An RC curve is an alternative noise rating system that prioritizes low-frequency sound assessment, using octave bands from 31.5 Hz to 4,000 Hz to evaluate building acoustical quality. Developed specifically for HVAC and mechanical noise, RC ratings include low-frequency rumble that NC curves may underemphasize. RC curves are particularly valuable for assessing vibration-related noise in buildings with sensitive operations or sleeping occupants.
Technical Details
RC ratings range from RC 25 to RC 70, with 5-dB increments between levels, similar to NC numbering. The RC procedure includes a percentage loudness comparison at each frequency band to identify whether noise is balanced or has a low-frequency or high-frequency bias. RC curves extend lower than NC curves, capturing the 31.5 Hz and 63 Hz bands where HVAC equipment typically generates the most problematic energy.
Practical Significance
A system may pass NC 40 but fail RC 40 if low-frequency rumble dominates, indicating deficient vibration isolation despite adequate overall sound levels. Hotels, hospitals, and recording studios frequently specify RC ratings alongside NC values. RC assessment guides isolation design choices, directing investment toward elastomeric pads, spring isolators, or equipment relocation rather than duct silencers.