Noise Criterion – NC – level is a standard describing relative loudness of a space within a range of frequencies. It was established in the U.S. for rating indoor noise such as the noise from air-conditioning equipment or similar. In Europe it is common to use the Noise Rating Curve – NR.

The method consists of a set of criteria curves ranging 63 to 8000 Hz and a tangency rating procedure. The criteria curves define the limits of octave band spectra that must not be exceeded to meet the occupant’s acceptance in the actual spaces.

NC-30 represents a specific noise curve that provides a comfortable level of noise of any given environment.