Day-night average sound level
The day-night average sound level (Ldn or DNL) is the average noise level over a 24-hour period. The noise between the hours of 10pm and 7am is artificially increased by 10 dB. 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 (DENL) commonly used in Europe or Community Noise Exposure Level (CNEL) used in California legislation, that is the DNL with the addition of an evening period from 7 PM to 10 PM when noises are boosted 5 dB to account for the approximate decrease in background community noise by 5 dB 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 The FAA says that a maximum day-night average sound level of higher than 65 dB is incompatible with residential communities. Communities in affected areas may be eligible for mitigation such as soundproofing, and airports
- "Noise Monitoring". Massport. Retrieved 31 January 2014.