Noise Criteria (NC) is a rating system that specifies acceptable indoor background noise levels for different building types and room functions. The NC rating provides a single-number descriptor that accounts for human perception across multiple frequency bands, making it the standard for HVAC noise performance requirements in North America. NC ratings range from NC 15 (very quiet libraries) to NC 70 (industrial facilities).
Technical Details
Each NC curve represents maximum allowable sound pressure levels at octave band frequencies from 63 Hz to 8,000 Hz. Residential living spaces typically target NC 30-35, while office environments aim for NC 35-45. NC criteria were established by ASHRAE in 1957 and refined through ISO 6954 international standards. A single NC violation at any frequency band requires the entire rating to step up to the next level.
Practical Significance
Specifying appropriate NC levels during HVAC design prevents tenant complaints and productivity losses linked to background noise. Systems exceeding target NC values require ductwork redesign, silencer installation, or equipment isolation modifications costing 5-15 percent of base HVAC budgets. NC compliance verification occurs during final commissioning before occupancy.