What is DNL in Terms of Aircraft Noise?
The Federal Aviation Administration (FAA) established DNL as the primary metric for aircraft noise analysis and expressing aircraft noise exposure in the United States. "DNL" is the acronym for Day-Night Average Sound Level, which represents the total accumulation of all sound energy, but spread out uniformly over a 24-hour period.
DNL has been widely accepted as the best available method to describe aircraft noise exposure and is the noise descriptor required by the FAA for use in aircraft noise exposure analyses and noise compatibility planning. It also has been identified by the U.S. Environmental Protection Agency as the principal metric for airport noise analyses.
The calculation for DNL considers the time of day an aircraft operated and applies a 10-decibel penalty on aircraft arriving or departing between 10 p.m. and 7 a.m.; the output is a numeric value in decibels that represents a 24-hour average noise exposure value. The current federally-established threshold of significance is 65 dB DNL.
While DNL also may be used for non-aviation purposes, the FAA's use of DNL is specific to aircraft noise. The Metropolitan Airports Commission publishes Aircraft Day-Night Level values as "ADNL" to avoid confusion with other uses of "DNL."