NC Curve (Noise Criteria)

NC curves are the most widely used background noise rating method in North America. Each NC curve defines maximum acceptable octave-band sound pressure levels across eight bands from 63 Hz to 8 kHz. The curves slope downward with frequency, reflecting the fact that low-frequency noise is less annoying than mid and high-frequency noise at the same SPL. The NC rating procedure: measure the octave-band SPL from 63 Hz to 8 kHz, plot the values against the NC curve family, and report the lowest NC curve that completely envelopes the measurement. If the measured 500 Hz band touches NC-35 but all other bands are below NC-30, the NC rating is NC-35. This "worst band governs" approach means a single problematic frequency band determines the rating. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) publishes recommended NC values for different space types. Representative targets: NC-15 for recording studios and concert halls, NC-25 for churches and courtrooms, NC-30 for private offices and classrooms, NC-35 for open plan offices, NC-40 for lobbies and corridors, NC-45 for computer rooms and workshops. These targets assume the HVAC system is the dominant noise source. NC curves have limitations. They do not account for tonal noise (a hum or whine at a specific frequency can be very annoying even if the broadband level meets the NC target). The PNC (Preferred Noise Criteria) and RC (Room Criteria) methods were developed to address this by adding requirements for spectral balance and tonal content. In practice, NC ratings should be measured with the space unoccupied but with all mechanical systems running at normal operating conditions. Short-term measurements (30-60 seconds of steady-state Leq per band) are sufficient for steady HVAC noise. SonaVyx automatically computes the NC rating from octave-band SPL measurements and overlays the relevant NC curves on the display.