Noise Criteria

Noise criteria are standardized octave-band spectral curves used to rate and specify acceptable background noise levels in occupied spaces. The three main systems are NC (Noise Criteria, ANSI), NR (Noise Rating, ISO), and RC (Room Criteria, ASHRAE). Each system defines a family of curves across octave bands from 63 Hz to 8000 Hz. A measured background noise spectrum is compared against the curves, and the highest curve touched or exceeded defines the rating. NC curves were developed for office and commercial environments, NR curves are used internationally (ISO 1996), and RC curves include quality descriptors for spectral balance (neutral, rumbly, hissy). Typical design targets are NC/NR 25–30 for classrooms, NC/NR 30–35 for offices, and NC/NR 15–20 for concert halls. The choice of criteria system depends on the applicable building code and regional practice. All three systems emphasize that low-frequency noise is more disruptive than the A-weighted level alone suggests.