Acoustic Glossary


A : Sound and Vibration Definitions, Terms, Units, Measurements ...

% Noise Dose

1/1 octave

1/3 octave

1/n octave analysis is performed on a fractional part of an octave where n is the variable. Commonly used values are 1/1, 1/3, 1/12 and 1/24-octave.

A/D Converter, converts a analogue signal to a digital signal.

A-weighting also known as the A curve, A filter and A network

Absorption under Sound Absorption

AC Coupling is the connection of a signal from one circuit to another in a manner that rejects DC components.

See also • DC coupling.

Accelerance is the ratio of acceleration to the applied force

Acceleration is the rate of change of velocity and is a vector quantity.

The SI units are m/s2 or if using Imperial units then 'g' = 9.80665 m/s2 = 386.089 in/s2

Acceleration, velocity, displacement and angular frequency ω = 2·π·f, are related as follows
Velocity = a/ω and Displacement = v/ω so at 159 Hz an acceleration of 10 m/s2 = 0.01 m/s and = 10 μm.
This works for all frequencies, we chose 159 Hz to keep the numbers simple. We also have a vibration nomogram for downloading.

If 1 N = 1 Kg·m/s2 it follows that m/s2 acceleration also equals N/kg Newtons per kilogram.

Amax is the maximum RMS acceleration.

Amin is the minimum RMS acceleration.

Amp is the maximum Peak acceleration.

aw is the time-averaged, frequency-weighted, single-axis vibration acceleration.

See also • angular accelerationparticle acceleration, used in acoustic wave theory.

Acceleration Definition IEC 113-01-38, vector quantity a = dv/dt, where v is velocity and t is time
Note 1 the acceleration is related to a point described by its position vector. The point may localize a particle, or be attached to any other object such as a body or a wave.
Note 2 the acceleration depends on the chosen reference frame.
Note 3 the coherent SI unit of acceleration is metre per second squared, m/s².

The human response to acceleration covers a wide range from a few μm/s2 to tens of metres per second squared. The dB scale reduces this immense range to a manageable set of numbers, see the acceleration level units below.

Acceleration Level dB (La) = 20 lg (a/ao) dB re 1 µm/s2.

Acceleration Reference Level (ao) = 1 μm/s2 ≡ 0 dB (also defined in ISO 1683) *

An increase or decrease in acceleration of 20 dB = a factor of 10
An increase or decrease in acceleration of 40 dB = a factor of 100
An increase or decrease in acceleration of 60 dB = a factor of 1000 etc.

(Vibratory) Acceleration Level Definition IEC 801-22-09, logarithm of the ratio of a given (vibratory) acceleration to the reference acceleration. Acceleration level in decibels is 20 times the logarithm to the base ten of the ratio.
Note 1 unless otherwise specified, the reference acceleration is 1 μm/s2 *
Note 2 unless otherwise specified, the accelerations are understood to be expressed in RMS values.

* ISO 1683 also states 'in connection with structure-borne sound, a vibratory acceleration reference value of 10 μm/s2 is also in use'

See other • reference levels and the IEC Definition of Level

Acceleration Equivalent Level (Aeq) a single number to represent the equivalent acceleration energy as it varies over a working day, measured in m/s2. For example Aeq8 = 2.5 m/s2 indicates a equivalent level of 2.5 m/s2 measured over an 8 hour shift.

To calculate the equivalent value for other periods use the formulae Aeq8 = a √T/8 where T = hours

Aeq4 and Aeq16 are also used in some vibration exposure applications.

See also • hand arm vibrationvibration at work regulationsvibration dose valuewhole body vibration


a vibration sensor whose electrical output is directly proportional to the acceleration component of the vibration. The two most common accelerometer types are the traditional charge type and the IEPE : integrated electronic piezoelectric type with a built-in line-drive amplifier to enable the output signal to be transmitted over 'longer cable runs'.

ACGIH : American Conference of Industrial Hygienists

Acoustic Absorption under sound absorption

Acoustic Admittance Definition

IEC 801-25-46, reciprocal of acoustic impedance

See also acoustic impedance and related topics

Acoustic Calibrator

, an instrument providing a reference noise source used to calibrate and check the performance of sound level meters.

Acoustic Compliance Definition

IEC 801-25-45, reciprocal of acoustic stiffness

Acoustic Coupler Definition

IEC 801-28-03, a cavity of predetermined shape and volume used, for example, for the calibration of earphones or microphones in conjunction with a calibrated microphone adapted to measure the sound pressure developed within the cavity.

