A : Sound and Vibration Terms, Definitions, Units, Measurements ...
% Noise Dose
, 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 see sound absorption
Absorptivity under 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.
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 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
Acceleration Equivalent Level (Aeq)
, a single number to represent the equivalent acceleration energy
as it varies over a working day, measured in m/s². For example Aeq8 = 2.5 m/s² indicates a equivalent level of 2.5 m/s² measured over an 8 hour shift.
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.
exposure action and limit values •
hand arm vibration •
vibration at work regulations •
vibration dose value •
whole body vibration
, 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
See also acoustic impedance and related topics
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.
is the resistance to the flow of sound through a medium
and is also known as the acoustic ohm
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/m3 (pascal-second per metre-cubed).
Acoustic Impedance Definition IEC 801-25-40,
at a specified surface, quotient of sound pressure
by volume velocity
through the surface.
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.
For example the specific acoustic impedance (z)
at a point in a sound field
is the ratio of the sound pressure
to the particle velocity
i.e. z = p ÷ v and the SI unit is the Pa·s/m
(pascal-second per metre).
acoustic admittance •
acoustic ohm •
acoustic reactance •
acoustic resistance •
characteristic acoustic impedance •
characteristic impedance of a medium •
complex acoustic impedance •
conjugate impedances •
driving point 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 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
See also acoustic impedance and related topics
See also audible sound • oscillation
Acoustic Radiation Factor
Acoustic 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
See also acoustic impedance and related topics
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
See also audible sound • oscillation
Acoustical Society of America, the ASA publish standards and information related to the knowledge and applications of acoustics
Acoustics Definition, concerning the cause, nature, production, measurement transmission and effects of sound and vibration.
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
See also acoustic admittance
Age Related Threshold Shift
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
industrial and port noise •
railway noise •
road traffic noise
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.
anti-aliasing filter •
Ambient Noise Definition IEC 801-21-12,
encompassing sound, at a given place, being usually a composite of sounds from many sources near and far. See also ambient sound
cited in the British Standard 4142
Other noise descriptors,
background noise •
broadband noise •
gaussian noise •
narrowband noise •
pink noise •
pseudo random noise •
random noise •
residual sound •
specific sound •
white noise •
, 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 |
Angular, physical properties or quantities measured with reference to an angle, especially those associated with rotation
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
Apparent Sound Power Level
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
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.
, 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.
, the human ear can respond to minute sound pressure
variations in the air if they are in the frequency
range, roughly 20 Hz - 20 kHz and wavelength
17 m to 0.017 mm.
Audible Sound Definition IEC 801-21-02,
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.
auditory masking •
masked threshold •
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.
Auditory Masking occurs when the perception of one sound is affected by the presence of another sound
critical bands •
threshold of hearing.
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.
Average and Averaging
, some sound levels only last a second or two and may or may not be repeated on a regular basis and if they are, the levels and intermittency may be different.
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
Average Speech Power
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 •
Glossary Search •
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