A single-number noise rating system published in 1957 to rate steady-state continuous noise in a room from all types of equipment, including fans, mixing boxes, diffusers, etc.
Noise criteria curves evaluate existing listening conditions by measuring sound levels (preferably at ear level) at the loudest locations in a room. Noise criteria may also be referred to dBA levels.
In acoustics, normally used to describe a rating system for evaluating noise from heating, ventilating and air conditioning ( HVAC) systems. It has come to be used as a basis for evaluating and establishing requirements for background noise in listening spaces. Based on a set of sound level vs. frequency curves known as the NC criteria. The lowest curve that the spectrum of the measured sound does not exceed is defined as the noise criterion (e.g. NC-30) for that room. There are other variations on this theme, adopted for different purposes, or in different parts of the world (e.g. NR - noise rating - in Europe).