Castle Group Ltd

Percentile levels are used greatly when measuring environmental noise, ie that which may pollute the environment.

Ln, where n may be anything from 1 to 99, is that noise level exceeded for n% of the measurement time. By definition of percentiles, L1 must be greater than or equal to L2, which must be greater than or equal to L3, etc. It is often the case that only a few Ln values are ever used.

An example of how Ln values look in a graphical format:

L90 is frequently taken as the Lp of the background level. L10-L90 is often used to give a quantitative measure as to the spread or "how choppy" the sound was.

L10 is the noise level exceeded for 10% of the time of the measurement duration. This is often used to give an indication of the upper limit of fluctuating noise, such as that from road traffic.

L50 is the noise level exceeded for 50% of the measurement duration. It is the middle point and has been incorporated in some American Community Noise Assessments.

L90 is taken to be the ambient or background noise level as used, for example, in BS 4142 'Rating Industrial Noise Affecting Mixed Residential and Industrial Areas'.

L10-L50 is often used in such noise indices as Traffic Noise Index: