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NOVA Acoustics

Acoustics Glossary

A-Z list of terminology commonly used within acoustics.


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.


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.


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).


D level difference: the difference in sound levels in two spaces separated by a partition, measured as part of a sound insulation test according to BS EN ISO 140.

Dnc, Dne, Dnf,: standardized level difference values for small elements (e.g. ventilators), Dne, suspended ceiling, Dnc, and raised floors, Dnf, defined in parts 10,12 and 9 respectively of BS EN ISO 140.

Dn normalised level difference: a measurement of airborne sound insulation, corrected according to BS EN ISO 140-4 for receiving room characteristics (sound absorption); a complete set of measurements consists of 16 third octave band values, from 100 Hz to 3150 Hz.

DnT standardised level difference: a measurement of airborne sound insulation, corrected according to BS EN ISO 140-4 for receiving room characteristics (reverberation times); a complete set of measurements consists of 16 third octave band values, from 100 Hz to 3150 Hz.

DnT,w weighted standardised level difference: a single-figure value of airborne sound insulation performance, derived according to procedures in BS EN ISO 717-1 used for rating and comparing partitions and based on the values of D at different frequencies; values of DnT,w are specified in the Building Regulations.

Damping: a process whereby vibrational energy is converted into heat through some frictional mechanism, thus, causing the level of vibration to decrease.

Damping ratio: the ratio of the amount of damping in a vibration system to the amount of damping when critical.

Day-evening-night level, Lden: the LAeq over the period 00.00-24.00, but with the evening values (19.00-23.00) weighted by the addition of 5 dBA, and the night values (23.00-07.00) weighted by the addition of 10 dBA.

Day-night level: Ldn an index of environmental noise which is a 24 h Leq, but with a 10 dB weighting added to the night-time noise levels (22.00 to 07.00) to allow for increased sensitivity to noise during the night-time.

dBA: the A-weighted sound pressure level; see under A-weighting.

Decade: a range of ten to one, e.g. from 100 Hz to 1000 Hz.

Decibel (dB): the decibel scale is a scale for comparing the ratios of two powers, or of quantities related to power, such as sound intensity; on the decibel scale the difference in level between two powers, W1 and W2 is N dB, where N = 10log10 (W1/W2); the decibel scale may also be used to compare quantities, whose squared values may be related to powers, including sound pressure, vibration displacement, velocity or acceleration, voltage and microphone sensitivity; in these cases the difference in level between two signals, of magnitude S1 and S2 is given by N = 20 log10 (S2/S1); the decibel scale may be used to measure absolute levels of quantities by specifying reference values which fix one point in the scale (0 dB) in absolute terms; a decibel is one tenth of a bel.

Degrees of freedom: the number of degrees of freedom of a mass-spring model of a vibrating system is the minimum number of coordinates required to specify all the different possible modes of vibration of the system.

Deterministic: a deterministic signal is one whose value can be predicted with certainty from a knowledge of its behavior at previous times, as opposed to a random signal, where this is not possible.

Dielectric: a material which is an electrical non-conductor or insulator; it is used between the plates of a capacitor.

Diffraction: the process whereby an acoustic wave is disturbed and its energy redistributed in space as a result of an obstacle in its path; the relative sizes of the sound wavelength and the object are always important in diffraction; reflection may be considered to be a special case of diffraction when the size of the obstacle is very large compared to the wavelength; the combined effects of diffraction from an irregular array of objects in the path of the sound is also known as scattering; diffraction theory deals with all aspects of the interactions between matter (i.e. obstacles) and waves, so it also determines the directional patterns of sound radiation from vibrating objects.

Diffuse reflection: reflection of sound at a rough irregular profiled surface, which scatters sound in different directions (as opposed to specular reflection).

Diffuse sound field: a sound field of statistically uniform energy density in which the directions of propagation of waves are random from point to point.

Diffuser: an object or surface profile designed to scatter sound in random directions and so to minimize specular reflection (according to the law of reflection).

Diffusion: the scattering of sound wave in many different directions by an object or a surface.

Digital signal: a signal having a discrete number of values, which can be represented as a sequence of numbers; see also analogue-to-digital converter and digital-to-analogue converter.

Digital audio tape recorder (DAT): a tape recorder which includes an ADC (and a DAC) and which records analogue signals on tape in coded digital form.

Digital-to-analogue converter (DAC): an electronic device, which converts digital signals into analogue signals.

Dipole: A sound source, which has the characteristics of two monopole sources of equal amplitude but opposite phase a short distance apart. An example is the sound radiated by an unbaffled loudspeaker cone, which approximates to a dipole source.

Directivity factor: the ratio of the sound intensity at a given distance from the source, in a specified direction, to the average intensity over all directions, at the same distance.

Directivity index: the directivity factor (DF) of a source expressed in decibels, i.e. 10 log10 (DF).

Direct sound: sound which arrives at the receiver having travelled directly from the source, without reflection.

Direct sound field: that part of the sound field produced by the source where the effects of reflections may be neglected.

Distortion: a lack of faithfulness in a signal, such as the introduction of harmonics into the frequency spectrum, introduced, for example, because of non-linearity or of overload of some component of the measurement system.

Disturbance: an effect of noise which may be indicated by some change of behaviour, e.g. closing windows, interruption to speech, moving bedrooms; the objectively measurable effect of noise on the performance of a task or activity, such as listening to speech, or getting to sleep (as compared with annoyance which can only be measured by asking people questions, e.g. via a questionnaire).

Doppler effect: the change in the observed frequency of a wave caused by relative motion between source and receiver.

Dose-response: the relationship between the human response to noise or vibration and the received exposure (or dose) of noise or vibration received.

Dynamic magnification factor (Q factor): a quantity which is a measure of the sharpness of resonance of an oscillating system (either mechanical or electrical); it is related to the amount of damping in the system.

Dynamic range: the range of magnitudes of a signal which a measuring system, or component of a system, can faithfully record, process or measure, from highest to lowest; usually expressed in decibels.

Dynamic stiffness: the ratio of change of force to change of displacement in a vibrating system; it may be different from the static stiffness of the system.


Ear defenders or ear protectors: earmuffs or earplugs worn to provide attenuation of sounds reaching the ear and reduce the risk of noise induced hearing loss.

Early sound reflections (or early sound): reflections of sound from surfaces in a room, which arrive at the receiver within a certain time after the direct sound. This is usually considered too be around 50 ms for speech or about 80 ms for music. Early sound is treated by the listener’s ear/brain system as being helpful in reinforcing the direct sound signal and therefore acoustic designers will seek to encourage the production of early sound reflections while discouraging late reflections, which produce an unfavorable response in the listener.

Echo: a sound reflection whose magnitude and time delay are such that it is perceived as a separate, distinguishable repetition of the direct sound, as opposed to reverberation, which is perceived as part of the original sound.

Electret or prepolarised microphone: a type of condenser microphone in which a prepolarised layer of electret polymer is used as a dielectric between the diaphragm and backing-plate, which form the condenser.

