Noise Rating (NR) and other curves
‘A’ weighted levels can not be used to define a spectrum or to compare sounds of different frequencies. NR curves convey frequency information in a single-figure index by defining the highest measured or specified level at each frequency. To measure the noise rating of a given environment, the SPL is measured in octave or one-third octave bands and the noise rating is then the highest NR curve touched by the measured levels.
NC curves are similar to NR curves and are more commonly used outside Europe. They are not defined at 31.5 Hz. NR and NC curves were derived theoretically from the response of the human ear under laboratory conditions. More recent research has found that at low noise levels, noise spectra matching these curves allow too much noise at very high and very low frequencies compared with the levels at mid-frequencies. PNC curves have begun to be used more widely in the USA and UK for noise control in concert halls and theatres.