NC Curve

A Noise Criteria (NC) curve is a standardized octave-band spectral contour for rating background noise levels in occupied rooms, developed by Beranek in 1957 and widely used in North American practice. NC curves span octave bands from 63 Hz to 8000 Hz and are rated by the highest curve that the measured spectrum touches or exceeds. The NC system was designed to ensure that HVAC background noise does not interfere with normal activities. Typical NC targets include NC 25 for private offices, NC 30–35 for open offices, NC 15 for concert halls, and NC 40 for computer rooms. While simpler than the RC method, NC curves do not evaluate spectral quality (rumbly, hissy) or very low frequencies below 63 Hz. NC is referenced in ASHRAE Handbook and many US building codes. For new designs, ASHRAE recommends RC Mark II, but NC remains the most commonly specified criterion.