Electrostatic actuator: a device, which fits over a microphone, close to the diaphragm, and is used for remote calibration.

Emission: a measure of sound energy produced by a source of sound and radiated into the environment (see also immission).

EPNdB (effective perceived noise decibel): a metric used internationally in the noise certification of aircraft.

Equal loudness contours: a standardized set of curves which show how the loudness of pure tone sounds varies with frequency at various sound pressure levels.

Equivalent continuous noise level: see under continuous equivalent noise level, LAeq.

Environmental noise: the European Environmental Noise Directive 2002/49/EC defines environmental noise as ‘unwanted or harmful outdoor sound created by human activities, including noise emitted by means of transport, road traffic, rail traffic, air traffic, and from sites of industrial activity’.

Eustachian tube: the passage from the middle ear to the back of the throat, which serves to equalize the pressure across the eardrum.

eVDV: estimated vibration dose value; a measure of a cumulative amount of vibration based upon weighted RMS acceleration values and durations; for signals of limited crest facto, the eVDV approximates to the vibration dose value, VDV; see also under root mean quad and vibration dose value.

Eyring’s formula: a modified version of Sabine’s formula, for reverberation time, which takes into account the discrete nature of sound reflections; also known as he Norris-Eyring formula.

Exposure limit value: a noise exposure level defined in the 2005 Control of Noise at Work Regulations requiring action from employers (a personal daily (or weekly) noise exposure level of 87 dBA or peak sound pressure level of 140 dBC).


F (fast) time weighting: an averaging time used in sound level meters, and defined in BS EN ISO 61672-1.

Façade noise level: a noise level measured close to (less than 3 m away) from the façade of a building, which contains a contribution arising from reflection of sound at the façade. The difference between the façade level and the free field level (in the absence of the façade) is called the façade correction factor.

Far field: of a sound source; that part of the sound field of the source where the sound pressure and acoustic particle velocity are substantially in phase, and the sound intensity is inversely proportional to the square of the distance from the source.

Fast Fourier transform (FFT): an algorithm or calculation procedure for the rapid evaluation of Fourier transforms; an FFT analyser is a device which uses FFTs to convert digitized waveform signals into frequency spectra, and vice versa.

Field measurements: measurements carried out on-site, away from controlled laboratory conditions; the results of field tests of sound insulation may include the effects of flanking paths as well as direct sound transmission, which would not be the case for laboratory tests.

Filter: a device, which transmits signals within a certain band of frequencies but attenuates all other frequencies; filters may be electrical, mechanical or acoustical.

Flanking transmission: the transmission of airborne sound between two adjacent rooms by paths other than via the separating partition between the rooms, e.g. via floors, ceilings and flanking walls.

Flutter echo: a series of repeating echoes caused by parallel reflecting surfaces.

Forced vibration: steady-state vibration of a system caused by a continuous external force.

Form factor: the ratio of the RMS value of a signal to the mean value between two successive zero-crossings.

Fourier analysis/series/spectrum: Fourier’s theorem shows that any periodic function may be broken down (or analysed) into a series of discrete harmonically related frequency components which may be represented as a line spectrum.

Fourier transform: a mathematical process which transforms a non-periodic function of time into a continuous function of frequency, and vice versa (in the case of the inverse function).

Fractional dose: a fractional component of a total noise exposure, defined in connection with assessment of workplace noise exposure in connection with the Noise at Work Regulations.

Free field conditions: a situation in which the radiation from a sound source is completely unaffected by the presence of any reflecting boundaries; see also under anechoic.

Frequency: of a sinusoidally varying quantity such as sound pressure or vibration displacement; the repetition rate of the cycle, i.e. the reciprocal of the period of the cycle, the number of cycles per second; measured in hertz (Hz).

Frequency analysis: the separation and measurement of a signal into frequency bands.

Frequency response: of measurement system or component of such system, e.g. a sound level meter or microphone; the variation in performance, e.g. sensitivity, with change of frequency.

Frequency spectrum: a graph resulting from a frequency analysis and showing the different levels of the signal in the various frequency bands.

Frequency weighting: an electronic filter built into a sound level meter according with BS EN ISO 61672-1; see also under A- and C-weighting.

Fundamental frequency: the lowest natural frequency of a vibrating system; the repetition rate of a harmonic waveform.


Ground attenuation: an attenuation of sound at a distance from a receiver caused by interference between sound waves travelling directly from source to receiver and sound arriving at receiver after reflection at the ground.


Haas effect: a psycho-acoustic phenomenon in which precedence is given to the direction of the first arrival of direct sound in attributing the direction from which the sound is coming.

Hair cells: biological cells in the cochlea of the inner ear where vibration is turned into a neural signal, which is transmitted to the brain.

Hard ground: ground such as concrete, most other paving materials, and water which is considered to be an acoustically reflecting surface, in contrast with ‘soft ground’ which is considered to be sound absorbing.

Harmonic: a signal having repetitive pattern.

Harmonoise: an integrated method for predicting environmental noise throughout Europe, which may replace individual national methods such as CRTN and CRN.

Hearing level: a measured threshold of hearing, expressed in decibels relative to a specified standard threshold for normal hearing.

Hearing loss: any increase of an individual’s hearing levels above the specified standard of normal hearing.

Helmholtz resonator: a vibrating system having a single degree of freedom; it consists of an air-filled enclosure connected to the open air by a narrow column; the air in the enclosure acts as the spring and the air in the column acts as the mass.

Henry (H): the unit of electrical inductance.

Hertz (Hz): the unit of frequency; the number of cycles per second.

High pass filter: a filter, which transmits frequency components of a signal that are higher than a certain cut-off frequency but which attenuates those below the cut-off.

HML method: a method for estimating the A-weighted sound pressure level at the ear when hearing protector is being worn based on the attenuation provided by the protector at high (H), medium (M) and low (L) frequencies.

Hyperacusis: unusual sensitivity and discomfort caused by sounds which are usually tolerable to other listeners. The condition can sometimes be associated with hearing loss.

Hysteresis damping: a type of damping that occurs within materials as a result of phase changes which occur between stress and strain during the vibration cycle.


Immission: a measure of sound energy received at a particular location in the environment.

Impact noise: sound resulting from the impact between colliding bodies.

Impact sound insulation: the resistance of a floor to the transmission of impact sound; measured according to BS EN ISO 140-7.

Impedance: see under acoustic impedance.

Impedance matching: the use of a device to act as a buffer between a system, or component of a system, with a high output impedance and a system, or component system, with a low input impedance. An example of electrical impedance matching is the use of a preamplifier between a condenser microphone and the signal processing electronics in a sound level meter. An example of middle ear to match the very different acoustic impedances of air (in the outer ear) and cochlear fluid (in the inner ear): see also under input and output impedance.

Impulse: a transient signal of short duration; impulsive noise is often described by words such as bang, thump, clatter.

