Ln, where n may be anything from 1 to 99, is the sound pressure level exceeded for n% of the measurement time.
L90 is frequently understood as background sound level. L10-L90 is often used to give a quantitative measure of the spread or range or "how choppy" the sound level is.
L10 is the sound 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 is used in some Community Noise assessments.
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