Appendix 4.2 : Definitions of Noise Units and Criteria

For convenience, the guidelines are given in terms of external noise levels. It should be noted, however, that these are based largely on standards within buildings, since this is where the main activities which are affected by noise occur.

Noises from different sources are measured in different ways and alternatives are often available, even for one type of noise. In each case, the noise scales chosen provide the best measure of how people are affected. All environmental noise is measured using the A-weighted sound level - dB(A). The following descriptors take the variability of the noise and the response of the community into account:

NEF Noise Exposure Forecast. A complex composite measure of exposure to aircraft noise taking into account the peak noise level, the duration of flyover, the tonal characteristics and the number of aircraft movements in both the daytime and night-time period.

Lmax The maximum noise level during a designated time interval or a noise event.

Leq(24 hour) Equivalent continuous sound level. A measure of energy level of a time-varying noise.

L10(1 hour) The noise level exceeded for 10% of the one-hour period. Generally used for road noise at peak traffic flow.

L90(1 hour) The noise level exceeded for 90% of the one-hour period. Generally used as a measure of the background noise level.

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   Last revision date : May 2014