COMPOSITE NOISE RATING (CNR)
A U.S. noise measurement system introduced in the early 1960's, and designed to evaluate land use near airports and predict annoyance levels from aircraft operations. Although still in use, the CNR has been superseded in some places by the more recent NOISE EXPOSURE FORECAST (NEF) system (which is similar but adds corrections for duration and pure tones) and the COMMUNITY NOISE EQUIVALENT LEVEL (which is based on dBA readings to avoid computer calculation).
The measurement is based on the maximum PERCEIVED NOISE LEVEL (PNLmax) in PNdB plus consideration of the number of flights during the day and night. The basic equation is:
CNR = PNLmax + 10 log10 (ND + 16.7 NN) - 12 (dB) where PNLmax is the approximate energy mean of the maximum perceived noise levels at a given point, and ND and NN are the number of flights during the day (0700 to 2200) and night (2200 to 0700) respectively. The factor 16.7 represents a 10-to-1 weighting of night flights over day ones.
In general, the CNR is related to the NEF and the NNI (see NOISE AND NUMBER INDEX) by the constant values:
CNR @ NEF + 72 CNR @ NNI + 56
As observed by R.A. Barron, in The Tyranny of Noise (p. 50), the CNR "supposedly predicts whether a given noise will lead to no response, or provoke a rising degree of protest, culminating in vigorous legal action. (quoting L. Goodfriend) 'The objective is not to produce an enjoyable or even a suitable environment. It is merely to prevent complaints.' " See: SOUND INTRUSION.
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