THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

Composite Noise Rating13.9 Rating of Aircraft Noise 333The composite noise rating CNR traces its history as far back as the early 1950s.The CNR, originally used as a basic parameter, measures the level rank on thebasis of a seta set of curves placed about 5 dB apart in the mid-frequency range,in nearly the same fashion as the NC and NR curves. The level rank was obtainedby superposing the noise spectrum on the curves and determining the highest zoneinto which the spectrum protruded. The rank thus found was then modified toinclude algebraic corrections to reflect the spectrum characteristics, peak factor,repetitive nature, background noise level, time of day, adjustment to exposure,and even public relations factors. The realized value of CNR was associated with arange of community annoyance categories established by case histories, which canrange from no annoyance through mild annoyance, varying degrees of complaints,threats of legal action, to downright vigorous community response.The CNR had to be modified to meet the advent of jet aircraft (first militaryand then commercial) in the late 1950s. Instead of being assigned a level rank, themilitary aircraft noise was converted into an equivalent sound pressure level (SPL)in the 300–600 Hz range on the basis of a set of curves similar to the level-rankcurves. The time-varying SPL was averaged and modified by corrections similarto those described in the preceding paragraph. When commercial jetcraft arrivedon the scene, the CNR was modified to use the perceived noise level (PNL) insteadof SPL. The final version of CNR is of the form:CNR = PNL max + N + Kwhere PNL max denotes the average maximum perceived noise level for individualaircraft flyover events (either landing or take-off) for a 24-h period, N is a correctionfactor for the number of aircraft flyovers, and K is an arbitrary constant. The CNRmethodology has proven to be useful for predicting community response to aircraftnoise, ranging on a scale from no reaction to vigorous community response. Eachground point can be represented byCNR ij = PNL ij + 10 log(N d,ij + 16.7N n,ij ) − 12 (13.8)where N d,ij and N n,ij represent the number of daytime and nighttime events foreach aircraft class iand flight path j. A penalty of 10 dB is imposed on the nighttimeflights, and the factor of 16.7 takes into account that there are fewer nighttime hoursthan daytime. The following expression gives the total CNR for the ground point:CNR = 10 log ∑ i∑jantilog CNR ij10(13.9)But Equation (13.9), the final version of CNR, does not correct for backgroundnoise, prior history, public relations, or other factors such as the occurrence ofpure tones. We discussed CNR here even though it is no longer used, because thisparameter serves as the precursor of currently used noise measures and descriptorssuch as NEF and NNI, which are described below.