NC tells you how loud the background noise is. RC tells you how loud it is and what it sounds like. The Room Criteria system was developed to address a fundamental shortcoming of NC and NR: two rooms can have the same NC rating but sound completely different — one with a low-frequency rumble that vibrates furniture, another with a high-frequency hiss from air diffusers. RC separates the level from the character, giving designers and building operators a much more useful diagnostic tool.
TLDR
RC (Room Criteria) is a noise rating system developed by ASHRAE that rates background noise by both level (a single number, similar to NC) and spectral quality (a letter descriptor: N for neutral, R for rumble, H for hiss, RV for rumble with vibration). The RC level is the arithmetic average of sound pressure levels at 500, 1000, and 2000 Hz. The quality descriptor is determined by comparing low-frequency energy (16 to 250 Hz) and high-frequency energy (2 to 4 kHz) against the RC reference spectrum. If low-frequency levels exceed the reference by more than 5 dB, the noise has a "rumble" character (R). If high-frequency levels exceed by more than 3 dB, it has a "hiss" character (H). An "N" (neutral) rating means the spectrum is well-balanced. RC is the preferred ASHRAE metric for mechanical system noise and is referenced in the ASHRAE HVAC Applications Handbook.
Real-World Analogy
Think of rating a restaurant's noise with just a decibel number versus a more descriptive review. "It was 72 dB" tells you the level but nothing about the character. "It was moderately loud with a constant low-frequency hum from the kitchen extract fan" tells you both. NC is the decibel number. RC is the descriptive review — it tells you the level (RC-35) and the character (R for rumble, H for hiss, N for neutral). A building manager hearing "RC-35(R)" immediately knows the noise is moderate level with a dominant low-frequency rumble, probably from the air handling unit or chiller. That diagnosis is far more useful than just "NC-35."
Technical Definition
The RC Reference Spectrum
The RC system defines a reference spectrum — a straight line with a slope of -5 dB per octave band from 16 Hz to 4 kHz. This slope approximates the spectral shape of "neutral" broadband noise that occupants find least objectionable. The RC level is defined as:
RC = (L₅₀₀ + L₁₀₀₀ + L₂₀₀₀) / 3
Where L₅₀₀, L₁₀₀₀, and L₂₀₀₀ are the measured octave-band sound pressure levels at 500, 1000, and 2000 Hz respectively. This mid-frequency average represents the perceived "loudness" of the noise.
Spectral Quality Assessment
After determining the RC level, the spectrum is compared against the RC reference curve (positioned at the RC level) in two regions:
Low-frequency region (16 to 250 Hz):
- Calculate the energy average of measured levels at 16, 31.5, 63, 125, and 250 Hz
- Compare to the corresponding RC reference curve levels
- If the measured levels exceed the reference by more than 5 dB in any low-frequency band: R (Rumble)
- If they exceed by more than 10 dB and are above 65 dB at 16 or 31.5 Hz: RV (Rumble with perceptible Vibration)
- Calculate the energy average of measured levels at 2000 and 4000 Hz
- Compare to the corresponding RC reference curve levels
- If the measured levels exceed the reference by more than 3 dB: H (Hiss)
Example
A room with measured octave-band levels:
| 63 Hz | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz |
|---|---|---|---|---|---|---|
| 58 | 50 | 44 | 38 | 34 | 32 | 31 |
RC level = (38 + 34 + 32) / 3 = 34.7, rounded to RC-35
Comparing low-frequency bands: 63 Hz at 58 dB vs. RC-35 reference at 53 dB = +5 dB (borderline rumble). 125 Hz at 50 dB vs. reference at 48 dB = +2 dB (within tolerance).
Result: RC-35(R) — moderate level with a slight low-frequency rumble. The 63 Hz band controls, suggesting the air handling unit or ductwork resonance needs attention.
RC Mark II
An updated version, RC Mark II, was developed by Blazier to refine the quality assessment thresholds and extend the analysis to include the 16 Hz and 31.5 Hz octave bands more explicitly. RC Mark II is referenced in ASHRAE guidelines but RC (original) remains more widely used in practice.
Why It Matters for Design
The quality descriptor transforms RC from a specification tool into a diagnostic tool. When commissioning measurements reveal RC-35(R), the engineer immediately knows to investigate low-frequency sources: AHU casing breakout, fan imbalance, duct resonance, or structural vibration from rooftop equipment. An RC-35(H) diagnosis points to diffuser noise, valve noise, or thin ductwork radiating high-frequency turbulence.
This diagnostic power makes RC especially valuable during commissioning — the phase where a completed building is tested against its acoustic specification. NC or NR would report the room as passing (NC-35, within spec) even if the noise character is objectionable. RC catches the spectral imbalance that makes occupants complain even when the overall level is acceptable.
ASHRAE's HVAC Applications Handbook recommends RC as the preferred metric for specifying and evaluating mechanical system noise in buildings. In practice, many specifications still use NC (because it is simpler and more widely understood), but sophisticated clients and acoustic consultants increasingly specify RC for critical spaces.
How AcousPlan Uses This
AcousPlan's noise criteria module computes the RC level and quality descriptor from your octave-band noise input. The results dashboard displays the RC rating with its quality letter (N, R, H, or RV) and highlights which frequency bands are causing the spectral imbalance. This lets you identify whether the noise problem is a level issue (overall too loud) or a quality issue (spectrally unbalanced), and the AI co-pilot recommends appropriate mitigation for each case.
Related Concepts
- What is an NC Rating? — Simpler level-only system
- What is an NR Rating? — European level-only system
- What is a Background Noise Survey? — How RC data is collected
- Noise Criteria (NR/NC/RC) Explained — Full comparison of all three systems
- What is STI? — Speech intelligibility affected by noise character
Calculate Now
Determine the RC rating and spectral quality of your room's background noise. AcousPlan computes RC level, quality descriptor, and controlling bands from your octave-band data.