**1/n octave** analysis is performed 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.

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

**AC Coupling** is the connection of a signal from one circuit to another in a manner that rejects DC components.

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 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.

**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 Level** *dB (La)* = 20 lg (a/ao) dB re 1 µm/s^{2}.

**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 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.

* 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.

See also • hand arm vibration • vibration at work regulations • vibration dose value • whole body vibration

**ACGIH : American Conference of Industrial Hygienists**

See also acoustic impedance and related topics

**Acoustic Impedance (Z)** is the ratio of the sound pressure to the volume velocity so Z = p/U

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 impedance • specific acoustic reactance • specific acoustic resistance • specific wall admittance • specific wall impedance • transfer impedance

**Acoustic Louvres** includes sound-attenuating baffles for reduction of airborne sound.

● Note

See also • acoustic impedance and related topics

See also • audible sound • oscillation

● Note the unit is the radian per metre

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.

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

**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.

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

**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** physical properties or quantities measured with reference to an angle, especially those associated with rotation

**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

**ANSI : American National Standards Institution**

● Note : the quantity that is the measure of response must be indicated; for example, velocity anti-resonance.

See also • resonance

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

Articulation Index

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

Atmospheric Sound Absorption

● Note the unit is the Neper per metre.

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.

See also • critical bands • threshold of hearing.

**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.

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

Average Sound Level

Average Sound Pressure - under effective sound pressure

Average Sound Pressure Level - in a room

Average Speech Power

*aw* under acceleration

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

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