The day-night average sound level (Ldn or DNL) is the average noise level over a 24-hour period. The noise level measurements between the hours of 22:00 and 07:00 are artificially increased by 10 dB before averaging. This noise is weighted to take into account the decrease in community background noise of 10 dB during this period. There is a similar metric called day-evening-night average sound level (Lden or DENL) commonly used in other countries, or community noise exposure level (CNEL) used in California legislation; that is, the DNL with the addition of an evening period from 19:00 to 22:00 when noise level measurements are boosted 5 dB (or 4.77 dB in the case of CNEL) to account for the approximate decrease in background community noise during this period.
In the US, the Federal Aviation Administration has established this measure as a community noise exposure metric to aid airport noise analyses under Federal Aviation Regulation Part 150.[1] The FAA says that a maximum day-night average sound level of higher than 65 dB is incompatible with residential communities.[2] Communities in affected areas may be eligible for mitigation such as soundproofing.