A-weighting under frequency-weightings
Absolute Threshold of Hearing under threshold of hearing.
AC Coupling is the connection of a signal from one circuit to another in a manner that rejects DC components.
See also DC coupling.
Acceleration (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).
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.
Acceleration Level, 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 reference acceleration of 1 μm/s² ≡ 0 dB (defined in ISO 1683) *
Level of Acoustic Acceleration (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.
Acoustical Society of America (ASA), publish standards and information related to the knowledge and applications of acoustics, we include ASA Definitions where appropriate in this glossary
Acoustics concerns the cause, nature, production, measurement transmission and effects of sound and vibration.
Acoustic Calibrator, an instrument providing a reference noise source used to calibrate and check the performance of sound level meters.
Acoustic Coupler (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 Impedance is the resistance to the flow of sound through a medium, also known as the acoustic ohm.
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
Acoustic Louvres include sound-attenuating baffles for the reduction of airborne sound.
Acoustic Mass (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.
Acoustic Radiation Pressure (IEC 801-21-42) unidirectional steady pressure exerted on a surface by an acoustic wave.
Acoustic Reference Wind Speed (IEC 415-06-06) wind speed of 8 m/s at reference conditions (10 m height, roughness length equal to 0,05 m) used in the calculation of the apparent sound power level to provide a uniform basis for the comparison of apparent sound power levels from different wind turbine generator system.
Note : the acoustic reference speed is generally expressed in meters per second.
Acoustics the branch of physics concerned with the production, control, transmission, reception and effects of sound.
Acoustic Streaming (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 Vibration (IEC 801-21-01) movement of particles in an elastic medium about an equilibrium position and is also known as acoustic oscillation and sound definition.
See also audible sound • oscillation
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
Airborne Sound reaches the point of interest through the 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.
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, 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.
Ampere (A) 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 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 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.
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
Anechoic Chamber, 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.
Angular Deviation Loss (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.
Anti-aliasing Filter 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.
Antiresonance (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 antiresonance.
See also resonance
Apodization Function also called tapering or windowing. 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.
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 m²See also cross section area
Arithmetic Mean 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.
Artificial Ear (IEC 801-28-05) device for the calibration of earphones, incorporating a calibrated microphone for the measurement of sound pressure and an acoustic coupler such that the overall acoustic impedance is similar to that of the normal human ear in a given frequency band. Also known as an ear simulator
Artificial Mastoid (IEC 801-28-08) device which simulates the mechanical impedance of the average human mastoid where a bone vibrator may be applied to permit calibration of the vibrator. Also know as a mastoid simulator
Artificial Mouth (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.
Artificial Reverberation is reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., to make the sound more lifelike.
Atmospheric Pressure (atm) also known as barometric pressure, is 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.
See also static pressure
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.
Autocorrelation 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'.
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.
Autospectrum for FFT measurements, the fourier transform of a time signal is complex as it has magnitude and phase. The autospectrum is the average of the squared magnitude.
For 1/n-octave constant percentage bandwidth measurements, it is the mean square of the filter output.
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.
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