1/1 octave bands

1/3 octave bands

*Absorption* • sound absorption

*Absorptivity* • 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**,
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** 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.

● 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 Level (La)** = 20 lg (a/ao) dB re 1 µm/s², where

a is the acceleration level in m/s², and

ao is the **acceleration reference level of 1 μm/s² ≡ 0 dB** (defined in ISO 1683) *

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.

● 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

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.

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

See also • acoustic impedance and related topics

**Acoustic Impedance (Za)** is the ratio of the sound pressure (p) to the sound volume velocity (U) = p/U acoustic ohms, the SI unit is the Pa·s/m^{3} (pascal-second per metre-cubed).

**Acoustic Impedance** is a complex subject due to the wide range of sound and vibration situations found in the infrasound, hearing range and ultrasound fields not to mention structure borne noise and underwater acoustics. Further complications are introduced by phase differences when dealing with both vectorial and scalar quantities. See the list of related links below, developed to deal with **specific situations**.

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

● Note :

See also • acoustic impedance and related topics

See also • audible sound • oscillation

● Note : the unit is the radian per metre

*Acoustic Radiation Factor* under radiation factor

*Acoustic Radiation index* under radiation index

Acoustic Reference Levels

Acoustic Reference Wind Speed

See also • audible sound • oscillation

See also phase cancellation

Active Sound Fields

Active Sound Intensity

See also • acoustic admittance

Indicators • LAeq,16h • Lday • Levening.

Air Density

See also • anti-aliasing filter • nyquist frequency

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

**Ambient Sound Level, La** = LAeq,T, the equivalent continuous A-weighted sound pressure level at a given time T.

1 ampere is equivalent to 1 Coulomb per second.

**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.

See also • reverberation

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} |

See also • nyquist frequency.

Antinode

● 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)

**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 averaging sound levels • decibel calculations

Articulation Class (AC)

Articulation Index (AI)

ASA • Acoustical Society of America

Atmospheric Sound Absorption

**Atmospheric Pressure (atm)** 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.

● 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.

*Audible Threshold* • under threshold of hearing.

See also • auditory masking • masked threshold • threshold shift

See also • critical bands • threshold of hearing.

It is frequently used for analysing time domain functions. It is the cross-correlation of a signal with itself.

For 1/n-octave constant percentage bandwidth measurements, it is the mean square of the filter output.

For example shooting ranges, quarry basting, bell ringers practicing once or more times a week. Traffic noise alongside motorways compared with intermittent local traffic and a multitude of industrial noises.

Over the years instrumentation engineers have developed various ways to measure and represent the *average* values of level vs time histories. Further down this page we list some related topics. Another well established method of describing fluctuating noise levels is the L10, L90, Ln etc., statistical noise level descriptors.

*Average Sound Level Meters* - see integrating sound level meters

*Average Sound Pressure* - under effective sound pressure

*Average Sound Pressure Level in a room*, under sound insulation

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

Axial Modes