Noise Criteria Curves: NC, NR, PNC, and RC Compared
TL;DR
Four noise rating systems are in common use: NC (Noise Criteria, USA), NR (Noise Rating, Europe/ISO), PNC (Preferred Noise Criteria, updated NC), and RC (Room Criteria, quality + level). NC uses 8 octave bands (63-8000 Hz) with a tangency method — the NC rating equals the lowest NC curve not exceeded by any band. NR uses the same method with different curve shapes (ISO 1996 reference). PNC adds a low-frequency extension and modified high-frequency requirements. RC adds spectral quality assessment (neutral, rumble, hiss) alongside the level rating. For live sound venues, NC-25 (concert halls) to NC-35 (worship spaces) is typical. For studios, NC-15 to NC-20. The SonaVyx SPL meter computes NC and NR ratings automatically from octave band data.
Why Multiple Rating Systems Exist
Background noise in occupied spaces affects speech intelligibility, recording quality, listener comfort, and hearing health. A single dBA number is insufficient because the spectral content matters — 45 dBA of low-frequency HVAC rumble is less annoying (and less damaging to intelligibility) than 45 dBA of broadband hiss.
Noise criteria curves define acceptable levels per octave band, creating a spectral envelope that accounts for human sensitivity and annoyance at different frequencies.
NC: Noise Criteria (Beranek, 1957)
The most widely used system in the USA. NC curves are defined at 8 octave band center frequencies: 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz.
Method: Measure the octave band SPL at each frequency. Plot on the NC chart. The NC rating is the highest (loudest) NC curve that is tangent to or exceeded by any single measured band.
Example: If all bands fall at or below the NC-30 curve except the 250 Hz band which touches NC-35, the room rates NC-35.
NC Target Values by Room Type
| Room Type | NC Target | Approximate dBA |
|---|---|---|
| Concert hall | NC-15 to NC-20 | 25-30 dBA |
| Recording studio | NC-15 to NC-20 | 25-30 dBA |
| Theatre / Drama | NC-20 to NC-25 | 30-35 dBA |
| Church / Worship | NC-25 to NC-30 | 35-40 dBA |
| Classroom | NC-25 to NC-30 | 35-40 dBA |
| Conference room | NC-25 to NC-35 | 35-45 dBA |
| Open office | NC-40 to NC-45 | 45-50 dBA |
| Restaurant | NC-40 to NC-45 | 45-55 dBA |
NR: Noise Rating (ISO 1996)
The European equivalent, defined in the ISO 1996 series. NR curves are similar in concept to NC but have different shapes — NR curves are generally steeper at low frequencies, making NR ratings typically 2-5 points higher than NC for the same measured noise.
NR is referenced in European building codes, HVAC specifications, and the Noise at Work Regulations. The curves use the same 8 octave bands (63-8000 Hz) and tangency method.
PNC: Preferred Noise Criteria (Beranek, 1971)
PNC was developed to address limitations in NC — specifically, NC allowed too much low-frequency energy (63-250 Hz rumble) and too much high-frequency energy (4000-8000 Hz hiss) that listeners found objectionable even when the NC rating was met.
PNC curves are approximately 3-5 dB lower than NC at 31.5 and 63 Hz, and 3-5 dB lower at 4000 and 8000 Hz. This creates a more stringent standard for spectral quality. PNC saw limited adoption and has been largely superseded by RC.
RC: Room Criteria (Blazier, 1981; ASHRAE)
RC is the most sophisticated system, assessing both level and spectral quality. It is the ASHRAE-recommended rating method for HVAC system noise.
RC level: The arithmetic mean of the 500, 1000, and 2000 Hz octave band levels.
RC quality: After computing the RC level, the method checks for spectral imbalance:
- RC(N): Neutral — all bands within ±5 dB of a reference spectrum. This is the ideal result.
- RC(R): Rumble — low-frequency bands (16-500 Hz) exceed the reference by more than 5 dB. Indicates HVAC mechanical noise.
- RC(H): Hiss — high-frequency bands (1000-4000 Hz) exceed the reference by more than 3 dB. Indicates air turbulence or diffuser noise.
- RC(RV): Rumble + Vibration — very high low-frequency energy. May indicate structural vibration transmission.
Which System to Use
| Context | Recommended System | Reason |
|---|---|---|
| US building codes | NC | Most commonly referenced |
| European standards | NR | ISO standard, regulatory requirement |
| HVAC design | RC | ASHRAE recommendation, includes quality |
| Studio / critical listening | NC or RC | Either works; RC adds quality assessment |
| Quick assessment | NC | Simple, well-understood, widely published targets |
Relationship to dBA
As a rough approximation: dBA ≈ NC + 10 (within ±3 dB for typical spectra). However, this relationship breaks down for spectra dominated by low-frequency or high-frequency energy. Always measure the full octave band spectrum and compute the NC/NR rating properly rather than estimating from dBA.
SonaVyx Implementation
The SPL meter computes NC and NR ratings in real-time from octave band analysis data. The 1/1 octave band display shows the measured spectrum overlaid with the applicable NC curve set. The NC rating is displayed as a number (e.g., "NC-32") with color coding: green (meets target for selected room type), amber (within 5 NC points), red (exceeds target by more than 5 points).
Venue type targets are built into the treatment calculator, which can estimate the NC rating after proposed treatment and predict whether the target will be achieved. The AI diagnostic includes noise floor assessment as one of its six analysis categories, using NC curves as the reference standard.
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Frequently Asked Questions
Last updated: March 19, 2026