A-weighting: A frequency weighting devised to attempt to take into account the fact that human response to sound is not equally sensitive to all frequencies; it consists of an electronic filter in a sound level meter, which attempts to build this variability into the indicated noise level reading so that it will correlate, approximately, with human response.

Absorption: see under sound absorption.

Absorption coefficient: see under sound absorption coefficient.

Acceleration: rate of change of velocity (in m/s²).

Accelerometer: a transducer, which measures acceleration.

Acoustic calibrator: a device for producing an accurately known sound pressure level; used for the calibration of sound level meters.

Acoustic enclosure: a structure built around a machine to reduce noise.

Acoustic impedance: of a surface or acoustic source; the (complex) ratio of the sound pressure averaged over the surface to the volume velocity through it; the volume velocity is the product of the surface area and acoustic particle velocity; see also under characteristic, mechanical and specific acoustic impedance.

Acoustic lagging: materials applied externally to the surface of pipes and ducts to reduce the radiation of noise; not to be confused with thermal lagging.

Acoustic model: a device for predicting acoustic parameters such as sound pressure level or reverberation time. Often used in connection with predicting acoustic performance of rooms such as concert halls and other performance spaces. Scale modelling involves creating a physical model of 1/Nfull scale and modelling the acoustic characteristics of sound sources, microphones and material surfaces at N times the frequencies to be encountered in the full scale version of the space. Computer modelling involves creating a virtual three dimensional models of the space and predicting its acoustic performance using software packages which use ray tracing, beam (or cone) tracing or source/image techniques (or combination of these)

Acoustics particle velocity: the velocity of a vibrating particle in an acoustic wave.

Acoustic reactance: the imaginary part of the complex acoustic impedance

Acoustic resistance: the real part of the complex acoustics impedance.

Acoustic trauma: sudden permanent hearing damage caused by exposure to a burst of high-level noise.

Acoustics: (1) the science of sound; (2) of a room: those factors, which determine its character with respect to the quality of the received sound.

Action value: a noise exposure level in the workplace above which certain actions are required under the 2005 Control of Noise at Work Regulations.

Active filter: a filter, which contains transistors, integrated circuits or other components requiring a power supply.

Active noise control: a noise control system, which uses antiphase signals from loudspeakers to reduce noise by destructive interference.

Agglomeration: An area having a population in excess of 100,000 persons and a population density equal to or greater than 500 people per km² and which is considered to be urbanised (a term in noise mapping and defined in the EU Noise Directive)

Airborne sound: sound or noise radiated directly from a source, such as a loudspeaker or machine, into the surrounding air (in contrast to structure-borne sound).

Airborne sound insulation: the reduction or attenuation of airborne sound by a solid partition between source and receiver; this may be a building partition, eg. a floor, wall or ceiling, a screen or barrier or an acoustic enclosure.

Aliasing: introduction of false spectral lines (aliases) into a spectrum by having the maximum frequency of the signal greater than one-half the digital sampling frequency.

Ambient noise: the totally encompassing noise in a given situation at a given time; it is usually composed of noise from many sources, near and far (defined in BS 4142).

Amplitude: the maximum value of a sinusoidally varying quantity.

Analogue signal: an analogue signal is one in which continuous variations in an electrical signal faithfully represents (i.e. is analogous to) the variation in some physical variable such as temperature or sound pressure.

Analogue-to-digital converter: (A/D converter or ADC), a device which samples and digitalises analogue signals, preparatory for digital signal processing; the continuously varying analogue signal is converted into a finite number of discrete steps or levels then represented as a series of numbers.

ANCON: UK civil aircraft noise computer model developed by the Environmental Research and Consultancy Department of the Civil Aviation Authority (CAA). ANCON calculated contours from data describing aircraft movements, routes, noise generation and sound propagation.

Angle of view: a term used in the prediction of road traffic or railway noise giving the angle of view of the road or railway line subtended at the receiving point.

Anechoic: literally ‘without echo’, i.e. without any sound reflections. An anechoic room is one in which all the interior surfaces (wall, floor and ceiling) are lined with sound absorbing materials so that there are no reflections; it provides a standard environment for acoustic tests.

Angular frequency: the product 2∏ times frequency; symbol ω; measured in radians per second.

Annoyance: Noise annoyance is a feeling of displeasure caused by noise.

Anti-aliasing filter: A low pass filter inserted in an instrument, before the ADC, in order to prevent aliasing.

Antinode: a point, line or surface of an interference pattern at which the amplitude of the sound pressure or particle velocity is at a maximum.

Antivibration (AV) mounts: springs or other resilient materials used to reduce vibration (and noise) by isolating the source from its surroundings.

Apparent sound reduction index (R’): a term relating to the sound insulation performance of partitions, defined in BS EN ISO 140-4, measured in octave or third octave frequency bands.

Articulation index (AI): a measure of the intelligibility of speech; the percentage of words correctly heard and recorded in an intelligibility test, or predicted from the levels of speech, background noise and reverberant sound in a room. The value of AI varies between 0 and 1 (representing 100% score or perfect intelligibility).

Attenuation: a general term used to indicate the reduction of noise or vibration, by whatever method or for whatever reason, and the amount, usually in decibels, by which it is reduced.

Attenuator: a devise introduced into air or gas flow systems in order to reduce noise; absorptive types contain sound absorbing materials; reactive types are designed to tune out noise at particular frequencies.

Audibility: the ability of a sound to be heard; the concept of audibility has been used as a criterion for setting limits to noise levels, particularly from amplified music; it is a subjective criterion, i.e. one which can only be determined by the ear of the listener, not by measurement of sound levels; also used as a criterion to determine the degree of privacy between rooms (e.g. offices).

Audibility threshold: the minimum sound pressure which can just be heard at a particular frequency by people with normal hearing; usually taken to be 20 μPA at 100Hz.

Audible range: frequencies from 20 Hz to 20 kHz (approx.); sound pressures from 20 mPa to 100 Pa (approx.).

Audiogram: a chart or graph of hearing level against frequency.

Audiometer: an instrument, which measures hearing sensitivity.

Audiometry: the measurement of hearing.

Auditory cortex: the region of the brain, which receives signals from the ear.

Aural: of or relating to hearing or the hearing mechanism.

Axial mode: the room modes associated with each pair of parallel surfaces.