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Glossary

Our definitions for the terms below come from our experience, acoustic text books, and BS, EN & ISO Standards.  Some acoustic terms have different meanings in different contexts or Standards – so be aware.  If you can’t find the term you are looking for please do ask us – we will add it for your and others’ benefit.

Absorption coefficient
The proportion of sound lost when incident at a surface.

Ambient noise
The total sound at a given place, usually a composite of sounds from many sources near and far. Should not be confused with “background noise”.

Attenuation
Noise reduction.

Attenuator
Noise-reducing device.

A-weighting
The sensitivity of the human ear is frequency dependent. The weighting of a signal with the A-weighting network approximates the frequency response of the human ear at moderate noise levels. Although other networks, such as C-weighting, better approximate the response of the human ear at higher noise levels, in practice the A-weighting network is still used for convenience.

Background noise
Typically the noise due to sources other than that under specific consideration. Usually quantified in standards using L90.statistical data.

Breakout
The escape of sound from any source-enclosing structure such as ductwork, metal casings and building envelopes.

Crosstalk
The transfer of airborne noise from one area to another via secondary air paths such as ventilation ductwork or ceiling voids.

Continuous equivalent A-weighted sound pressure level, LAeq,T
The value of the A-weighted sound pressure level in decibels (dB) of a continuous, steady sound that within a specified time interval, T, has the same mean squared sound pressure as the sound under consideration that varies with time.

Decibel, dB
A description of level, ten times the logarithm of the ratio between the value of a quantity and a reference value. For sound pressure, Lp, the reference quantity is 2 x 10-6 N/m2. The threshold of hearing is approximately 0 dB, and the threshold of pain around 120 dB. Hence the human perception of sound has a dynamic range of twelve orders of magnitude, equivalent to measuring the breadth of a hair and the diameter of the earth with the same apparatus. The logarithmic scale enables this wide range of scales to be described conveniently, with a similar relative magnitude to the experience of loudness.
Although sound pressure has units of N/m2, the decibel has no dimensional units.

Excitation frequency
A frequency at which a machine produces vibration. Often the speed of rotation of the machine.

Flanking transmission
Transmission of sound from a source room to an adjacent receiving room but not via the common partition.

Flutter echo
Rapid but nearly even succession of echoes originating from the same sound source. Often occurs in empty rooms. An echo is defined as a sound wave that has been reflected and arrives with such a magnitude and time interval after the direct sound as to be distinguishable as a repetition of it.

Free sound field
Sound field in a homogeneous isotropic medium where boundaries exert a negligible effect on the sound waves.

Frequency (Hz)
The number of sound waves to pass a point in one second.

Hertz (Hz)
The unit of frequency equivalent to one cycle per second.

Insertion loss
The reduction of noise level by the introduction of a noise control device; established by the substitution method of test.

Insulation (sound)
The property of a material or partition of opposing sound transfer through its thickness.

Inverse square law
The reduction of noise with distance. In terms of decibels, it means a decrease of 6 dB for each doubling of distance from a point source when no reflective surfaces are present.

Isolation (vibration)
The reduction of vibrational force into a structure.

Isolation efficiency
The amount of vibration force absorbed by an isolator and thus prevented from entering the supporting structure, expressed as a percentage of the total force applied to the isolator.

Leq
See “Continuous equivalent sound pressure level”

L’nT,w
See “Weighted standardised impact sound pressure level”

L90
See “Background noise”

Masking noise or sound conditioning
Extra noise introduced into an area to reduce the variability of fluctuating noise levels and increase speech privacy.

Mass law
Heavy materials stop more noise passing through them than light materials. For any airtight material there will be an increase in its “noise-stopping” ability of approximately 6 dB for every doubling of mass per unit area.

Natural frequency
Frequency of free oscillation of a system. For a multipledegree-of-freedom system, the natural frequencies are the frequencies of the normal mode of oscillation.

Near sound field
Sound field near a sound source where instantaneous sound pressure and particle velocity are substantially out of phase. The inverse square law does not apply in the near sound field.

Noise
1. Erratic or statistically random oscillation.
2. Disagreeable or undesired sound or other disturbance.

Noise criterion (NC) curves
A US set of curves based on the sensitivity of the human ear. They give a single figure for broad-band noise. Used for indoor design criteria. They are similar to NR curves but have different frequency characteristics.

Noise rating (NR) curves
A set of curves based on the sensitivity of the human ear. They are used to give a single-figure rating for a broad band of frequencies. Used for interior design criteria. They are similar to NC curves but have different frequency characteristics.

Octave
Unit of logarithmic frequency interval: two sounds, the ratio of whose fundamental frequencies is 2, have a logarithmic frequency interval of 1 octave.

Octave bands
A convenient division of the frequency scale. Identified by their centre frequency, typically 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz.

Periodic sounds
A signal containing a finite number of pure tones which repeats itself at regular intervals.