Impulse response: a sound pressure versus time measurement showing how a device or room responds to an impulse.

Incus: or anvil, the middle of the three bones in the middle ear.

Inductance: the property of an electrical coil, or inductor, associated with the rate of change of magnetic field; measured in henrys (H).

Inertia base: a concrete slab used under antivibration mounts to provide additional mass, rigidity and stability.

Infrasound: acoustic waves with frequencies below the audible range, i.e. below about 20 Hz.

Initial noise: ambient noise prevailing in an area before any modification of the existing situation (used in BS 7445).

Input and output impedance: output impedance is an important property of any device, which delivers a signal, e.g. a battery, a microphone, accelerometer or an amplifier. Devices with low output impedance can deliver higher electrical currents (and more energy) than those with high output impedance. Input impedance is an important property of any device, which receives a signal (i.e. of any ‘load’), such as a loudspeaker, a level recorder or an amplifier. Devices with low input impedance draw a higher current (and more energy) from the source than those with a high input impedance. If a source device with a very high output impedance is connected to a receiving device with a low input impedance the output voltage will be reduced because the source will be unable to deliver the current (or energy) demanded by the receiver. This situation may be improved by the use of an impedance matching device interposed between source and receiver: see also under impedance matching.

Insertion loss: a measure of the effectiveness of noise control devices such as silencers and enclosures; the insertion loss of a device is the difference, in dB, between the noise level with and without the device present.

Insulation: see under sound insulation.

Integrated impulse response method: a method for determining room acoustics parameters such as reverberation time, based on measurement of the impulse response of the room and defined in BS EN ISO 3382 and BS EN ISO 354.

Integrated Pollution Prevention and Control (IPPC): Regulations introduced to comply with European Commission Directive 96/61 whereby a permit system is used for controlling pollution from industrial activities.

Integrating circuit: an electrical circuit which converts an acceleration signal into a velocity or displacement signal.

Integrating sound level meter: a sound level meter, which electrically integrates sound pressure signals to measure the equivalent continuous sound level, LAeq.

Intelligibility: of speech signals; the degree to which each individual syllable of speech can be identified and understood.

Interference: (1) the principle of interference governs how waves interact; the combined wave disturbance is the algebraic sum of the individual wave disturbances, leading to the possibility of constructive and destructive interference; (2) the disturbing effect of unwanted signals on the wanted signal, often electrical in nature.

IPPC: see under Integrated Pollution Prevention and Control Regulations.

ISO: International Organisation for Standardisation.

Isolation: see under vibration isolation.

Isolation efficiency: a measure of the effectiveness of a vibration isolation; isolation efficiency = (1-T) √ó100%, where T is the transmissibility of the system; see also under transmissibility.


Jerk: the rate of change of acceleration.

Joule (J): unit of energy; the amount of energy used in 1 second by a source of energy delivering power at a rate of 1 watt.

Just noticeable difference (JND): a concept used in psycho-acoustic measurement; the difference between two (acoustic) stimuli, which is just noticeable in some defined condition.


No terminology available.


L (level): sound pressure level, SPL; in general, it implies the use of decibels related to the ratio of powers, or power-related quantities such as sound intensity or sound pressure.

LA: see under A-weighted sound pressure level.

LAE: see under sound exposure level, SEL.

LAeq,16hour: the LAeq over the period 07.00-23.00, local time (for strategic noise mapping this is an annual average).

LAeq,T: see under continuous equivalent sound level.

LAmax: the maximum RMS A-weighted sound pressure level occurring within a specified time period; the time weighting, Fast or Slow, is usually specified.

LAN,T: percentile level, i.e. the sound pressure level in dBA which is exceeded for N% of the time interval T, e.g. in LA10 and LA90.

Lday: the LAeq over the period 07.00-19.00, local time (for strategic noise mapping this is an annual average)/

Lden: (day-evening-night-level): the LAeq over the period 00.00-240.00, but with the evening values (19.00-23.00) weighted by the addition of 5 dBA, and the night values (23.00-07.00) weighted by the addition of 10 dBA.

LEP,d: see under personal daily noise exposure level.

Levening: the LAeq over the period 19.00-23.00, local time (for strategic noise mapping this is an annual average).

Lnight: the LAeq over the period 23.00-07.00, local time (for strategic noise mapping this is an annual average).

L’nT: see under standardised impact sound pressure level.

L’nT,w: see under weighted standardised impact sound pressure level.

Lpeak: see under peak sound pressure level

LW: see under sound power level.

Late sound reflections (or late sound): reflections of sound from surfaces in a room which arrive at the receiver within 50 ms of the arrival of the direct sound signal (for speech) or within 80 ms for music. Late sound is treated by the listener’s ear/brain system as being unpleasant and discordant and therefore acoustic designers will seek to discourage the production of late sound reflections while encouraging early reflections which are considered to be helpful in reinforcing the direct sound signal response in the listener.

Level difference, D: BS EN ISO 140-4 uses the difference in level between the two rooms as the basic measure of airborne sound insulation.

Level recorder: an instrument for registering and measuring the variation of signals, such as sound pressures, with time.

Linear: a measurement device is linear if its output is directly proportional to its input; in the case of a microphone, for example, this means that the sensitivity is constant and does not change with sound pressure level; linear SPL means unweighted.

Linearity: the degree to which a device is linear.

Logarithmic decrement (δ): a measure of the amount of damping in a vibrating system, based on the rate of background noise increases.

Lombard effect: an effect whereby a speaker will often raise the level of his/her voice when the level of background noise increases.

Longitudinal wave: a wave in which the vibratory movement of the particles in the medium is parallel to the direction in which the wave is travelling; compressional waves in a fluid medium are longitudinal.

Long-term average rating level: average over the long-term time interval of the rating levels for a series of reference time intervals, carried out as described I BS 7445-2 (ISO 1996-2).

Long-term average sound level: average over the long-term time interval of the equivalent continuous A-weighted sound pressure levels for a series of reference time intervals comprised within the long-term time interval, carried out as described in BS 7445-2 (ISO 1996-2).

Long-term sound level, long-term average rating level, long-term time interval: terms used in connection with the description and measurement of environmental noise, and defined in BS 7445.

Long-term time interval: specified time interval for which the results of the noise measurement are representative. Note that the long-term time interval consists of a series of reference time intervals and is determined for the purpose of describing the environmental noise and is generally designated by competent authorities.

Loss factor: a term used to describe the amount of damping in a system or material; it is twice the damping ratio.

Loudness: the measure of the subjective impression of the magnitude or strength of a sound.

Loudness level: the loudness level of a sound is the sound pressure level of a standard pure tone, of specified frequency, which is equally loud, according to the assessment of a panel of normal observers.

Lower exposure action value: a noise exposure level defined in the 2005 Control of Noise at Work Regulations requiring action from employers and employees (a personal daily (or weekly) noise exposure level of 80 dBA, or peak sound pressure level of 135 dBC).

