(n.) A unit of measurement for the ambient or background noise level of an occupied indoor space. The measured sound pressure levels (SPL) in octave bands are compared to a series of standard noise criteria (NC) curves to determine the “NC level” of a space. The standard NC curves take into account the equal loudness contours of normal hearing to accurately reflect the listening experience. Each NC curve is assigned a number (in 5 dB increments) corresponding to the octave band SPL measured over the octave centered at approximately 1500 Hz. A space with a background noise level of “NC-30” or lower is very quiet.
noise criterion curve [NC curve]
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