Aviation Terms->Day-Night Average Sound Level
DNL or Ldn. The average noise level over a 24 hour period except that noise occurring at night (between the hours of 10PM and 7AM) are artificially increased by 10 dB. This weighting reflects the added intrusiveness of night noise events attributable to the fact that community background noise typically decreases by 10 dB at night. Under Federal Aviation Regulation (FAR) Part 150, the FAA has established Ldn/DNL as the cumulative noise exposure metric for use in airport noise analyses, and has developed recommended guidelines for noise/land use compatibility evaluation.