Low frequency noise: a term generally used to refer to sound below a frequency of about 100 to 150 Hz. It is much less well attenuated during transmission both outdoors and indoors than sounds of higher frequencies, and is therefore often heard at considerable distances from its source particularly late at night when other background noise from other sounds has decreased. In such cases it can often five rise to annoyance because it is often tonal in nature.

Low pass filter: a filter, which transmits signals at frequencies below a certain cut-off frequency and attenuates all higher frequencies.

Lumped parameter model: a model of a vibrating system in which mass, stiffness and damping are represented as discrete elements.


Magnetic tape recorder: a device for capturing, storing and replaying analogue signals onto a tape medium containing ferromagnetic metal oxide particles.

Malleus: or hammer; one of the three bones of the middle ear, connected to the eardrum.

Masking: the process whereby the threshold of hearing for one sound is raised due to the presence of another thus rendering the first sound inaudible.

Mass law: an approximate relationship for predicting the sound reduction index of panels and partitions, based only on the surface density of the panel and the frequency of the sound.

Mean free path: a term used in the statistical treatment of sound in rooms, relating to the average distance between reflections.

Measurement time interval: a term used in standards on the measurements and rating environmental noise (BS 4142 and BS 7445) to indicate the total time over which measurements should be taken or predicted, for assessment purposes. Note that this may consist of the sum of a number of non-contiguous, short-term measurement time intervals.

Measurement uncertainty: see under uncertainty.

Mechanical filter: a resilient pad or layer, which prevents the transmission of high frequency vibration acts as a low velocity at a point in a vibrating system.

Mel: a unit of pitch; the pitch of any sound judged by listeners to be n times that of a 1 mel tone is n mels; 1000 mels is the pitch of a 1000 Hz tone at a sensation level of 40 dB.

Micron (μm): one thousandth of a millimetre or one millionth of a metre.

Microphone: a transducer, which converts acoustic signals into electrical (voltage) signals.

Middle ear: an air-filled space which connects the eardrum of the outer ear to the oval window of the inner ear by three small bones, called ossicles.

Milli-: a standard metric prefix meaning one thousandth.

MLS (maximum length sequence): a method for measuring room acoustics parameters featuring the use of maximum length sequence, pseudo-random but deterministic signals.

Modal analysis: a method for investigating models of vibration of structures involving mapping of the amplitude and phase of the vibration when the structure is excited into vibration.

Mode of vibration: a pattern of vibration of a vibrating system, characterised by a series of nodes and antinodes.

Mode shape: the shape of a particular mode of vibration is usually represented as the maximum displacement of the system from its mean or equilibrium position.

Modulus of elasticity: the stress divided by the strain for an elastic medium; an important factor in determining the speed of elastic waves in the medium; there are different types of elastic modulus, e.g. shear modulus, compression or bulk modulus and torsional modulus, for the different types of elastic deformation.

Monopole: a model or idealised point source of sound, which radiates spherical waves.

Music noise level (MNL): the A-weighted continuous equivalent sound level of the music noise measured at a particular location. Defined in the 1995 Noise Council Code of Practice on Environmental Noise Control at Concerts.


Nano-: a standard metric prefix meaning one thousand millionth (i.e. 10‚Äæ9).

Narrowband filter: a band pass filter with a small bandwidth, i.e. less than one thirds octave.

Natural frequency: the frequency of free or natural vibrations of a system.

Near field: of a sound source; the region or space surrounding the source where sound pressure and acoustic particle velocity are not in phase, and the sound pressure varies with position in a complex way.

Neighbour or neighbourhood noise: noise from domestic premises: household appliances, radios, televisions, music systems, noisy pets, DIY activities, intruder alarms, parties or similar events.

Newton (N): the SI unit of force; the force required to produce an acceleration of 1 m/s² in a mass of 1 kg.

Node: a point, line or surface in a standing-wave pattern where some characteristic of the vibration, e.g. the displacement, is zero.

Noise: unwanted sound or unwanted signal (usually electrical) in a measurement or instrumentation system.

Noise and track keeping (NTK) system: a computerised system used at some major airports whereby radar is used to monitor flightpaths (tracks) of arriving and departing aircraft in order to identify aircraft which are not keeping within required limits, and to match noise events from nearby noise monitoring terminals with aircraft.

Noise criteria (NC) curves: a method devised by Beranek in the 1940s for rating or assessing internal (mainly office) nose. It consists of a set of curves relating octave band sound pressure level to octave band centre frequencies; each curve is given an NC number, which is numerically equal to its value at 1000 Hz. The NC value of a noise is obtained by plotting the octave band spectrum against the family of curves. In order to meet a particular NC specification the noise level must be either below or equal to the SPL in each octave band.

Noise dose: an amount of noise energy, usually A-weighted, received by a person, resulting from a combination of sound pressure level and exposure time; see also under personal daily noise exposure level, LEP,d.

Noise exposure category: a term used in Planning Policy Guidance Note 24 Planning and Noise.

Noise exposure forecast (NEF): a noise index used mainly in the United States for aircraft noise.

Noise immission: the amount of noise exposure received at a particular location.

Noise index: a method of evaluating or rating a noise, usually by assigning a single number to it, based on some combination of its physical characteristics (sound pressure level, frequency, duration) and other factors such as time of day, tonal characteristics and impulsive characteristics.

Noise limit: a maximum or minimum value imposed on a noise index, e.g. for some legal purpose or to determine eligibility for some benefit.

Noise mapping: the production of computer software generated maps showing how the predicted levels of outdoor noise levels vary with location, e.g. from street to street in an area. They show ‘sound (or noise) immission contours’ (see also noise immission) and may be used (according to the EU Noise Directive) to help generate noise action plans.

Noise nuisance: has been defined by the World Health Organisation as ‘a feeling displeasure evoked by noise’. Statutory nuisance has a more specific meaning and is subject to legal action under the Environmental Protection Act 1990.

Noise pollution level, LPN: an index devised in the 1960s for assessing environmental noise, based on a combination of its LAeq value and its variability, expressed in terms of standard deviation; it is now rarely used.

Noise rating (NR) curves: a method of rating noise which is similar to the NC system but intended to be applicable to a wider range of situations; the method was defined in ISO R1996, now withdrawn, but the NR system continues to be used, particularly for offices and is used in BS 8233.

Noise reduction coefficient: a single-figure number sometimes used to describe the performance of sound absorbing materials, based on a combination of its absorption coefficient at various frequencies.

Noise zone: region where the long-term average rating level lies between two specified levels such as, for example, between 65 and 70 dB. The noise zone number for this example is 65-70 dB (defined in BS 7445)

Non-linear: in general there is a non-linear relationship between two quantities if they are not directly proportional to each other; if in measurement and instrumentation systems the input exceeds the linear range, then non-linearity results in a distorted output.

Normalised: corrected or standardised in some way, as in normalised level difference, Dn, defined in BS EN ISO 140-4, where the measured level difference is corrected on the basis of the amount of sound absorption in the receiving room.

