustics Glossary

A-Z list of terminology commonly used within acoustics.



C; Ctr: spectral, adaptation terms used in connection with the measurement and assessment of airborne sound insulation and defined in BS EN ISO 7172-1.

C-weighting: one of the frequency weightings defined in BS EN ISO 61672-1; it corresponds to the 100 Phon contour and is the closest to the linear or unweighted value.

Capacitor: one of the basis elements of an electrical circuit consisting of two conducting plates separated by a gap containing an insulator, or dielectric, it has the property of capacitance, measured in farads (F), microfarads (μF) or picofarads (pF).

Centre frequency: the centre of a band of frequencies; in the cases of octave or one-third octave it is the geometric mean of the upper and lower limiting frequencies if the band.

Characteristic acoustic impedance: (of a medium) the ratio of sound pressure to acoustic particle velocity at a point in the medium during the transmission of a plane wave; it is the product (ρc) of the speed of sound (c) in the medium and its density (ρ), measured in rayls (Nsm ‾³, Pas/m or kg/m²s).

Charge amplifier: a type of preamplifier suitable for use with piezoelectric accelerometer; it gives an output which is proportional to the electric charge present in the input signal.

Cochlea: a coiled, snail-shaped structure in the inner ear; it is fluid-filled and contains a complex arrangement of membranes and hair cells which convert mechanical vibrations of the fluid into electrical impulses transmitted to the brain.

Coherence, coherent: two sounds are coherent if there is a constant phase difference between their two waveforms (usually because they have originated from the same source).

Coincidence effect: an effect which leads to increased transmission of sound by panels and partitions when the speed (and wavelength) of flexural waves in the panel coincide with the speed (and wavelength) of the sound waves exciting the panel.

Colouration: some change to a sound from the original version, detectable to a listener, caused for example by the sound reproduction system or by the room in which the sound is produced.

Community noise: defined by the World Health Organisation as ‘Noise emitted from all noise sources except noise at the industrial workplace’.

Compressional wave: an elastic wave in a fluid or solid in which the elements of the medium are subjected to deformations which are purely compression, i.e. which do not contain any element of rotation or shear, and of which sound waves in air are an example.

Condenser: see under capacitor.

Conductive deafness: hearing loss, which is caused by some defect of fault in the outer or middle ear.

Continuous descent approach (CDA): a noise abatement technique for arriving aircraft in which the pilot, when given clearance by air traffic control (ATC), will descend along a path which as far as is practicable corresponds to a continuous descent (rather than periods of level flight in between periods of descent). The idea is to remain as high as possible for as long as possible during descent. CDA has benefits in terms of both reduced noise and improved fuel economy.

Continuous equivalent noise level, LAeq: of a time varying noise; the steady noise level (usually in dBA) which, over the period of time under consideration, contains the same amount of (A-weighted) sound energy as the time varying noise, over the same period of time; also called time averaged sound level.

Continuous spectrum: a sound or vibration spectrum whose components are continuously distributed over the particular frequency range, for example random noise; contrast with a line spectrum from a harmonic sound.

Cortex: see under auditory cortex.

Coulomb damping: a form of damping in which the damping force is constant, independent of either displacement or velocity (also called dry friction damping)/

Crest factor: of a signal; the ratio of the peak to the root mean square (RMS) value.

Criterion: the basis on which a noise or vibration is to be judged, e.g. damage to hearing, interference with speech, annoyance.

Critical band: in human hearing, only those frequency components within a narrow band, called the critical band, will mask a given tone. Critical bandwidth varies with frequency but is usually between 1/6 and 1.3 octaves.

Critical damping: the amount of viscous damping in a system, which will allow the system to return to its equilibrium position, in the minimum time, without overshoot, i.e. without oscillation; the boundary between overdamping and underdamping.

Critical frequency: the lowest frequency at which the coincidence effect takes place for a particular panel or partition, and above which the sound insulation performance starts to deteriorate.

CRN (Calculation of Railway Noise): the UK method for predicting rail (i.e. train) noise in terms of LAeq and Lden. May be replaced by integrated Europe-wide methods such as Harmonoise.

Crosstalk: a signal from one track, channel or circuit, which is transmitted, unwanted, into another track, channel or circuit.

CRTN (Calculation of Road Traffic Noise): the UK method for predicting road traffic noise, in terms of LA10,18hour. First published in 1968 and revised in 1998 it was developed as a method for determining eligibility for sound insulation under the Noise Insulation of the Land Compensation Act, but is more widely used for Noise Impact Assessment Regulations and for planning applications. May be replaced by integrated Europe-wide methods such as Harmonoise.

Curie point: the temperature above which a piezoelectric material becomes polarized, and loses its piezoelectric properties.

Cycle: of a periodically varying quantity; the complete sequence of variations of the quantity, which occurs during one period.

Cycle per second: unit of frequency; one cycle per second is one hertz (Hz).