TL;DR
Background noise in buildings — predominantly from HVAC systems — must be specified, measured, and controlled. Three rating methods dominate practice: NR (Noise Rating, ISO-based, common in Europe), NC (Noise Criteria, ANSI-based, common in North America), and RC (Room Criteria, ANSI/ASA S12.2, recommended for quality-sensitive spaces). All three compare measured octave-band noise levels against a family of reference curves, but they differ in frequency range, curve shape, and the quality information they provide. Specifying the wrong system for your region, or confusing NR 35 with NC 35 (they are not equivalent), leads to HVAC systems that are either too noisy or over-engineered. This article explains all three methods, provides worked examples, and gives clear guidance on which to specify for each building type.
The Hospital Where the HVAC Was Too Quiet
A private hospital in Munich specified background noise levels of NC 25 for patient rooms — the lowest practical NC rating, intended to provide an ultra-quiet healing environment. The HVAC contractor delivered exactly that: NC 24 measured at commissioning. The hospital was delighted, briefly.
Within three months, patient complaints about "hearing everything" became a pattern. Footsteps in corridors, conversations at nursing stations, doors closing, equipment alarms from adjacent rooms — all were clearly audible because the background noise floor was too low to provide any masking. Patients in rooms nearest the nursing station reported difficulty sleeping, not from the HVAC noise they could not hear, but from the human and equipment noise that the low background noise level exposed.
The facility eventually installed a sound masking system to raise the background level to NC 30 — deliberately adding noise to a space where they had paid a premium to remove it. The lesson: background noise is not inherently bad. The goal is the right level of the right spectral shape, not the lowest possible level.
NR: Noise Rating (ISO)
The NR system was developed by Kosten and Van Os in 1962 and adopted into ISO standards. It uses a family of curves covering octave bands from 31.5 Hz to 8000 Hz. The NR rating of a measured spectrum is determined by the highest NR curve that is reached or exceeded by any octave band.
NR Curve Values (Selected)
| NR | 31.5 Hz | 63 Hz | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz | 8 kHz |
|---|---|---|---|---|---|---|---|---|---|
| 20 | 64 | 46 | 32 | 22 | 17 | 14 | 12 | 11 | 10 |
| 25 | 67 | 49 | 36 | 27 | 22 | 19 | 17 | 16 | 15 |
| 30 | 70 | 52 | 40 | 32 | 27 | 24 | 22 | 21 | 20 |
| 35 | 73 | 56 | 44 | 37 | 32 | 29 | 27 | 26 | 25 |
| 40 | 76 | 60 | 48 | 42 | 37 | 34 | 32 | 31 | 30 |
| 45 | 79 | 63 | 52 | 47 | 42 | 39 | 37 | 36 | 35 |
Strengths
- Includes 31.5 Hz band — captures very low frequency rumble
- Widely used in European standards (BS 8233, DIN 4109, SIA 181)
- Simple: the NR rating is the highest curve touched by any band
Weaknesses
- No quality assessment — does not indicate whether excess is at low or high frequency
- A spectrum that touches NR 35 at one band and is below NR 25 at all others still rates NR 35
NC: Noise Criteria (ANSI)
NC was developed by Leo Beranek in 1957 and revised in ANSI/ASA S12.2. It uses a similar family of curves but originally covered 63 Hz to 8000 Hz (omitting the 31.5 Hz band). The curves are slightly different in shape from NR, with more relaxed low-frequency limits.
NC Curve Values (Selected)
| NC | 63 Hz | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz | 8 kHz |
|---|---|---|---|---|---|---|---|---|
| 20 | 51 | 40 | 33 | 26 | 22 | 19 | 17 | 16 |
| 25 | 54 | 44 | 37 | 31 | 27 | 24 | 22 | 21 |
| 30 | 57 | 48 | 41 | 36 | 31 | 29 | 28 | 27 |
| 35 | 60 | 52 | 45 | 40 | 36 | 34 | 33 | 32 |
| 40 | 64 | 56 | 50 | 45 | 41 | 39 | 38 | 37 |
| 45 | 67 | 60 | 54 | 49 | 46 | 44 | 43 | 42 |
NR vs NC: Not Interchangeable
For the same measured spectrum, the NR rating is typically 1-5 points higher than the NC rating at low frequencies, and approximately equal at mid-frequencies. Specifying "NR 35 or NC 35" as if they are equivalent is incorrect — NC 35 is approximately 2-3 dB more lenient at low frequencies than NR 35.