Normal mode: a natural mode of a vibrating system.

Normal threshold of hearing: the modal value of the thresholds of hearing of a large number of ontologically normal observers between 19 and 25 years of age.

Noy: a unit of noisiness related to the perceived noise level in PNdB by the formula: PNdB = 40 + 10 log2 (Noy).

NPRs (noise preferential routes): specified aircraft departure routes at some airports designed to minimise noise exposure to major centres of population near to the airport. Compliance with NPRs is monitored by noise and track keeping (NTK) systems.

Nyquist frequency: the frequency, which corresponds to half the sampling rate of digitized data, above which aliasing occurs.


Octave: the range between two frequencies whose ratio is 2:1.

Organ of Corti: a complex structure in the cochlea of the middle ear, supported by the basilar membrane and containing the hair cells.

Oscillation: a to-and-fro motion; a fluctuation of a quantity or value about a mean.

Oscilloscope: a device for displaying oscillatory signals on a cathode-ray screen.

Ossicles: the three small bones of the middle ear, which connect the eardrum with the oval window in the cochlea.

Otitis media: inflammation and/or infection of the middle ear leading to build-up of fluid and blockage of the Eustachian tube and may result in temporary or permanent hearing loss (also sometimes called ‘glue ear’).

Otoacoustic emission: sounds emitted by the eardrum and detected by a miniature microphone in the ear canal arising from activity in the cochlea in response to the stimulus of external sound.

Outer ear: the outer part of the hearing mechanism, which collects and guides airborne sound down the ear canal to the eardrum.

Output impedance: the impedance of a device measured at its output.

Oval window: diaphragm connecting the cochlea to the middle ear.

Overdamping: (1) an amount of damping, in excess of critical, which is sufficient to prevent oscillation in a mass-spring system; (2) producing a damping ratio greater than one.

Overload: a situation in which a component or system is used beyond its range of linearity.

Overload indicator: a device, which indicates when an instrument is likely to read incorrectly because it is being overloaded.

Overtone: a higher (i.e. not the lowest) harmonic or natural frequency of a vibrating system.


P-wave: a longitudinal compression wave in an elastic medium.

Particle velocity: see under acoustic particle velocity.

Pascal: a unit of pressure equal to 1 N/m².

Pass band: a band of frequencies, which are transmitted by a band pass filter.

Passive: a device, which does not require a source of power for its operation, e.g. a passive filter or a passive noise control (cf. active).

Peak: the maximum deviation of a signal from its mean value within a specified time interval.

Peak-to-peak: the algebraic difference between the extreme values of a signal occurring within a specified time interval.

Perceived noise level: of a sound; the sound pressure level of a reference sound which is assessed by normal observers as being equally noisy; the reference sound consists of a band of random noise centred on 1000 Hz.

Percentile level, LAN,T: the sound level, in dBA which is exceeded for N% of the time interval T, for example in LA10 and LA90. The time weighting (F or S) should always be specified, e.g. LAF10 or LAS10. Note that percentile levels determined over a certain time interval cannot generally be extrapolated to other time intervals.

Period: a repetitive signal; the time for one cycle.

Periodic signal: one which, repeats itself exactly.

Permanent threshold shift: the component of threshold shift, which shows no progressive reduction with the passage of time when the apparent cause has been removed.

Personal daily noise exposure level, LEP,d: that steady or constant level which, over eight hours, contains the same amount of A-weighted sound energy as is received by the subject during the working day.

Phase: of a sinusoidal signal; an angle whose value determines the point in the cycle, i.e. the magnitude of the signal, at some reference time.

Phase difference: the difference between the phase angles (of two sinusoidal signals of the same frequency).

Phon: the unit of loudness level; the loudness level of a sound, in Phons, is the sound pressure level of a 1000 Hz pure tone judged by the average listener to be equally loud.

Piezoelectric: the behavior of certain crystalline materials whereby a deformation of the material (caused by force or stress) results in the production of electric charge on the stressed faces, and a voltage difference between them.

Pink noise: a random broadband signal which has equal power per percentage bandwidth and therefore has a flat, i.e. horizontal, frequency spectrum when plotted on a logarithmic frequency scale (cf. white noise).

Pinna: the external part of the ear leading to the ear canal.

Pitch: that attribute of auditory sensation in terms of which sound may be ordered on a scale related primarily to frequency; the unit of pitch is the mel.

Plane wave: a wave in which the wavefronts are plane and parallel everywhere, so the sound energy does not diverge with increasing distance from the source.

Plenum: a chamber or space used to collect air prior to its distribution via a duct system.

PNdB: the unit of perceived noise level.

Point source: an idealised concept of an acoustic source, which radiates spherical waves.

Polarisation: a property of transverse waves but not longitudinal waves; it relates to the direction of the particle displacement in the plane normal to the direction of propagation.

Power spectral density: the amount of power per unit of frequency in a signal. It is measured in W/Hz, or more generally for a voltage signal in V²/Hz (since power is proportional to V²).

Ppv, peak particle velocity: a measure of vibration, usually measured in mm/s, used for assessing likelihood of damage to buildings from vibration.

Preamplifier: a circuit which acts as an electrical impedance matching device between a transducer with a high output impedance, such as a microphone or accelerometer, and the signal processing circuits of the sound level or vibration meter.

Precedence effect: see under Haas effect.

Pre-completion testing (PCT): sound insulation tests required of newly built (and some converted) dwellings, in order to test compliance with the performance requirements of the 2002 Building Regulations (unless built in accordance with Robust Standard details).

Preferred speech interference level: the arithmetic average of the sound pressure levels in the three octave bands 500 Hz, 1000 Hz and 2000 Hz.

Prepolarised microphone: see under electret microphone.

Presbycusis: hearing loss, mainly of high frequencies, that occurs with advancing age.

Progressive wave: a wave that travels outwards, from its source, and is not being reflected.

Psycho-acoustics: the study of the relationship between the physical parameters of a sound and its human perception.

Pure tone: a sound for which the waveform is a sine wave, i.e. for which the sound pressure varies sinusoidally with time.

Pure tone audiometer: an instrument for measuring hearing acuity to pure tones by determination of hearing levels.


Q factor: a quantity which measures the sharpness of the resonance of a single degree of freedom mechanical or electrical vibrating system; in a mechanical system it is related to the damping ratio, the amplification produced at resonance and the shape of the resonance peak.


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

Rw and R’w: single figure frequency weighted values of R and R’ defined in BS EN ISO 717-1.

Random noise/vibration signal: a noise, vibration or signal, which has a random waveform, with no periodicity.

RASTI: rapid analysis speech transmission index; a measurement parameter for assessing the speech intelligibility in a room. IT has a value between 0 and 1: 1 representing perfect speech intelligibility and 0 representing zero speech intelligibility. RASTI is a shortened version of Speech Transmission Index, STI.

Rating level: LAr,Tr a noise index defined in BS 4142 and BS 7445; the specific noise level plus any adjustment for the characteristic features of the noise (e.g. tonal and impulsive features) during a specified time (see under specific noise level).

