Acoustic Glossary


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

1/n-Octave analysis made 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.

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

Accelerance complex ratio of acceleration to applied force.

Acceleration IEC Definition,

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.


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.
Amp is the Maximum Peak Acceleration.
aw is time-averaged, frequency-weighted, single-axis vibration acceleration.

See also : Angular AccelerationParticle Acceleration, used in acoustic wave theory.

The human response to vibration covers a wide range from a few μm/s2 to tens of metres per second squared. The dB scale is a useful way to represent the immense range using a manageable set of numbers, see Acceleration Level below.

Acceleration Level IEC Definition,

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.

Acceleration Level dB : La = 20 lg (a/ao) dB re 1 µm/s2 - also known as Vibration Acceleration Level
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.

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

See also 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

Accelerometer 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'.

Acoustic Admittance IEC Definition,

reciprocal of acoustic impedance

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

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

Acoustic Impedance IEC Definition,

at a specified surface, quotient of sound pressure by volume velocity through the surface


Acoustic Impedance is the opposition to the flow of sound through a medium.

Acoustic Impedance : Za at a surface is the complex ratio of the effective sound pressure averaged over the surface to the effective volume velocity through it.
Za = p/Q the SI Units are Pa·s/m5 (MKS units: acoustic ohms)

Acoustic Louvre a louvre designed with sound-attenuating baffles for reduction of airborne sound.

Acoustic Ohm is a unit of measurement of sound resistance, and is the ratio of the sound pressure to volume flow, m3/s = Pa s/m3.

These units got their name by analogy with electric resistance, which is measured in ohms.

See Acoustic Impedance above.

Acoustic Oscillation IEC Definition,

movement of particles in an elastic medium about an equilibrium position

Acoustic Phase Coefficient IEC Definition,

imaginary part of the linear exponent of sound propagation


Note : the unit is the radian per metre

Acoustic Power also known as sound power is the sound energy measured over a stated period of time.

Acoustic Radiation Pressure IEC Definition,

unidirectional steady pressure exerted on a surface by an acoustic wave.

Acoustic Reactance IEC Definition,

the imaginary part of acoustic impedance


Acoustic Resistance IEC Definition,

the real part of acoustic impedance

Acoustic Streaming IEC Definition,

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 Vibration IEC Definition,

movement of particles in an elastic medium about an equilibrium position

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.

Admittance IEC Definition,

reciprocal of impedance of stated kind

Airborne Sound sound that reaches the point of interest by propagation 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

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 is the maximum RMS acceleration.

Ambient Noise IEC Definition,

encompassing sound, at a given place, being usually a composite of sounds from many sources near and far.


See also: Background NoiseResidual NoiseSpecific Noise

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

Ampere : A 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 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.

Anechoic without echo, refers to the absence of sound reflections. It is not possible to create a truly anechoic environment, as the perfect sound absorber does not exist, an expensive Anechoic Chamber is as close as it gets. However measurements in a 'field' well away from any reflective surfaces and any other sound sources can approximate to semi-anechoic conditions.

Anechoic Chamber IEC Definition,

room with no appreciable reverberation, used for acoustic measurements


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 Room IEC Definition,

room whose boundaries absorb substantially all the sound incident thereon, thereby affording free-field conditions
Also known as an Free Field Room


Angle of Incidence
Angle of Reflection

Angles 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 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 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 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 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 analogue low pass filters 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 IEC Definition,

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.

Apodization Function also called a Tapering or Window Function. 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.

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

Artificial Ear 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 know as an Ear Simulator

Artificial Mastoid device which simulates the mechanical impedance of the average human mastoid where a bone vibrator may be applied to permit calibration of the vibrator.

Artificial Mouth 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.

Artificial Reverberation 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 a complex sound, usually emitted by an artificial mouth whose spectrum corresponds to that of the average human voice.

Atmospheric Absorption up to a few hundred metres atmospheric absorption can be ignored. At greater differences it can be significant particularly at frequencies above 1 kHz.

See also : Sound Absorption

Atmospheric Pressure : atm 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.

Attenuation Coefficient IEC Definition,

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 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 IEC Definition,
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 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.

Auditory Masking occurs when the perception of one sound is affected by the presence of another sound.
See also : Critical BandsThreshold of Hearing.

Autocorrelation 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 the 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 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 and the other Averaging entries below.

Averaging various methods of 'signal processing' are used in acoustics :
Ensemble Averaging
Exponential Averaging
Linear Averaging
RMS Averaging
Spatial Averaging
Spectrum Averaging
Time Domain Averaging
Time Weightings

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

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


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