Acoustic Emission

is the energy that is generated when materials are under stress or break.

Acoustic Energy under sound energy

Acoustic Impedance

is the resistance to the flow of sound through a medium, analogous to Ohm's law in electrical theory and sometimes referred to as the acoustic ohm.

Acoustic Impedance (Z) is the ratio of the sound pressure to the volume velocity so Z = p/U

Acoustic Impedance Definition

IEC 801-25-40, at a specified surface, quotient of sound pressure by volume velocity through the surface

See also • acoustic admittanceacoustic ohmacoustic reactanceacoustic resistancecharacteristic acoustic impedancecharacteristic impedance of a mediumcomplex acoustic impedanceconjugate impedancesdriving point impedanceimpedancespecific acoustic impedancespecific acoustic reactancespecific acoustic resistancespecific wall admittancespecific wall impedancetransfer impedance

Acoustic Insulation under sound insulation

Acoustic Intensity under sound intensity

Acoustic Louvres includes sound-attenuating baffles for reduction of airborne sound.

Acoustic Mass Definition

IEC 801-25-43, at a frequency for which inertia is dominant, quotient of sound pressure by the resulting in-phase volume acceleration during sinusoidal motion, also known as inertance.
Note acoustic mass has dimensions of mass divided by the square of area

Acoustic Ohm

, the a unit of sound resistance, the ratio of the sound pressure to volume flow, m3/s = Pa·s/m3

See also • acoustic impedance and related topics

Acoustic Oscillation Definition

IEC 801-21-01, movement of particles in an elastic medium about an equilibrium position, also known as acoustic vibration and sound.

See also • audible soundoscillation

Acoustic Phase Coefficient Definition

IEC 801-23-37, imaginary part of the linear exponent of sound propagation
Note the unit is the radian per metre

Acoustic Power under sound power

Acoustic Pressure under sound pressure

Acoustic Radiation Pressure Definition

IEC 801-21-42, unidirectional steady pressure exerted on a surface by an acoustic wave.

Acoustic Reactance Definition

IEC 801-25-42, the imaginary part of acoustic impedance

Acoustic Reference Levels

Acoustic Reference Wind Speed

Acoustic Resistance Definition

IEC 801-25-41, the real part of acoustic impedance

See also acoustic impedance and related topics

Acoustic Stiffness Definition

IEC 801-25-44, in a system in which friction and inertia are negligible, quotient of sound pressure by the resulting in-phase volume displacement during sinusoidal motion.

Acoustic Streaming Definition

IEC 801-23-43, unidirectional current in a fluid due to the presence of acoustic waves

Acoustic Trauma

, damage to the hearing mechanism caused by a sudden burst of intense noise, or by a blast. The term usually implies a single traumatic event.

Acoustic Velocity Level under particle velocity level

Acoustic Vibration Definition

IEC 801-21-01, movement of particles in an elastic medium about an equilibrium position, also known as acoustic oscillation and sound.

See also • audible soundoscillation

Acoustic Waves under sound waves

Active Noise Control, reducing unwanted sound electronically. When a sound wave of equal amplitude but opposite sign (180 degree out of phase) is added to the original sound - the result is sound cancellation.

Active Sound Fields

Active Sound Intensity

Admittance Definition

IEC 801-25-15, reciprocal of impedance of stated kind

See also • acoustic admittance

ADPCMadaptive pulse code modulation

Aeqacceleration equivalent value : Aeq4, Aeq8, Aeq16

Age Related Threshold Shift

AIarticulation index

Airborne Sound reaches the point of interest through air.

Airborne Sound Insulation Index (Ia''), former name for - weighted apparent sound reduction index, R'w

Air Condenser Microphones are widely used in noise measurements because they offer the best linearity, frequency range and high stability. Because of their importance we have prepared more details under measurement microphones

Air Density

Algorithm a specific procedure for solving problems. An FFT - fast fourier transform is an algorithm.

Aliasing digital sampling requires the analogue signal to be sampled at twice the frequency of interest otherwise aliasing occurs. If the signal is not filtered to eliminate the high frequencies, they appear as 'false' lower frequency signals. Once 'introduced' these aliased signals cannot be distinguished from valid sampled data.