Ray: a straight line representing the direction in which a sound is travelling, used in situations where the size of reflecting surfaces is large compared to the sound wavelength.

Rayl: the unit of specific acoustic impedance.

Rayleigh wave: a type of elastic wave, which propagates close to the surface of a solid.

Raynaud’s disease: a disorder affecting the blood vessels, nerves, connective tissues and bones of the fingers; one of its causes is prolonged exposure to high levels of vibration.

Ray tracing: a method of modelling room acoustic performance using computer software package (also called beam tracing).

Reactance: the complex component of impedance associated with energy being stored and converted from one form to another (e.g. from potential to kinetic, or from electrostatic to electromagnetic) rather than being converted to heat.

Reactive silencer: a silencer, which reduces sound levels by using changes in impedance instead of sound absorbing materials.

Real time, in: quickly enough to observe changes in a situation as they happen.

Real-time analyser: a device, which is capable of analysing signals (usually in the frequency domain) in real time.

Real world protection: the degree of sound level attenuation provided by hearing protectors under realistic working conditions, as opposed to that from laboratory tests; an allowance to be deducted from manufacturer’s test data to allow for real world conditions.

Recruitment: an aspect of certain forms of perspective deafness; an abnormally rapid increase in the sensation of loudness with increasing sound pressure level.

Reference time interval, Tr: the specified interval over which a noise index such as equivalent continuous A-weighted sound pressure level is determined for assessment purposes e.g. in BS 7445 and BS 4142. The value of Tr is 1 hour during the day and 5 minutes at night.

Reference value: standardised values used as the basis for decibel scales of sound pressure, sound intensity, sound power, vibration acceleration, velocity and displacement.

Reflection: the redirection of waves, which occurs at a boundary between media when the size of the boundary interface is large compared with the wavelength (see also diffuse reflection and specular reflection).

Refraction: the change in direction of waves caused by changes in the wave velocity in the medium.

Repeatability: the variability of measurements when repeated under the same measurement conditions.

Reproducibility: the variability of measurements when repeated under different measurement conditions.

Residual noise: The ambient noise remaining at a given position in a given situation when the specific noise source is suppressed to a degree such that it does not contribute to the ambient noise (defined in BS 4142 and BS 7445; see also specific noise and ambient noise).

Residual noise level, LAeq,T: the equivalent continuous A-weighted sound pressure level of the residual noise.

Resonance: the situation in which the amplitude of forced vibration of a system reaches a maximum at a certain forcing frequency (called the resonance frequency).

Resonance frequency: the frequency at which resonance occurs, i.e. at which the forced vibration amplitude in response to a force of constant amplitude is a maximum; for an undamped system the resonance frequency is the same as the natural frequency of the system; for a damped system the resonance frequency is slightly reduced.

Reverberant room: a standard acoustic test environment designed to produce diffuse sound conditions throughout the space.

Reverberant sound/reverberation: the sound in an enclosed space, which results from repeated reflections at the boundaries.

Reverberant sound field: the region in an enclosed space in which the reverberant sound is the major contributor to the total sound pressure level.

Reverberation time: the time required for the steady sound pressure level in an enclosed space to decay by 60 dB, measured from the moment the sound source is switched off.

Ringing: transient free vibration of bodies caused by impact.

Robust Standard details: a method of building new dwelllings according to prescribed details which guarantees sound insulation sufficient to meet the requirements of the 2002 Building Regulations and so avoid the need for pre-completion sound insulation testing.

Room constant, Rc: a constant used in the calculation of reverberant sound pressure level in a room: Rc = S /(1 РαAVG), where S= total area of room surfaces and αAVG =average absorption of room surfaces.

Room mode: a three-dimensional standing-wave sound pressure pattern, i.e. a mode shape, associated with one of the natural frequencies of a room.

Root mean quad (RMQ): the RMQ value of a set of numbers is the fourth root of the average of the fourth powers of the numbers; for a vibration waveform the RMQ value over a given time period is the fourth root of the average value of the fourth power of the waveform over that time period.

Root mean square (RMS): the RMS value of a set of numbers is the square root of the average of their squares; for a sound or vibration waveform the RMS value over a given time period is the square root of the average of the square of the waveform over that time period.

Round window: a diaphragm or membrane at the end of the cochlea, which connects with the middle ear.


S (slow) time weighting: one of the standard averaging times for sound level meter displays, defined in BS EN ISO 61672-1.

Sabin: unit of sound absorption; one sabin is the amount of absorption equivalent to one square metre of perfect absorber.

Sabine’s formula: a formula for predicting reverberation times of rooms.

Sampling frequency: of a digitalised signal; the number of samples per second (see Shannon’s sampling theorem).

Sampling interval: the time interval between samples.

Scalar: a quantity that may be completely defined by its magnitude alone, i.e. it has no direction.

Semi-anechoic: a room with anechoic walls and ceiling, but with a sound reflecting floor.

Semi-reverberant: a room which is neither completely anechoic nor reverberant, but somewhere in between.

Sensation value: of a specified sound; the sound pressure level when the reference sound pressure corresponds to the threshold of hearing for the sound.

Sensitivity: of a transducer; the ratio of output to input, e.g. for a microphone the sensitivity is measured as output voltage (V)/ input pressure (Pa).

Shannon’s sampling theorem: an important result of signal processing theory that states that a minimum of two samples per hertz is needed adequately to represent the highest frequency component in a signal. Therefore a sample ate of at least 40,000 samples per second would be needed adequately to represent a signal with frequency components of up to 20,000 Hz. It is important that this condition is met otherwise false frequencies, called aliases, start to appear in the analysis.

Shear wave: a transverse wave of shear stress propagating in an elastic medium.

Shock: a sudden transient disturbance to a vibrating system caused by a rapid change in force, displacement, velocity or acceleration.

Signal-to-noise ratio: a measure of the strength of a signal, indicating its magnitude relative to the background electrical noise in the measurement system; usually expressed in decibels.

Silencer: a device for reducing noise in air and gas flow systems; silencers are either absorptive or reactive; also called attenuators or mufflers.

Simple harmonic motion: a single frequency vibration, i.e. one in which the displacement varies sinusoidally with time.

Simple source: an idealised model of an acoustic source, which radiates spherical waves, under free field conditions; see also under point source, monopole.

Sine wave: the graph of a sinusoidal function, which indicates the simplest possible repeating waveform, characterised by a single frequency and constant amplitude.

Single degree of freedom system: a vibrating system consisting of only one mass, one spring and one dashpot (damper); such a system has one natural frequency and one mode of vibration; its motion can be completely described by one variable.

Sinusoidal: relating to a sine wave.

SNR (single noise rating): a single figure method for evaluating the attenuation performance of hearing protectors.

Snubber: a device used to restrict the maximum displacement of a vibrating system, e.g. at resonance.

Sociocusis: haring loss arising from everyday activities.

