(NC) A set of approximate equal loudness curves used to assess the acceptability of background sound in buildings. The curves are used along with a measured or estimated octave band spectrum to determine the NC rating of the spectrum. Criteria for acceptable sound in buildings are often expressed as ranges of acceptable NC ratings. NC criteria are generally applied to background sound in buildings produced by building heating, ventilating and air conditioning (HVAC) equipment. The NC rating of sound produced by an HVAC noise source involves overlaying the measured or estimated octave band spectrum on a series of noise criteria curves. The highest NC curve tangent to the measured spectrum in any octave band is the NC rating of that spectrum.