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An outdoor sound environment typical of a remote suburban setting, or a rural or public lands setting. Characteristic average day-night sound levels (DNL, represented by the symbol Ldn) would generally be less than 45 dB, and the everyday sounds of nature, e.g., wind blowing in trees and birds chirping would be a prominent contributor to the DNL.

Source: http://www.volpe.dot.gov/acoustics/docs/1990-1999/1999-1.pdf

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