Soft ground: ground such as grassland, cultivated land which is considered to be an acoustically absorbing surface, in contrast with ‘hard ground’ which is considered to be sound reflecting.

Sone: the unit of loudness; the tone scale is devised to give numbers which are approximately proportional to the loudness; it is related to the Phon scale as follows: P= 40 + 10log2S, where P represents Phons and S represents Sones.

Sound: (1) pressure fluctuations in a fluid medium within the (audible) range of amplitudes and frequencies, which excite the sensation of hearing; (2) the sensation of hearing produced by such pressure fluctuations.

Sound absorbing material: material designed and used to maximise the absorption of sound by promoting frictional processes; the most commonly used materials are porous, such as mineral fibre materials or certain types of open-cell foam polymer materials.

Sound absorption: (1) the process whereby sound energy is converted into heat, leading to a reduction in sound pressure level; (2) the property of a material, which allows it to absorb sound energy.

Sound absorption coefficient: a measure of the effectiveness of materials as sound absorbers; it is the ratio of the sound energy absorbed or transmitted (i.e. not reflected) by a surface to the total sound energy incident upon that surface; the value of the coefficient varies from 0 (for very poor absorbers and good reflectors) to 1 (for very good absorbers and poor reflectors).

Sound exposure level, SEL (LAE): a measure of A-weighted sound energy used to describe noise events such as the passing of a train or aircraft; it is the A-weighted sound pressure level which, if occurring over a period of one second, would contain the same amount of A-weighted sound energy as the event.

Sound insulating material: material designed and used as partitions in order to minimise the transmission of sound; the best materials are those which are dense and solid, such as wood, metal or brick, although lightweight panels can also be effective when in the form of double-skin constructions.

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

Sound intensity: the sound power flowing per unit area, in a given direction, measured over an area perpendicular to the direction of flow; its units are in W/m².

Sound intensity level, LI: sound intensity measure on a decibel scale: LI = 10log10(I/I0), where I0 is the reference value of sound intensity, 10–12 W/m¬≤.

Sound level: a frequency-weighted sound pressure level such as the A-weighted value.

Sound level meter: an instrument for measuring sound pressure levels.

Sound power: the sound energy radiated per unit time by a sound source, measured in watts (W).

Sound power level, LW: sound power measured on a decibel scale: LW = 10log10(W/W0), where W0 is the reference value of sound power, 10–12 W.

Sound pressure: the fluctuations in air pressure, from the steady atmospheric pressure, created by sound, measured in pascals (Pa).

Sound pressure level, SPL (Lp): sound pressure measured on a decibel scale: Lp = 20log10(p/p0), where p0 is the reference sound pressure, 20 x 10–6 Pa.

Sound propagation: the transmission or transfer of sound energy from one point to another.

Sound reduction index, R: a measure of the airborne sound insulating properties, in a particular frequency band, of a material in the form of a panel or partition, or of a building element such as a wall, window or floor; it is measured in decibels: R = 10log10(1/t), where t is the sound transmission coefficient; it is measured under laboratory conditions according to BS EN ISO 140-4; also known as transmission loss.

Soundscape: the total sound environment at a particular location, implying much more than can be described just in terms of sound level. A soundscape approach to improving the urban acoustic environment, i.e. how it may be made more pleasing to the ear, rather than simply on noise reduction.

Sound transmission: the transfer of sound energy across a boundary from one medium to another.

Sound transmission coefficient: the ratio of the sound energy transmitted by a partition, or across a boundary, to the sound energy incident upon the partition or the boundary.

Sound wave: a pressure wave in a fluid which transmits sound energy through the medium by virtue of the inertial, elastic and damping properties of the medium.

Specific acoustic impedance: at a point in a sound field; the complex ratio of sound pressure to the acoustic particle velocity. In terms of the fundamental units m, kg and s, the units of specific acoustic impedance are kgm/s2, but sometimes the quantity is expressed in terms of newtons (N) or pascals (Pa), as either NSM-3 or as Pas/m, and also as the rayl.

Specific noise: the particular component of the ambient noise which is under consideration or investigation, e.g. in connection with a planning application or noise complaint; defined in BS 4142.

Specific noise source: The noise source under investigation for assessing the likelihood of complaints (defined in BS 4142).

Specific noise level, LAeq,Tr: the equivalent continuous A-weighted sound pressure level at the assessment position produced by the specific noise source over a given reference time interval.

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

Spectrum: a frequency spectrum is a graph showing variation of sound pressure level (or other quantity) with frequency.

Specular reflection: sound reflection which obeys the law of reflection that angle of incidence equals angle of reflection and which occurs at surfaces which are smooth on a scale comparable with the wavelength of the sound (as opposed to diffuse reflection).

Speech intelligibility: the ability of speech to be understood; the ability of a listener to hear and correctly interpret verbal messages. The concept of intelligibility is used as a criterion to determine the degree of acoustic privacy between rooms.

Speech interference level: a measure of the ambient noise level in offices which gives an indication of the degree to which speech will be intelligible; it is based on the arithmetic mean of the octave band sound pressure levels 500 Hz, 1000 Hz and 2000 Hz, which are most significant for good speech intelligibility.

Speech privacy: the degree to which speech is unintelligible between offices. Three ratings are used: confidential, normal (non-obstructive) and minimal. It is inversely related to speech intelligibility, e.g. good speech intelligibility leads to poor acoustic privacy, and vice versa.

Speech transmission index (STI): a measurement parameter for assessing the speech intelligibility in a room. It has a value between 0 and 1: 1 representing perfect speech intelligibility and 0 representing zero speech intelligibility.

Spherical waves: an idealized model of how sound propagates in free field conditions, and used as the basis of certain sound level prediction methods.

Standard deviation: a measure of the deviation or scatter of a set of values (e.g. sound pressure level measurements) from the mean value.

Standardised impact sound pressure level, L’nt: a measurement of impact sound insulation, corrected according to BS EN ISO 140-7 for room characteristics; a complete set of measurements consists of 16 values, one for each third octave frequency band from 100 Hz to 3150 Hz.

Standardised level difference, DnT: a measurement of airborne sound insulation, corrected according to BS EN ISO 140-4 for receiving room characteristics; a complete set of measurements consists of 16 values, one for each third octave frequency band from 100 Hz to 3150 Hz.

Standing waves: a wave system characterized by a stationary pattern of amplitude distribution in space arising from the interference of progressive waves; also called stationary waves.

Stapes: or stirrup; one of the three bones of the middle ear, connected to the oval window of the inner ear.

Static deflection: the deflection produced in the spring of a mass-spring system by the weight of the mass; it is related to the natural frequency of the system and is used to specify the stiffness of springs for vibration isolation.

Stationary waves: see under standing waves.

Steady noise: noise for which the fluctuations in time are small enough to permit measurement of average sound pressure level to be made satisfactorily without the need to measure LAeq using an integrating sound level meter; defined in BS 4142.