Pure tone
A note of single frequency only.

Radius of distraction, rD
The radius of distraction may be defined as the distance at which the STI falls below 0.5 in an open plan office. This is based on work by Hongisto, Finnish Occupational Health Institute, that correlates improving work performance when the STI falls below an STI of 0.5. There is also a radius of privacy, where the STI falls below 0.2, but this dimension is typically larger than the available space in open plan offices.

Reactive attenuator
An attenuator in which the noise reduction is brought about typically by changes in cross-section, chambers and baffle volumes, for example a car exhaust silencer.

Regeneration
The noise generated by airflow turbulence. The noise level usually increases with flow speed.

Resonance
State of a system in forced oscillation such that any changes, however small, in the frequency of excitation result in a decrease in a response of the system. The peak of the response curve with respect to frequency.

Resonant frequency (Hz)
Frequency at which resonance exists.

Reverberation
The sound that persists in an enclosed space, as a result of repeated reflection or scattering, after the source of the sound has stopped.

Reverberation time
Of an enclosure, for a sound of a given frequency or frequency band. The time that would be required for the sound pressure level in the enclosure to decrease by 60 dB, after the source has been stopped.

Room constant
The sound-absorbing capacity of a room, usually expressed in m2.

R’w
See “Weighted apparent sound reduction index”

Sabine formula
Predicts the reverberation time of a room or enclosure from known room volume and absorption characteristics. Becomes inaccurate when absorption is high, or when absorption is unevenly distributed.

Sound insulation
The property of a material or partition to oppose sound transfer through its thickness.

Sound level meter
An instrument for the measurement of sound level, with a standard frequency weighting and standard exponentially-weighted time-averaging.

Sound Pressure Level, Lp
The sound pressure level is a measure of the acoustic pressure fluctuations caused by the passing of sound energy. Energy is transmitted by the vibration of the medium. In fluids such as air, these pressure fluctuations may be measured with a microphone. The RMS value of the pressure is usually converted to a decibel level in order to conveniently express the magnitude. Although sound pressure has units of N/m2, the sound pressure level expressed in decibels has no dimensional units.

Sound Power Level, Lw
The sound power level is a measure of the acoustic power of a source. Acoustical energy is transported away from the source by the vibration of a medium. Hence the sound power emitted by a source may change if the physical coupling of the source is changed. Sound power has units of Watts, although sound power level expressed in decibels has no dimensional units. The reference power level for sound is 10-12 Watts.

Sound Intensity Level, LI
The sound intensity level is a measure of the acoustic power per normal unit area. Acoustical energy is transported away from the source by the vibration of a medium. Hence the sound intensity due to a single source radiating freely decreases with distance from the source. Sound intensity has units of W/m2, although sound intensity level expressed in decibels has no dimensional units. The reference intensity level for sound in air is 10-12 W/m2.

Sound reduction index (SRI)
Of a partition, for a specified frequency band. Difference in decibels between the average sound pressure levels in the reverberant source and receiving rooms, plus ten times the logarithm to the base 10 of the ratio of the area of the common partition to the total sound absorption in the receiving room.

Sound spectrum
Representation of the magnitudes (and sometimes of the phases) of the components of a complex sound as a function of frequency.

Spatial decay rate of speech, DL2
The decay in decibels of a speech sound source for a doubling of the distance from the source, at distances greater than 4 metres in an open plan office. Described as the spatial decay rate of speech, or the spatial decay of the A-weighted speech level per doubling of distance.

Speech transmission index (STI)
A parameter used for the quantification of audio systems to describe the intelligibility of speech. A high value STI indicates a high degree of speech intelligibility. A measure of speech transmission that correlates well with the intelligibility, and also the distracting effect of unwanted speech. Taking values between zero and one, with values above 0.6 rated as “good”, and values below 0.2 indicating a reasonable level of acoustic privacy.

Standing wave
Periodic wave having a fixed distribution in space that is the result of interference of progressive waves of the same frequency and kind. Such waves are characterised by the existence of nodes or partial nodes and antinodes that are fixed in space.

Static deflection
The distance that vibration isolators compress when loaded.

STI
See “Speech transmission index”

Third-octave bands
A small division of the frequency scale, three to each octave. Enables more accurate noise analysis.

Transmissibility
The amount of vibratory force that is transferred to the structure through an isolator, expressed as a percentage of the total force applied.

Vibration dose value (VDV)
A parameter used to reflect the disturbance and/ or annoyance caused by intermittent vibration.

Vibration isolation
Any of several means of reducing the transfer of vibrational force from the mounted equipment to the supporting structure, or vice versa.

Wavelength
The length between two points on a wave that are in phase.

Weighted apparent sound reduction index (R’w)
A single-number index which characterises the frequency dependent airborne sound insulation performance of building elements.

Weighted standardised impact sound pressure level, (L’nT,w)
A single-number index which characterises the frequency-dependent impact sound insulation performance of building elements.

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