See also • anti-aliasing filternyquist frequency

Amax under acceleration

Ambient Noise Definition

IEC 801-21-12, encompassing sound, at a given place, being usually a composite of sounds from many sources near and far.

Other noise descriptors • background noisebroadband noisegaussian noisenarrowband noiseperiodicpink noisepseudo random noiserandom noiseresidual noisespecific noisewhite noisewideband noise

Ambient Pressure

the pressure of the surrounding medium. The SI unit is the Pascal, Pa which is very small relative to the atmospheric pressure.

Amin under acceleration

Amp under acceleration

Ampere (A)

is the basic SI unit of electric current; the constant current that, when maintained in two parallel conductors of infinite length and negligible cross section placed 1 metre apart in free space, produces a force of 2 x 10-7 Newton per metre between them.

1 ampere is equivalent to 1 Coulomb per second.

Amplification Factor (Q)

the mechanical gain of a structure when excited at a resonant frequency. The amplification factor is a function of the system damping. For a damping ratio = 0 (no damping) the amplification factor is infinite, for = 1 (critically damped) there is no amplification.


is the magnitude of an oscillating quantity, for example sound pressure or vibration level. In the case of a vibrating object, the amplitude is measured and expressed in three ways displacement, velocity and acceleration. Amplitude is also the y-axis of the vibration time waveform and spectrum; it helps define the severity of the vibration.

Amplitude Distribution

a representation of time-varying noise indicating the percentage of time that the noise level is present in a series of amplitude intervals.

Amplitude Probability used to investigate the amplitude distribution of signals, also known as probability amplitude.

Analogue continuously variable physical quantity, such as a sound or vibration wave.

Analogue to Digital Converter, converts an analogue signal to a digital one. US spelling is analog.


(without echo), refers to the absence of sound reflections. It is almost impossible to create a truly anechoic environment, as the perfect sound absorber does not exist. However the anechoic chamber can come close.

Anechoic Chamber

, echo free room, within specified limits. The walls are lined with sound absorbent wedges to minimize reflections and create free-field conditions, so direct sound measurements of test objects may be made. Low frequency measurements are restricted by the room dimensions and the sound absorbing materials 'wedges' used.

Anechoic Chamber Definition

IEC 723-03-31, room with no appreciable reverberation, used for acoustic measurements

Anechoic Room Definition

IEC 801-31-18, room whose boundaries absorb substantially all the sound incident thereon, thereby affording free-field conditions, also known as free field room.

Angle of Incidence

IEC 705-0403, at a point on a surface, the acute angle between the normal to this surface and the direction of propagation of an incident wave

Angle of Reflection the angle measured from the reflected wave to the surface normal

Angle of Refraction see sound wave refraction.

Angles are usually expressed in degrees or radians. Less often as Grads or Gradians (except on some electronic calculators)

Units Values
Degrees 30° 45° 60° 90° 180° 270° 360°
Radians 0 π/6 π4 π/3 π/2 π 3 π/2
Grads 0g 100g/3 50g 200g/3 100g 200g 300g 400g

Angular physical properties or quantities measured with reference to an angle, especially those associated with rotation

Angular Acceleration

(α) is the rate of change of angular velocity with time, units radians per second squared, rads/s2

Angular Deviation Loss Definition

IEC 801-25-69, sensitivity level of the transducer on the principal axis minus the sensitivity level of the transducer for a specified direction

Angular Displacement (θ) is measured in radians rather than degrees. This is because it provides a very simple relationship between distance travelled around the circle and the distance r from the centre. θ = s/r = length of arc divided by the radius in radians

Angular Frequency (ω) is the frequency expressed in radians per second (rad/s). To convert a frequency in hertz to an angular frequency multiply by 2·π . For an oscillation with period T, the angular frequency ω = 2·π / T

Angular Momentum (L) is the quantity of rotation of a body, which is the product of the moment of Inertia and it's angular velocity, units newton metre seconds (N·m·s)

Angular Velocity (v) is the rate of change of angular position of a rotating body, units radians per second, rads/s

ANSI : American National Standards Institution

Anti-aliasing Filter

an analogue low pass filter used before analogue to digital conversion to filter out the frequencies greater than half the sampling frequency and prevent aliasing . See also • nyquist frequency.


Anti-resonance Definition

IEC 801-24-07, phenomenon of a system in forced oscillation such that any change in the frequency of excitation, however small, results in an increase in a response of the system
Note : the quantity that is the measure of response must be indicated; for example, velocity anti-resonance.