STI: see speech transmission index.

STIPA: a version of STI for public address systems.

STITEL: a version of STI for telecommunications systems.

Strain: the fractional change in shape due to an elastic deformation in a material caused by an applied stress.

Stress: force per unit area, measured in N/m2; stress applied to elastic materials causes strain.

Structure-borne sound: sound which reaches the receiver after travelling from the source via a building or machine structure; structure-borne sound travels very efficiently in buildings, and is more difficult to predict than airborne sound.

Subjective: depending upon the response of the individual.

Superposition: according to the principle of superposition, the wave disturbances in a medium caused by different sources may be combined algebraically.


Tangential mode: a room mode, which involves reflections between two pairs of parallel surfaces (e.g. walls).

Temporary threshold shift: the component of threshold shift, which shows progressive reduction with the passage of time, when the apparent source has been removed.

Threshold of hearing: of a given listener the lowest sound pressure level of a particular sound that can be heard under specified measurement conditions, assuming the sound reaching the ears from other sources is negligible.

Threshold of pain: for a given listener the minimum sound pressure level of a specified sound, which will produce the sensation of pain in the ear.

Threshold shift: the deviation, in decibels, of a measured hearing level from one previously established.

Timbre: the quality of a sound, which is related to its harmonic structure.

Time averaged sound level: see under continuous equivalent sound level, LAeq,T.

Time constant: of a process or quantity, which decays exponentially with time; the time required for the value to reduce by a factor of 1/e, where e is the exponential number 2.7183…

Time weighting: one of the standard averaging times (F, S, I) used for the measurement of RMS sound pressure level in sound level meters, specified in BS EN ISO 61671-1.

Tinnitus: a subjective sense of noise in the head or ringing in the ears for which there is no observable cause.

Tonality: the degree to which a noise contains audible pure tones: broadband noise is generally less annoying than noise with identifiable tones.

Tone: a sound, which produces the sensation of pitch; see also under pure tone.

Traffic noise index: and index used for the assessment of environmental noise in the 1960s, based on a combination of the LA10 value and the LA90 value; it is now rarely used.

Transducer: a device for converting signals from one form to another; frequently, the requirement is to convert changes in some physical variable, such as temperature or sound pressure, into analogous changes in electrical voltage or charge.

Transfer function: of a vibrating system; the ratio of the output or response of the system to the input excitation, usually expressed as a complex function of frequency.

Transfer standard: a calibrated noise source designated to fit over a microphone.

Transient: a noise or vibration signal, which is not continuous but which decreases to zero then remains zero.

Transmissibility: of a vibrating system; the non-dimensional ratio of vibration amplitude at two points in the system; frequently, the two points are on either side of springs used as antivibration mounts, and the transmissibility is used as an indicator of the effectiveness of the isolation.

Transmission coefficient: see under sound transmission coefficient.

Transmission loss: see under sound reduction index.

Transverse sensitivity: of an accelerometer; the sensitivity to vibration in a direction perpendicular to the axis of the accelerometer.

Transverse wave: a wave in which the direction of vibration of the particles of the medium is perpendicular to the direction of wave travel; an example is a shear wave in a solid medium.

Triboelectric effect: the production of electric charge as a result of vibration in an accelerometer cable, leading to electrical noise in the accelerometer signal, unless the cable is secured to prevent movement.


Ultrasonics: the study of ultrasound.

Ultrasound: acoustic waves with frequencies, which are too high to be heard by human ears.

Uncertainty: an estimate of the degree of variability or dispersion associated with the result of a measurement.

Unweighted sound pressure level: a sound pressure level, which has not been frequency weighted, sometimes known as the linear sound pressure level; symbol Lp(see also Z-weighting).

Upper exposure action value: a noise exposure level defined in the 2005 Control of Noise at Work Regulations requiring action from employers and employees (a personal daily (or weekly) noise exposure level of 85 dBA, or peak sound pressure level of 137 dBC).


Vector: a quantity, which has a direction as well as a magnitude, e.g. force, displacement, velocity and acceleration.

Velocity: the rate of change of displacement, measured in m/s or mm/s.

Vibration: a to-and-fro motion; a motion, which oscillates about a fixed equilibrium position.

Vibration dose value (VDV): a measure of vibration exposure; the fourth root of the integral, over the measurement period, of the fourth power of the frequency-weighted time-varying acceleration; see also under eVDV and root mean quad.

Vibration isolation: the reduction of vibration and structure-borne sound by thhe use of resilient materials inserted in the transmission path between source and receiver.

Vibration white finger: blanching of the fingers and other symptoms caused by exposure to hand-transmitted vibration; see also under Raynaud’s disease.

Viscous damping: damping of the sort, which occurs in viscous fluid layers, in which the damping force is proportional to the velocity of the fluid element.

Volt (V): the unit of electrical potential; the difference in electrical potential between two points on an electric conductor which is carrying a constant electric current of one ampere (A) when the power dissipated between the points is one watt (W).


Watt (W): the unit of power; the power dissipated when one joule of energy is expended in one second.

Wave: in an elastic medium; a mechanism whereby a disturbance, and the energy associated with it, is propagated through an elastic medium; the disturbance results in vibrations of the particles of the medium, vibrations transmitted to nearby regions as a result of the elastic and inertial nature of the medium, resulting in a disturbance which is a function of both position and time.

Waveform: a graph showing how a variable at one point in a wave (e.g. sound pressure or particle velocity) or vibration varies with time.

Wavefront: the leading edge of a progressive wave, along which, the vibration of the particles of the medium are in phase.

Wavelenght: the minimum distance between two points that are in phase within a medium transmitting a progressive wave.

Weber-Fechner law: a law of psychology, which states that the change of subjective response to a physical stimulus is proportional to the logarithm of the stimulus.

Weighted sound reduction index, Rw: a single figure value of sound reduction index, derived according to procedures given in BS EN ISO 717-1, used for rating and comparing partitions and based on the values of sound reduction index at different frequencies.

Weighted standardized impact sound pressure level, L’nT,w: a single-figure value of impact sound insulation performance, derived according to procedures in BS EN ISO 717-2, used for comparing and rating floors and based in the values of L’nT at different frequencies; values of L’nT,w are specified in the Building Regulations.

Weighted standardized level difference, DnT,w: a single-figure value of airborne sound insulation performance, derived according to procedures given in BS EN ISO 717-1, used for rating and comparing partitions based on the values of D at different frequencies; sound insulation performance requirements in the Building Regulations are specified in terms of values of DnT,w + Ctr.

Weighting: see under frequency weighting and time weighting.

White finger: see under vibration white finger.

White noise: a random broadband noise which contains equal power per unit bandwidth, so it has a flat, i.e. horizontal, frequency spectrum when plotted on a linear frequency scale (cf. pink noise).

Whole-body vibration: vibration transmitted to the body as a whole.


No terminology available.


No terminology available.


Z-weighting: a (zero) frequency weighting defined in BS EN ISO 61672-1.


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