Definition
A measure of community noise exposure that averages all sound energy over a 24-hour period, with a 10-decibel penalty added to noise occurring between 10:00 p.m. and 7:00 a.m. to reflect the greater impact of nighttime noise. Expressed in decibels (dB), it is used by the FAA and EPA to assess aircraft noise around airports for land-use planning, environmental review, and noise mitigation programs.
Plain English
An average noise level over a full day, with nighttime noise counted more heavily because it bothers people more when they are trying to sleep.
Context Anchor
Seen in airport noise studies, airport planning documents, and discussions of noise-abatement procedures around airports.
Derivation
The name describes its method: it averages noise across day and night, with night weighted more heavily. 'Equivalent' means a single steady level that represents the same total sound energy as all the varying noise events combined.
Why Pilots Care
Pilots and airport operators use DNL values to determine when noise mitigation measures or operating restrictions may be required.
Grounding Statement
If an airport had loud jet departures all day and a few at 2:00 a.m., the late-night ones count for ten times more in the DNL calculation than the same flights in daytime.
Intuition Check
DNL is not the loudness of one aircraft at one moment. It is a full-day noise measure, with nighttime sounds counted more heavily.
Example Sentence 1
The airport's 65 dB DNL contour was used to determine which neighborhoods qualified for sound insulation funding.
Example Sentence 2
When updating the noise exposure map, the consultant recalculated the DNL contours using the latest flight track data.