What is Day-Night Average Noise Level (DNL)?
DNL is the total accumulation of aircraft noise spread out uniformly throughout the day (i.e., over a 24-hour period). DNL is an annualized metric representing the noise of a typical day of the year. To compensate for the added annoyance created by nighttime aircraft activity, DNL adds a 10-decibel weighting (i.e., a “penalty”) to night operations (between 10 pm and 7 am). The weighting effectively incorporates 10 nighttime operations for each actual operation between 10 p.m. and 7 a.m. Federal regulations require the use of DNL, rather than other noise metrics, to determine if aircraft noise impacts are “significant”. The FAA uses a DNL of 65 decibels to determine if non-compatible activities exist in the vicinity of an airport.
