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/s^{2} or if using Imperial units then 'g' = 9.80665 m/s^{2} = 386.089 in/s^{2}

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/s^{2} = 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/s^{2} it follows that m/s^{2} 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.

The human response to vibration covers a wide range from a few
μm/s^{2} 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.

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/s^{2} *
● 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/s^{2} - also known as Vibration Acceleration Level Acceleration Reference Level : ao = 1 μm/s^{2} ≡ 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/s^{2} is also in use'

Acceleration Equivalent Level : Aeq a single number to represent the equivalent acceleration energy as it varies over a working day, measured in m/s^{2}.
For example Aeq8 = 2.5 m/s^{2} indicates a equivalent level of 2.5 m/s^{2} 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.

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 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 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/m^{5} (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, m^{3}/s = Pa s/m^{3}.

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

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.

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.

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 underMeasurement 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.

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.

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.

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

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 m^{2}, 1 m^{2} = 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 m^{2}

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 Reverberationreverberation 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.

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 SoundIEC 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 Bands •
Threshold 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.