Noise Criteria (NC) Calculator

NC is a single-number rating of background noise in a room, defined by a family of NC curves across the octave bands from 63 Hz to 8000 Hz. The NC rating is the lowest NC curve that lies entirely above the measured octave-band levels, determined by the tangency method. Lower NC values mean a quieter space; offices typically target NC 30 to 40 and concert halls NC 15 to 25.

Plot the measured sound pressure level in each octave band and compare it to the standard NC curves. The NC rating equals the lowest-numbered curve such that no band level exceeds that curve. This calculator implements the tangency method by interpolating each band against the published NC curve values and taking the maximum required NC across all bands.

A single dB(A) number hides the spectral balance of noise. Two rooms with the same dB(A) can feel very different if one has more rumbling low-frequency content. NC curves are shaped to reflect the ear's tolerance at each frequency, so the NC rating captures whether the noise is well balanced or dominated by a particular band such as HVAC rumble or hiss.

The NC system uses the eight octave bands centred at 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz. Each NC curve specifies a permitted level in each of these bands. The 31.5 Hz band is not included in the classic NC method, which is one reason newer metrics like RC and NCB were developed for low-frequency rumble.

Typical design goals are NC 25 to 30 for private offices and classrooms, NC 30 to 40 for open offices and restaurants, NC 35 to 45 for lobbies and corridors, and NC 15 to 25 for recording studios, concert halls and bedrooms. Mechanical engineers use these targets to size and silence HVAC systems.