The 24-hour energy average of the A-weighted sound pressure level, with the levels during the period 10:00 p.m. to 7:00 a.m. the following day increased by 10 dBA before averaging.

DNL is a method of averaging total sound energy over a 24-hour period. In the DNL calculation, all flights between 10 p.m. and 7 a.m. have a 10 dB (decibel) penalty added to them before they are averaged in with the daytime flights. This is because nighttime flights are more intrusive than similar daytime events. Since DNL is an average, it does not reflect how loud an individual flyover is at any given house. Homes that are currently located in the 65 - 69 DNL contour on the approved Noise Exposure Map may be eligible for sound insulation.

A method for predicting, by a single number rating, cumulative aircraft noise that affects communities in airport environs. The DNL value represents decibels of noise as measured by an A-weighted sound-level meter. In the DNL procedure, the noise exposure from each aircraft takeoff or landing at ground level around an airport is calculated, and these noise exposures are accumulated for a typical 24-hour period. (The 24-hour period often used is the average day of the peak month for aircraft operations during the year being analyzed.) Daytime and nighttime noise exposures are considered separately. A weighting factor equivalent to a penalty of 10 decibels is applied to \ operations between 10 p.m. and 7 a.m. to account for the increased sensitivity of people to nighttime noise. The DNL values can be expressed graphically on maps using either contours or grid cells. DNL may also be used for measuring other noise sources, such as automobile traffic, to determine combined noise effects.