Acoustic Glossary


A : Sound and Vibration Terms and Definitions ...

% Noise Dose

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

A(8) Daily Vibration Exposurehand-arm vibrationwhole-body vibrationacceleration equivalent level

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

A-weighting under frequency-weightings

Absolute Threshold of Hearing under threshold of hearing.

Absorption under sound absorption and related terms
Absorptivity under sound absorption factor

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 (metre per second-squared) or if using Imperial units then 'g' = 9.80665 m/s² = 386.089 in/s²

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 the m/s² (metre per second-squared).

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.

Amax, the maximum rms acceleration.

Amin, the minimum rms acceleration.

Amp, the maximum peak, acceleration.

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

Reference Acceleration = 1 μm/s² ≡ 0 dB

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

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

Acceleration Level (La) = 20 lg (a/ao) dB re 1 µm/s², where a is the acceleration level in m/s² and ao is the reference acceleration of 1 μm/s² ≡ 0 dB (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 per second-squared 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 formula Aeq8 = a √T/8 where T = hours. Aeq4 and Aeq16 are also used in some vibration exposure applications.

See also exposure action valueexposure limit valuehand arm vibrationvibration at work regulationsvibration dose valuewhole body vibration

Accelerometer, a vibration sensor/transducer 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

Acoustics concerns the cause, nature, production, measurement transmission and effects of sound and vibration.

Acoustic Absorption

Acoustic Admittance Definition (IEC 801-25-46) reciprocal of acoustic impedance.

See also acoustic impedance and related terms

Acoustical Society of America (ASA), publish definitions and information related to the knowledge and applications of acoustics. We include ASA Definitions where appropriate in this glossary

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

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 Damping
Acoustic Dissipation

Acoustic Emission, the energy that is radiated when materials are under stress, crack or break.

Acoustic Energy

Acoustic Environment, sound from all sound sources as modified by the environment, BS ISO 12913-1 refers.

Acoustic Impedance is the resistance to the flow of sound through a medium, also known as the acoustic ohm.

Acoustic Impedance can change dramatically when sound waves 'meet' and transverse media of different densities, with a corresponding change in the sound wave particle velocity.

Acoustic Impedance Definition (IEC 801-25-40) at a specified surface, quotient of sound pressure by volume velocity through the surface, the SI unit is the pascal second per metre cubed (Pa·s/m3)

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

Acoustic Insulation
Acoustic Intensity

Acoustic Louvres include sound-attenuating baffles for the 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.
Note : acoustic mass has dimensions of mass divided by the square of area.

Acoustic mass is also known as inertance

Acoustic Ohm, the unit of acoustic impedance, the ratio of the sound pressure to acoustic volume flow, m3/s. The acoustic ohm units are pascal second per cubic-cubed (Pa·s/m3)
See also acoustic impedance and related terms

Acoustic Oscillation Definition (IEC 801-21-01) movement of particles in an elastic medium about an equilibrium position. Acoustic Oscillation is also known as sound and acoustic vibration.

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.

See also sound propagation

Acoustic Particle Velocity
Acoustic Power
Acoustic Pressure

Acoustic Radiation Factor
Acoustic Radiation Index

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

See also acoustic impedance and related terms

Acoustic Reference Wind Speed Definition (IEC 415-06-06) wind speed of 8 m/s at reference conditions (10 m height, roughness length equal to 0.05 m) used in the calculation of the apparent sound power level to provide a uniform basis for the comparison of apparent sound power levels from different wind turbine generator system.
Note : the acoustic reference speed is generally expressed in meters per second.

Acoustic Reference Levels

Acoustic Resistance Definition (IEC 801-25-41) the real part of acoustic impedance.

See also acoustic impedance and related terms

Acoustics the branch of physics concerned with the production, control, transmission, reception and effects of sound.

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.

See also acoustic compliance

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, explosion etc..

Acoustic Velocity Level

Acoustic Vibration Definition (IEC 801-21-01) movement of particles in an elastic medium about an equilibrium position. Acoustic Vibration is also known as sound and acoustic oscillation.

