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  1. Average sound exposure over a 24-hour period is often presented as a day-night average sound level (Ldn). Ldn values are calculated from hourly Leq values, with the Leq values for the nighttime period (10:00 p.m.–7:00 a.m.) increased by 10 dB to reflect the greater disturbance potential from nighttime noises.
  2. (DNL, denoted by the symbol Ldn): A 24-hour time-averaged sound exposure level, adjusted for average-day sound source operations. In the case of aircraft noise, a single operation is equivalent to a single aircraft departure, approach, etc. The adjustment includes a 10 dB penalty for operations occurring between 2200 and 0700 hours, local time. Source: http://www.volpe.dot.gov/acoustics/docs/1990-1999/1999-1.pdf

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