**1/n-Octave** analysis made 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 Filter

A Network

A-weighting

See also sound absorption

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

**Accelerance,** complex ratio of acceleration to applied force.

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.

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 metre per second squared, m/s².

**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.

● Note 1 : unless otherwise specified, the reference acceleration is 1 μm/s

● 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 hand arm vibration • vibration at work regulations • vibration dose value • whole body vibration

**ACGIH : American Conference of Industrial Hygienists**

**Acoustic Calibrator,** an instrument providing a reference noise source used to calibrate and check the performance of sound level meters.

**Acoustic Emission,** the energy that is generated when materials are under stress or break.

**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)

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

**Acoustic Louvre** a louvre designed with sound-attenuating baffles for reduction of airborne sound.

● Note : acoustic mass has dimensions of mass divided by the square of area

See also acoustic impedance above.

See also audible sound • oscillation

● Note : the unit is the radian per metre

**Acoustic Power** also known as sound power is the sound energy measured over a stated period of time.

**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.

Acoustic Velocity Level under Particle Velocity Level

Active Sound Field

Active Sound Intensity

See also acoustic admittance

ADPCM : Adaptive Pulse Code Modulation

Aeq : Acceleration Equivalent Value Aeq4 : Aeq8 : Aeq16

Age Related Threshold Shift : ARTS

**Airborne Sound** sound that reaches the point of interest by propagation 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 mathematical 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** is the maximum RMS acceleration.

Other noise descriptors : ambient noise • background noise • broadband noise • narrowband noise • periodic • pink noise • pseudo random noise • random noise • specific noise • white noise • wideband noise

**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.

**Amin** is the minimum RMS acceleration

**Amp** is the maximum peak acceleration.

**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.

Angle of Incidence

Angle of Reflection

**Angles** 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** 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**:

● 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'

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.

Articulation Index and Intelligibility

Also know as an *Ear Simulator*

ARTS : Age Related Threshold Shift

**ASHRAE :** American Society of Heating, Refrigeration and Air-Conditioning Engineers.

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

**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, see also 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** 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.

See also critical bands • threshold of hearing.

**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.

See also ensemble averaging • exponential averaging • linear averaging • power spectrum averaging • rms averaging • spatial averaging • spectrum averaging • time domain averaging • time weightings

Average Sound Level

Average Sound Pressure Level

Average Speech Power

**aw** is the time-averaged frequency-weighted, single-axis vibration acceleration.

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