HVAC Noise Criteria (NC/RC) Estimator
Estimates room NC rating from equipment octave-band sound power and duct/silencer attenuation, per ASHRAE Fundamentals Ch. 48
Estimates the resulting room noise criteria (NC) rating from equipment octave-band sound power levels, subtracting duct attenuation, silencer insertion loss, and a simplified room effect, then comparing the delivered spectrum against the standard Beranek NC curves used throughout ASHRAE HVAC acoustics guidance.
How NC/RC estimation works
HVAC equipment sound power is published per octave band by manufacturers (AHRI-rated). As that noise travels through ductwork to the occupied space, it is attenuated by the duct itself (more so if internally lined), any silencer or sound trap installed, and diffusion into the room volume. Comparing the resulting delivered spectrum band-by-band against the standard NC curves — and finding the lowest curve the spectrum does not exceed in any single band — gives the room's NC rating, the long-standing ASHRAE method for describing HVAC background noise.
Frequently asked questions
What's the difference between NC and RC?
NC (Noise Criteria) rates a spectrum against fixed reference curves and is governed by whichever single band is worst; RC (Room Criteria) instead averages the mid-frequency bands and also flags spectral imbalance (excess rumble or hiss) that NC alone doesn't capture.
What NC target should I design to?
Common ASHRAE-referenced targets: NC 25-30 for private offices and classrooms, NC 30-35 for open offices, NC 35-40 for lobbies and corridors, and NC 20-25 for recording studios or similarly critical-listening spaces.
Why does lined duct help so much more at mid frequencies?
Acoustic duct liner is most effective in the 500 Hz-2 kHz range where its thickness is best matched to the sound wavelength; very low frequencies (63/125 Hz) pass through with much less attenuation regardless of lining.