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Noise - Terms - Statistical Analysis

When assessing environmental noise it is generally useful to establish what is the general or "background" noise level in the area; this is best represented by the LA90 [also written dBA L90 or L90 dB(A)], which is the level exceeded for the 90% of the time under consideration. Such measurements are designated say 36.7 dBA LA90 or 36.7 LA90. Typical daytime background noise levels range from 18 LA90 in a remote rural areas, through 30 to 40 LA90 in "typical" or "quiet" suburban areas, to 50 to 60 LA90 for busy urban areas.

Note that some standards use LA95 as the descriptor of background noise.  For most noise climates there is unlikely to be a large (>1 dBA) difference between the LA90 and LA95.

It has also been found that the LA10 is the best method of representing traffic noise for the UK.

If we look again at our noise trace, the position of the LA90 (lower horizontal line) and LA10 (upper line) can be plotted.

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