Enter the sound pressure level in dB for each octave band. All eight bands are recommended for an accurate NC rating.
Band-by-Band Rating Margin
Full Calculation Table
| Frequency Band | Measured Level | Governing NC Curve Value | Margin |
|---|
How to Determine the NC Rating of a Space
Enter the measured or predicted sound pressure level in each of the eight standard octave bands from 63 Hz to 8000 Hz.
Click Calculate NC Rating to compare each band level against the full family of standard NC curves.
Review which single octave band determines the overall NC rating, since the rating is set by the highest curve touched by any one band, not an average.
Check the resulting NC rating against the recommended range for the space type using the noise criteria selector.
Typical Recommended NC Ranges by Space Type
| Space Type | Typical NC Range |
|---|---|
| Broadcast / recording studio | NC 10–20 |
| Concert hall, theatre | NC 15–20 |
| Private office, small conference room | NC 25–35 |
| Classroom | NC 25–30 |
| Hospital patient room | NC 25–35 |
| Open-plan office | NC 35–40 |
| Restaurant, retail space | NC 35–45 |
| Mechanical / equipment room (occupied briefly) | NC 50–60 |
General design guideline ranges. Confirm the specific target for your project against the applicable building code, client specification, or acoustical consultant recommendation. Use the noise criteria selector for a more detailed lookup by space type.
Understanding the NC (Noise Criteria) Rating System
The Noise Criteria rating system provides a standardized way to describe steady-state background noise in occupied spaces using a single number, derived from a full octave band spectrum rather than a single overall sound level reading. NC curves were developed to correlate with subjective judgments of acceptable indoor background noise, particularly for HVAC system noise, which tends to be broadband and steady rather than tonal or impulsive.
How the NC Rating Is Determined
Each standard NC curve, numbered from roughly NC 15 through NC 65 in increments of 5, specifies a maximum permitted sound pressure level at each of the eight standard octave bands from 63 Hz to 8000 Hz. This tangency method, comparing a measured spectrum against the family of curves and identifying the highest curve any single band touches, is defined in ANSI/ASA S12.2, Criteria for Evaluating Room Noise. To find the NC rating of a measured or predicted spectrum, each octave band level is compared against the family of curves, and the rating is set by the single highest-numbered curve that any one band touches or exceeds. This is a worst-case method, not an average: if seven bands comfortably meet NC 30 but one band reaches NC 40, the overall rating for the space is NC 40, driven entirely by that one governing band.
Why Low-Frequency Bands Often Govern the NC Rating
HVAC equipment noise, particularly from fans and air handling units, tends to concentrate energy in the lower octave bands, often 63 Hz, 125 Hz, and 250 Hz. NC curves are relatively flat and permissive at these low frequencies, meaning it can take a substantial sound pressure level increase at 63 Hz before that band starts driving the rating, but once it does, that low band frequently becomes the governing constraint. This is a common finding when troubleshooting a space that "sounds noisy" despite an acceptable-looking overall dB(A) reading: the culprit is often a single low-frequency band that a simple overall measurement does not reveal. Use the octave band calculator first if you need to combine multiple sources' spectra before running the NC comparison here.
NC vs. RC: When to Use Each System
The RC (Room Criteria) rating system was developed partly to address a limitation of NC: because NC curves flatten out at low frequencies, a spectrum can pass an NC rating while still containing perceptibly excessive low-frequency rumble. RC curves slope downward more steeply at low frequencies and include an explicit quality assessment (labelled Neutral, Rumble, or Roar/Vibration depending on the spectrum shape) that flags this condition even when the numeric rating looks acceptable. Many current acoustical consultants and specifications favour RC over NC for exactly this reason, particularly for an office HVAC project where an open-plan floor's target sits at the noisier end of the acceptable NC range. Use the RC rating calculator alongside this NC tool to get both perspectives on the same spectrum, and the NR curve calculator for projects that reference the international NR system instead.
Using the NC Rating for HVAC Design Decisions
Once you know the NC rating and which band is governing it, the appropriate corrective action depends on the source. If the governing band traces to fan noise travelling down a duct, the attenuator sizing calculator can determine the insertion loss needed at that specific frequency. If it traces to duct velocity noise at a diffuser or fitting, the duct self-noise calculator identifies the velocity threshold to stay under. If it traces to breakout noise through duct walls into an adjacent space, the duct breakout noise calculator is the relevant next step. The target NC value itself should be set before mechanical design begins, as with a school HVAC project targeting NC 25 to 30 for classrooms. Quiet-space design targets like these exist because sustained background noise is a recognized workplace concern, an area NIOSH researches from an occupational hearing and concentration perspective, even though its primary mandate concerns hazardous noise exposure rather than office comfort criteria specifically.
Frequently Asked Questions
The NC rating is determined by plotting the measured or predicted sound pressure level in each of the eight octave bands against the full family of standard NC curves, then identifying the single lowest-numbered NC curve that is not exceeded by any of the eight band levels. In practice, the room's rating is set by whichever individual band comes closest to, or exceeds, a given curve, even if every other band is well below it. A single high band, most often in the low-frequency range from an oversized or poorly selected fan, can drive the entire room's rating higher than the other seven bands would suggest. Use the octave band calculator first if you need to combine multiple sound sources into one spectrum before rating it here.
Recommended NC ranges vary by space type. Private offices and small conference rooms typically target NC 25 to NC 35. Open-plan offices are often designed to NC 35 to NC 40. Classrooms typically target NC 25 to NC 30 for speech intelligibility. Hospital patient rooms typically target NC 25 to NC 35. Broadcast and recording studios require much lower ratings, often NC 15 to NC 20 or lower. Use the noise criteria selector for a fuller lookup by specific space type, and confirm against your project's building code or specification.
The NC method intentionally uses a worst-case, single-band approach rather than an average because a narrow, prominent band of noise, especially low-frequency rumble or a distinct tonal whine, is perceived as far more objectionable than the same total energy spread evenly across the spectrum. Averaging would allow a very loud, narrow-band tone to be masked by several quiet bands, producing a rating that looks acceptable on paper but sounds noticeably annoying in the space. Using the worst governing band ensures the rating reflects the most disturbing component of the noise, not just its total energy.
Both systems compare octave band levels against standard curves and report a single number, but they differ in curve shape and low-frequency treatment. NC curves flatten out below about 500 Hz, making them relatively permissive of low-frequency rumble as long as it stays under the curve. RC curves slope downward more steeply at low frequencies and include an explicit quality assessment that flags excessive low-frequency or high-frequency content, even when the overall number is acceptable. Many acoustical consultants now prefer RC for this more complete low-frequency evaluation. Run the same octave band data through the RC rating calculator to compare both results directly.
Related Noise & Acoustics Tools
Room Criteria with quality suffix
International Noise Rating curve
Sum spectral sound data
Target levels by space type
Silencer insertion loss by band
Velocity-generated noise