Noise rating curves  --  An agreed set of empirical curves relating octave band sound pressure level to the center frequency of the octave bands, each of which is characterized by a "noise rating" (NR), which is numerically equal to the sound pressure level at the inter section with the ordinate at 1,000 Hz. The "noise rating" of a given noise is found by plotting the octave band spectrum on the same diagram and selecting the highest noise rating curve to which the spectrum is tangent.