Check your background noise levels → AcousPlan Noise Calculator
RC: Room Criteria (ANSI/ASA S12.2)
The RC method, developed by Blazier in 1981 and updated in ANSI/ASA S12.2-2008, addresses the fundamental limitation of both NR and NC: they tell you the noise is (or is not) too loud, but not whether it sounds acceptable.
How RC Works
- Calculate the arithmetic mean of measured levels at 500, 1000, and 2000 Hz. This is the RC rating number.
- Plot the measured spectrum against the RC reference line (a straight line at -5 dB/octave from the rating value at 1000 Hz).
- Assess spectral quality:
RC Quality Ratings
| RC Rating | Quality | Meaning |
|---|---|---|
| RC 32 (N) | Neutral | Spectrum follows the reference line — unobtrusive |
| RC 35 (R) | Rumble | Low-frequency excess — often caused by duct breakout or fan blade pass |
| RC 30 (H) | Hiss | High-frequency excess — often caused by air noise at diffuser or damper |
| RC 38 (RV) | Vibration | Very low frequency excess — risk of perceptible vibration |
Worked Example
Measured octave-band spectrum in a private office:
| Band (Hz) | 16 | 31.5 | 63 | 125 | 250 | 500 | 1 kHz | 2 kHz | 4 kHz |
|---|---|---|---|---|---|---|---|---|---|
| Level (dB) | 62 | 55 | 48 | 42 | 38 | 34 | 30 | 27 | 25 |
RC rating = mean(34, 30, 27) = 30.3 → RC 30
The RC reference line at RC 30 gives expected values of approximately [65, 60, 55, 50, 45, 40, 35, 30, 25] at each band. No band exceeds the reference by more than 5 dB (low) or 3 dB (high). The 16 Hz band (62 dB) is below the 75 dB vibration threshold.
Quality assessment: RC 30 (N) — Neutral spectrum, no quality concerns.
Recommended Background Noise Levels by Space Type
| Space Type | NR | NC | RC | Notes |
|---|---|---|---|---|
| Concert hall | 15-20 | 15-20 | 15-20(N) | Lowest practical limit |
| Recording studio | 15-20 | 15-20 | 15-20(N) | NC 15 extremely difficult to achieve |
| Hospital patient room | 25-30 | 25-30 | 25-30(N) | Too quiet can expose human noise |
| Private office | 30-35 | 30-35 | 30-35(N) | NC 30 for executive, NC 35 for standard |
| Open-plan office | 35-40 | 40-45 | 38-42(N) | Higher for speech masking |
| Classroom | 30-35 | 30-35 | 30-35(N) | ANSI S12.60 requires < 35 dBA |
| Restaurant | 35-45 | 40-45 | 40-45(N) | Higher levels acceptable |
| Retail | 40-45 | 40-45 | 40-45(N) | Background music usually dominates |
| Gymnasium | 40-50 | 45-50 | 40-45(N) | High activity noise masks HVAC |
Which System to Specify
European projects: Use NR. It aligns with BS 8233:2014, DIN 4109:2018, and most European building regulations.
North American projects: Use NC for standard applications, RC for quality-sensitive spaces (studios, hospitals, executive offices). RC provides the spectral quality assessment that NC lacks.
International projects: Specify NR or NC as appropriate for the governing building code, with an additional RC quality check for any space where occupant comfort is critical.
WELL v2 projects: WELL Feature 74 references ASHRAE Handbook targets, which are expressed as NC. Specify NC values per the WELL Sound Concept requirements.
Common HVAC Noise Sources and Their Spectral Character
| Source | Dominant Frequencies | NC/NR Impact | RC Flag |
|---|---|---|---|
| Fan blade pass | 63-250 Hz | Raises low-frequency bands | R or RV |
| Duct breakout | 125-500 Hz | Raises mid-low bands | R |
| Air turbulence at diffuser | 1000-4000 Hz | Raises high-frequency bands | H |
| Damper noise | 500-2000 Hz | Broadband hiss | H |
| Pump vibration (structural) | 31.5-125 Hz | Very low frequency excess | RV |
| Transformer hum | 100/120 Hz (± harmonics) | Tonal, narrow-band | R |
Summary
NR, NC, and RC are three standardised methods for rating background noise in buildings. All three use reference curves, but they differ in frequency range, curve shape, and quality information. The Munich hospital story illustrates that background noise control is not simply "lower is better" — the spectral shape and the overall level must be matched to the space's functional requirements. Specify the correct system for your region, include a target range (not just a maximum), and for quality-sensitive spaces, add an RC quality assessment to catch the rumble and hiss problems that NR and NC cannot detect.
Analyse background noise levels → AcousPlan Noise Criteria Calculator