Noise Criteria (NC) vs. Room Criteria (RC)

Quite often we are asked why does Thorburn Associates Inc. use Noise Criteria (NC) as opposed to Room Criteria (RC) during our mechanical system noise calculations. Both NC and RC are measures of mechanical system noise in a room. Noise Criteria curves were defined in 1957, by Leo Beranek of the Boston based Acoustical Consulting firm of BBN. Room Criteria curves were first proposed by Warren Blazier for the American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) in 1981.

Both sets of Criteria plot the noise level measured in a room against a set of curves across the different standard octave bands that are used in our acoustical work. For Noise Criteria we plot the octave band sound pressure levels in 8 bands and then select the highest curve that is crossed to come up with a single number to report. The Noise Criteria curve very closely resembles equal loudness curves that correspond to our sensitivity to higher frequency tones and our lack of sensitivity to lower frequency tones.

For Room Criteria we plot the sound pressure levels in the same 8 bands, and we also look at the curve, but now we must add a subjective decision to the curve to report the level. The RC curve is a straight line with a slope of 5 decibels per octave (in English, it does not correspond to how we hear).

So the short answer to the original question is – we use NC whenever we can because it does not require a “subjective” decision and it corresponds more closely to how a person hears.