A : Sound and Vibration Terms and Definitions

A : Sound and Vibration Terms and Definitions ...

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 Exposure • hand-arm vibration • whole-body vibration • acceleration 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 acceleration • particle 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.

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.

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)

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 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 definition • background noise • residual sound • specific 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	0°	30°	45°	60°	90°	180°	270°	360°
Radian	0	π/6	π4	π/3	π/2	π	3 π/2	2π
Grads	0^g	100^g/3	50^g	200^g/3	100^g	200^g	300^g	400^g

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.

Antinodes

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.

Acoustic Glossary

A : Sound and Vibration Terms and Definitions ...

Ambient Noise

Artificial Ear