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.

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 65 dB is incompatible with residential communities. Communities in affected areas may eligible for mitigation such as soundproofing.