day-night average sound level: Twenty-four hour average sound level for a given day, after addition of 10 decibels to levels from 0000 to 0700 hours and from 2200 (10 p.m.) to 2400 hours. Unit, decibel (dB); abbreviation, DNL; symbol, Ldn.


Day-night average sound level in decibels is related to the corresponding day-night sound exposure level, LEdn, according to:

Ldn = LEdn − 10 log (86,400/1)

where 86,400 is the number of seconds in a 24-hour day.

A-frequency weighting is understood, unless another frequency weighting is specified explicitly.

instantaneous sound pressure: Total instantaneous pressure at a point in a medium minus the static pressure at that point. Unit, pascal (Pa); symbol, p.

loudness level: Of a sound, the median sound pressure level, in a specified number of trials, of a free progressive wave having a frequency of 1,000 Hz that is judged as loud as the unknown sound when presented to listeners with normal hearing who are facing the source. Unit, phon.


The manner of listening to the unknown sound must be specified.

maximum sound level; maximum frequency-weighted sound pressure level: Greatest fast (125 ms) A-weighted sound level within a stated time interval. Alternatively, slow (1,000 ms) time-weighting and C-frequency-weighting may be specified. Unit, decibel (dB); abbreviation, MXFA; symbol, LAFmx (or C and S).

night average sound level: Time-average sound level between 0000 and 0700 hours and 2200 and 2400 hours. Unit, decibel (dB); abbreviation, NL; symbol, Ln.


Night average sound level in decibels is related to the corresponding night sound exposure level, LEn, according to:

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