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United States Department of Transportation United States Department of Transportation

What is DNL and why does the FAA use it?

The Day Night Average Sound Level (DNL or Ldn) noise metric is used to reflect a person's cumulative exposure to sound over a 24-hour period. DNL takes into account both the amount of noise from each aircraft operation as well as the total number of operations flying throughout the day and applies an additional 10dB weighting for nighttime flights between 10 p.m. and 7 a.m.

DNL is the FAA's required noise metric for the assessment of aircraft noise and was adopted through 14 Code of Federal Regulations Part 150 as required to meet the provisions of the Aviation Safety and Noise Abatement Act of 1979. It should be made clear that DNL is an agency metric, not just used for specific lines of business.

Part 150 applies broadly across the agency for this purpose not just for the airport specific provisions. Visit the Fundamentals of Noise and Sound page to learn more.