Acoustic Waves

Active Noise Control (ANC) can reduce low frequency noise levels significantly, when a sound of equal amplitude, but 180 degree out of phase, is added to the original sound, electronically - also known as active noise cancelling and active noise reduction.

See also phase cancellation

Active Sound Fields
Active Sound Intensity

Admittance Definition (IEC 801-25-15) reciprocal of impedance of stated kind.

See also acoustic admittance

ADPCM - adaptive pulse code modulation

Aeq - acceleration equivalent value - Aeq4, Aeq8, Aeq16

Age Related Threshold Shift

AI - Articulation Index

Airborne Sound reaches the point of interest through the air.

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

See also sound insulation and related terms

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

Aircraft Noise (UK Government Environmental Noise Regulations)
Indicators • LAeqLdayLevening.

See also effective perceived noise leveljudged perceived noise levelmaximum perceived noise levelnoise and number indexnoise exposure forecastperceived noise levelT10tone-corrected perceived noise level

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 soundspecific soundwhite 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.

See also static pressure

Ambient Sound, sound in a given situation at a given time, usually composed of sound from many sources near and far

Ambient Sound Level, La = LAeq,T, the equivalent continuous A-weighted sound pressure level at a given time T.

See also ambient noise definitionbackground noiseresidual soundspecific sound.

American National Standards Institution (ANSI) - set the USA Acoustic Standards

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.

Amplitude 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 and is also known as probability amplitude.

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

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

Anechoic (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, over a range of specified frequencies.
See also reverberation

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 and also known as a free field room.

Angle of Incidence Definition (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 sound wave

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

Angle of Refraction under 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°
Radian 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 radian per second-squared, rad/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 radian 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 radian

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

ANSI - American National Standards Institution - set USA acoustic standards

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.


Antiresonance 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 antiresonance.

See also resonance

Apodization Function also called 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
Apparent Sound Power Level

Area (A) is a scalar 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

Arithmetic Mean also known as the average or average level is obtained by summing all the measured levels then dividing by the number of levels measured. For example the average of 85, 90, 92 and 80 = 347/4 = 86.75. However if the measured levels were decibel levels then the average is very different. This is because the dB levels need to be converted back to absolute values before finding the arithmetic mean and then converting the answer back into decibels.

See also averaging sound levelsdecibel calculations

Articulation Class
Articulation Index

Artificial Ear, enables electroacoustic measurements on insert earphones and headphones

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 as 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., to make the 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.

ARTS - age related threshold shift

ASA : Acoustical Society of America, publish definitions and information related to the knowledge and applications of acoustics. We include ASA Definitions where appropriate in this glossary

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

Atmospheric Sound Absorption

Atmospheric Pressure (atm) also known as barometric pressure, is 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.

See also static pressure

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.

See also audible thresholdauditory maskingmasked thresholdthreshold of hearingthreshold shift

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

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

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

Audio Power is the electrical power delivered to a loudspeaker and not the sound power output from the speaker. For example 1 watt of sound power = 120 dB, i.e. very loud.

Auditory Masking occurs when the perception of one sound is affected by the presence of another sound
See also critical bandsthreshold of hearing.

Autocorrelation 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'.
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.

Autospectrum 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.

Average and Averaging some sound levels only last a second or two and may or may not be repeated on a regular basis and if they are, the levels and intermittency may be different.
For example shooting ranges, quarry basting, bell ringers practicing once or more times a week. Traffic noise alongside motorways compared with intermittent local traffic and a multitude of industrial noises.

Over the years instrumentation engineers have developed various ways to measure and represent the average values of level vs time histories. Further down this page we list some related terms. Another well established method of describing fluctuating noise levels is the L10, L90, Ln etc., statistical noise level descriptors.

Average Sound Level Meters see integrating sound level meters
Average Sound Pressure under effective sound pressure
Average Sound Pressure Level in a room under sound insulation
Average Speech Power

See also arithmetic meaneffective levelensemble averagingexponential averaginglinear averagingmean squarepower spectrum averagingrms averagingspatial averagingspectrum averagingtime-average sound leveltime-average sound pressure leveltime domain averagingtime-weightings

aw - under acceleration

Axial Modes

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