See also • resonance

Apodization Function

also called a Tapering or Windowing. It provides a smooth amplitude weighting of a signal to zero at the beginning and the end of the record to be sampled. This suppresses leakage which would otherwise be produced upon performing a discrete fourier transform.

Apparent Sound Insulation Index (R')
Apparent Sound Power Level
Weighted Apparent Sound Reduction Index (R'w)

Area (A) a quantity expressing the two-dimensional size of a defined part of a surface.

Surface Area, refers to the total area of the exposed surface of a 3-dimensional solid.

The derived SI unit is the square metre, symbol m2, 1 m2 = 1 m by 1 m or 2 m by 0.5 m, etc., 25 mm by 25 mm = 0.025 m by 0.025 m = 0.000625 m2

See also • cross section area

Articulation Index

Artificial Ear Definition IEC 801-28-05, device for the calibration of earphones, incorporating a calibrated microphone for the measurement of sound pressure and an acoustic coupler such that the overall acoustic impedance is similar to that of the normal human ear in a given frequency band. Also known a an ear simulator

Artificial Mastoid Definition IEC 801-28-08, device which simulates the mechanical impedance of the average human mastoid where a bone vibrator may be applied to permit calibration of the vibrator. Also know as a mastoid simulator

Artificial Mouth Definition IEC 801-28-06, device consisting of a loudspeaker unit mounted in a baffle or an enclosure so shaped as to have a radiation pattern similar to that of the average human mouth. Also known as a mouth simulator.

Artificial Reverberation is reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., Added to a signal to make it sound more lifelike.

Artificial Voice Definition IEC 801-28-07, complex sound, usually emitted by an artificial mouth whose spectrum corresponds to that of the average human voice, also known as a voice simulator

ARTSage related threshold shift
ASHRAE : American Society of Heating, Refrigeration and Air-Conditioning Engineers

Asymmetric, a waveform not identical on both sides of the mean or zero line, lacks symmetry.

Atmospheric Sound Absorption

Atmospheric Pressure (atm)

the force per unit area exerted on a surface by the weight of air above that surface.
Standard Atmospheric Pressure, at sea level, is equal to 101.325 kPa the preferred SI units or 8760 mmHg and 1013.25 millibars.

Attenuation Coefficient Definition

IEC 801-23-35, real part of the linear exponent of sound propagation.
Note the unit is the Neper per metre.

Audible Range

, the human ear can respond to minute sound pressure variations in the air if they are in the frequency range, roughly 20 Hz - 20 kHz and wavelength 17 m to 0.017 mm.

Audible Sound Definition

IEC 801-21-02,
a) acoustic oscillation of such character as to be capable of exciting a sensation of hearing
b) sensation of hearing excited by an acoustic oscillation or vibration

Audiogram a graph showing hearing loss as a function of frequency, measured with an audiometer.


an instrument for testing hearing. Standard equipment in ENT, Audiometry and Audiology Centres.

Audiometric Room

IEC 801-31-20, room insulated against outside noise and having some sound absorption, intended for testing of hearing.

Auditory Masking

occurs when the perception of one sound is affected by the presence of another sound

See also • critical bandsthreshold of hearing.


is the correlation between values of a signal at different times. A signal processing tool for finding repeating patterns, such as the presence of a periodic signal(s) 'buried in noise'. It is frequently used for analysing time domain functions. It is the cross-correlation of a signal with itself.

Auto Scale the axes of a graph used to display time signal, spectra, post-processed functions, etc., are automatically set by the software to fit the full display into the available viewing area.


for FFT measurements, the fourier transform of a time signal is complex as it has magnitude and phase. The autospectrum is the average of the squared magnitude. For 1/n-octave constant percentage bandwidth measurements, it is the mean square of the filter output.


in acoustics where dB levels are extensively used, average may not mean adding up the values and then dividing by the number of samples. See our discussion calculations using the decibel

See also • ensemble averagingexponential averaginglinear averagingpower spectrum averagingrms averagingspatial averagingspectrum averagingtime-average sound leveltime-average sound pressure leveltime domain averagingtime weightings

Average Sound Level
Average Sound Pressure - under effective sound pressure
Average Sound Pressure Level - in a room
Average Speech Power

aw under acceleration

Axial Mode, the room resonances associated with each pair of parallel walls in a rectangular room.

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