This is an excerpt from pages 26-27 of the Eastern Carolina Joint Land Use Study November 2002

Noise

In addition to addressing safety concerns, the AICUZ[Air Installations Compatible Use Zone] also addresses the issue of noise. Since 1974, the DoD[Department of Defense] has been using Day-Night Average Sound Level(Ldn) noise descriptor. As a minimum, contours for Ldn 65, 70, 75, and 80 are plotted on maps as part of AICUZ studies.

The use of Day-Night Average Sound Level has been criticized recently as not accurately representing community annoyance and land use compatability with aircraft noise. Much of the criticism stems from a lack of understanding of the basis for the measurement or calculation of Ldn. One frequent criticism is based on the inherent feeling that people react more to single noise events and not as much to "meaningless" time-average sound levels.

As an example of how Ldn is calculated, consider a case in which only one aircraft overflight occurs in daytime during a 24-hour period, creating a sound level of 100 dB for 30 seconds. During the remaining 23 hours, 59 minutes, and 30 seconds of the day, the ambient sound level is 50 dB. The Day-Night Average Sound Level for this 24-hour period is 65.5 dB. Assume, as a second example, that ten such 30-second overflights occur in daytime hours during the next 24-hour period, with the same ambient sound level of 50 dB during the remaining 23 hours, 55 minutes of the day. The Day-Night Average Sound Level for this 24-hour period is 75.4 dB. Clearly, the averaging of the noise over a 24-hour period does not ignore the louder single events and tends to emphasize both the sound levels and number of those events. This is the basic concept of a time-average sound metric, and specifically the Day-Night Average Sound Level.

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