An NC curve is a graphical representation of maximum acceptable sound pressure levels plotted across octave band frequencies from 63 Hz to 8,000 Hz, defining a specific Noise Criteria rating. Each NC curve establishes upper limits for sound energy at each frequency band, and sound must remain below the curve at all frequencies to achieve that NC rating. NC curves are fundamental tools for HVAC acoustic design and system commissioning.
Technical Details
NC curves increase in 5-dB increments at each frequency band as the rating number increases by 5 units. For example, NC 40 permits approximately 47 dB at 63 Hz but only 35 dB at 8,000 Hz, reflecting human sensitivity to higher frequencies. Technicians use real-time frequency analyzers to measure sound at each band and plot results against the target NC curve during system commissioning and troubleshooting.
Practical Significance
Violations at low frequencies (63-250 Hz) typically indicate equipment vibration transmission through structure, while violations at high frequencies (1,000-8,000 Hz) point to ductwork turbulence or blade noise. Understanding curve shape guides targeted noise remediation strategies, whether isolation mounts, duct silencers, or equipment relocation.