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

See also • auditory masking • masked threshold • threshold shift

See also • DC coupling.

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.

**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 acceleration • particle 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 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/s ^{2} 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 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.

**(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 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/s^{2} is also in use'

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 • exposure action and limit values • hand arm vibration • vibration at work regulations • vibration dose value • whole body vibration

**ACGIH : American Conference of Industrial Hygienists**

**Acoustic Admittance Definition IEC 801-25-46,** reciprocal of acoustic impedance

**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 Impedance ( Z_{a})** is the complex ratio of the sound pressure to the sound volume velocity.

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

See also • acoustic admittance • acoustic ohm • acoustic reactance • acoustic resistance • characteristic acoustic impedance • characteristic impedance of a medium • complex acoustic impedance • conjugate impedances • driving point impedance • impedance • specific acoustic admittance • specific acoustic impedance • specific acoustic reactance • specific acoustic resistance • specific wall admittance • specific wall impedance • transfer impedance • transmission impedance and radiation

**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

See also • acoustic impedance and related topics

*Acoustic Oscillation* is also known as Acoustic Vibration and Sound.

See also • audible sound • oscillation

**Acoustic Phase Coefficient Definition** IEC 801-23-37, imaginary part of the linear exponent of sound propagation

See also acoustic impedance and related topics

See also • audible sound • oscillation

**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

See also • acoustic admittance

*Aeq* ▫ acceleration equivalent value : Aeq4, Aeq8, Aeq16

Age Related Threshold Shift

**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

Indicators • LAeq,16h • Lday • Levening.

**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.

*Amax* under acceleration

Other noise descriptors • background noise • broadband noise • gaussian noise • narrowband noise • periodic • pink noise • pseudo random noise • random noise • residual noise • specific noise • white noise • wideband noise

*Amin* under acceleration

*Amp* under acceleration

1 ampere is equivalent to 1 Coulomb per second.

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

See also • reverberation

**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.

**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° |

Radians | 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 Acceleration** *(α)* is the rate of change of angular velocity with time, units radians per second squared, rads/s^{2}

**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

**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

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.

See also • cross section area

See also averaging sound levels • decibel calculations

Articulation Index (AI)

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

Atmospheric Sound Absorption

**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.

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.

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

See also • critical bands • threshold of hearing.

Average Sound Level Meters - see integrating sound level meters

Average Sound Pressure - under effective sound pressure

Average Sound Pressure Level - in a room

Average Speech Power

See also • arithmetic mean • effective level • ensemble averaging • exponential averaging • linear averaging • mean square • power spectrum averaging • rms averaging • spatial averaging • spectrum averaging • time-average sound level • time-average sound pressure level • time domain averaging • time weightings

Average Speech Power

Axial Modes

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