A : Sound and Vibration Terms, Definitions, Units, Measurements ...
% Noise Dose
under noise dose%
1/n octave band
analysis is performed on a fractional part of an octave
is the variable. Commonly used n
values are 1/1, 1/3, 1/12 and 1/24-octave.
A(8) Daily Vibration Exposure
acceleration equivalent level
, converts an analogue
signal to a digital
, also known as the A curve, A filter and A network
under frequency weightings
under threshold of hearing
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
is the rate of change of
and is a
. 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
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.
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).
, 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 acceleration reference level 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.
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
* 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
, 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
under sound absorption
Acoustic Calibrator, an instrument providing a reference noise source used to calibrate and check the performance of sound level meters.
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 Emission is the energy that is radiated when materials are under stress, crack or break.
under sound energy
, sound from all sound sources
as modified by the environment, BS ISO 12913-1
under sound insulation
under sound 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 and is also known as inertance
under sound power
under sound pressure
Acoustic Radiation Factor under radiation factor
Acoustic Radiation index under radiation index
Acoustic Radiation Pressure Definition IEC 801-21-42,
unidirectional steady pressure exerted on a surface by an acoustic wave.
Acoustic Reference Levels
Acoustic Reference Wind Speed
Acoustic Stiffness Definition IEC 801-25-44,
in a system in which friction and inertia are negligible, quotient of sound pressure
by the resulting in-phase volume displacement during sinusoidal motion.
Acoustic Streaming Definition IEC 801-23-43,
unidirectional current in a fluid due to the presence of acoustic waves.
Acoustic Trauma, damage to the hearing mechanism caused by a sudden burst of intense noise, explosion etc..
Acoustic Velocity Level
• under particle velocity level
under sound waves
Acoustical Society of America
, the ASA publish information and standards related to the knowledge and applications of acoustics
Active Noise Control (ANC)
can reduce low frequency noise levels significantly, when a sound of equal amplitude, but 180 degree out of phase, is added to the original sound, electronically - also known as active noise cancelling
and active noise reduction
See also phase cancellation
Active Sound Fields
Active Sound Intensity
ADPCM • adaptive pulse code modulation
Age Related Threshold Shift
under threshold shift
AI • articulation index
reaches the point of interest through air.
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
Aircraft Noise : UK Government Environmental Noise Regulations
a specific procedure for solving problems. An FFT - fast fourier transform
is an algorithm.
, 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.
See also •
anti-aliasing filter •
• under acceleration
, 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
• residual sound
• specific sound
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 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
, 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.
Angle of Refraction
, see sound wave refraction.
are usually expressed in degrees or radians, less often as Grads or Gradians (except on some electronic calculators)
| Degrees || 0° || 30° || 45° || 60°|| 90° || 180° || 270° || 360° |
| Radian || 0 || π/6 || π4 || π/3|| π/2 || π || 3 π/2 || 2π |
| Grads || 0g || 100g/3 || 50g || 200g/3 || 100g || 200g || 300g || 400g |
, 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
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
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.
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
Note : the quantity that is the measure of response must be indicated; for example, velocity anti-resonance.
See also • resonance
, also called tapering
. 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)
is a scalar 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.
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 • cross section area
, also known as the average
or average level
is obtained by summing all the measured levels then dividing by the number of levels measured. For example the average of 85, 90, 92 and 80 = 347/4 = 86.75. However if the measured levels were decibel levels then the average is very different. This is because the dB levels
need to be converted back to absolute values before finding the arithmetic mean
and then converting the answer back into decibels.
See also averaging sound levels • decibel calculations
Articulation Class (AC)
Articulation Index (AI)
Artificial Mouth Definition IEC 801-28-06,
device consisting of a loudspeaker unit mounted in a baffle or an enclosure so shaped as to have a radiation pattern similar to that of the average human mouth. Also known as a mouth simulator
generated by electrical or acoustical means to simulate that of concert halls, etc., to make the sound more lifelike.
ARTS • age related threshold shift
ASA • Acoustical Society of America
ASHRAE : American Society of Heating, Refrigeration and Air-Conditioning Engineers
, a waveform
not identical on both sides of the mean or zero line, lacks symmetry
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.
Audiogram, a 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.
is the correlation
between values of a signal at different times. A signal processing tool for finding repeating patterns, such as the presence of a periodic
signal(s) 'buried in noise'.
It is frequently used for analysing time domain functions. It is the cross-correlation of a signal with itself.
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.