| Absorption | the process by which sound energy is converted into heat |
| Absorption co-efficient | the fraction of incident sound energy that will be absorbed by a given surface |
| Airborne sound | sound radiated directly from a noise source into the surrounding air |
| Ambient noise | the totally encompassing noise in a given situation at a given time |
| Audible range | 20Hz to 20kHz (approximately) |
| A-weighting | a frequency weighting network corresponding approximately to the sensitivity of the human ear |
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| Background noise | as defined in BS4142, it is the A-weighted sound pressure level that is exceeded for 90% of a given time (LA90) |
| Broadband noise | sound energy distributed over a wide range of frequencies |
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| Crosstalk | the transfer of sound from one area to another - often used in connection with ducted ventilation systems |
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| dB | decibel - the basic unit of sound - most commonly used to refer to sound pressure level, but can equally refer to sound power level, sound intensity level etc. |
| dB(A) | the A-weighted dB level |
| DnT | the standardised level difference |
| DnTw | the weighted standardised level difference - a unit of sound insulation used, for example, in Approved Document E of The Building Regulations |
| Damping | a process whereby vibration energy is converted into heat through some form of frictional mechanism |
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| Echo | a delayed reflection of sound that is perceived by the ear as a separate distinguishable sound. |
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| Flanking transmission | the indirect transfer of sound between adjacent areas other than directly through the separating structure |
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| Gain | the increase in power level of a signal produced by an amplifier |
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| Harmonic | a signal having a repetitive pattern. |
| Helmholtz resonator | a reactive, tuned, sound absorber |
| Hertz (Hz) | the unit of frequency expressing the number of cycles per second |
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| Impact noise | noise resulting from the impact of one object on another |
| Insertion loss | a term usually applied to silencers which is simply the reduction in noise level achieved by the insertion of the noise control device between the noise source a "receiver" position |
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| L10 (LA10) | the (A weighted) sound pressure level exceeded for 10% of the time, commonly used as a measure of road traffic noise |
| L90, (LA90) | the (A weighted) sound pressure level exceeded for 90% of the time, used by BS 4142 and other standards to quantify the background noise level |
| Leq (LAeq) | the (A weighted) equivalent continuous sound pressure level, used to quantify the average noise level over a given period |
| LA | the A weighted sound pressure level |
| L'nT | standardised impact sound pressure level |
| L'nT,w | the weighted standardised impact sound pressure level |
| Loudness | a subjective quantity - a difference of 10 dB is very approximately equivalent to a doubling / halving of loudness |
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| NC (noise critieria) curves | a set of defined curves used to provide a single figure rating to a broadband noise - an American standard |
| NR (noise rating) curves | a set of defined curves used to provide a single figure rating to a broadband noise - a British standard |
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| Octave | the range between two frequencies whose ratio is 2:1 |
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| PPG24 | the planning policy guidance document used by Local Authorities to assess the acoustic implications of a planning application, particularly in respect of residential development |
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| Rw | the weighted sound reduction index (excludes the effects of flanking transmission) |
| R'w | the apparent weighted sound reduction index (includes the effects of flanking transmission) |
| Reverberation Time (RT) | the time required for a steady sound pressure level in an enclosed space to decay by 60dB |
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| Sound Intensity Level (SIL) | the sound intensity of a noise source, expressed as a decibel value |
| Sound Pressure Level (SPL) | the sound pressure level of a noise source, expressed as a decibel value |
| Sound Power Level (SWL) | the sound power level of a source, expressed as a decibel value |
| Sound Reduction Index (R) | the airborne sound insulation of a material in a particular frequency range, expressed as a decibel value |
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Last Revised: 7/08/02