Enter the sound pressure level in dB for each octave band. All eight bands are recommended for an accurate RC rating and quality assessment. The three highlighted bands (500–2000 Hz) form the RC number itself.
* Bands marked with an asterisk (500, 1000, 2000 Hz) determine the RC number itself. All eight bands are used for the quality assessment.
Spectrum Quality Assessment
Full Calculation Table
| Frequency Band | Measured Level | Neutral Reference | Deviation |
|---|
How to Calculate the RC 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 RC Rating to determine the RC number from the average of the mid-frequency bands and compare the full spectrum shape against the neutral reference curve.
Check whether the spectrum is rated Neutral, or flagged for excessive Rumble (low frequency) or Roar/Hiss (high frequency) based on how far bands deviate from the neutral curve shape.
Check the resulting RC number and quality letter against the recommended range for the space type using the noise criteria selector.
RC Quality Assessment Letters
| Letter | Meaning | Typical Cause |
|---|---|---|
| N (Neutral) | Spectrum closely follows the balanced neutral reference shape | Well-balanced HVAC system, no dominant frequency issue |
| R (Rumble) | Low-frequency bands exceed the neutral reference, perceptible as rumble | Oversized or poorly selected fan, duct-borne low-frequency noise |
| RV (Rumble, Vibration) | Severe low-frequency excess, potential to induce perceptible vibration | Significant fan or compressor low-frequency energy near structural resonance |
| H (Hiss) | High-frequency bands exceed the neutral reference, perceptible as hiss | High air velocity at diffusers, VAV box noise, or damper throttling noise |
Quality letters describe spectrum shape, not overall loudness. A room can have a low RC number and still receive an R or H flag if the spectrum shape deviates enough from neutral.
Understanding the RC (Room Criteria) Rating System
The Room Criteria rating system was developed to address specific limitations in the older NC method, particularly its relative insensitivity to low-frequency rumble. Unlike NC, which is set by whichever single octave band comes closest to exceeding a family of curves, the RC method calculates its headline number from an average of the mid-frequency bands, and then separately evaluates the entire spectrum's shape against a neutral reference to flag quality issues the numeric average alone would miss.
How the RC Number Is Calculated
The RC number is the arithmetic average of the sound pressure levels in the 500, 1000, and 2000 Hz octave bands, rounded to the nearest whole number. This mid-frequency average is chosen because it correlates well with the frequency range most associated with general loudness perception and speech interference, without being unduly influenced by either low-frequency rumble or high-frequency hiss, both of which are assessed separately.
The Quality Assessment: Reading the Letters After the Number
After computing the RC number, the full eight-band spectrum is compared against a neutral reference curve with a specific slope, roughly 5 dB per octave decreasing from low to high frequency. If the low-frequency bands (63 to 250 Hz) exceed this neutral reference by more than a defined threshold, the spectrum is flagged with an R for Rumble. A more severe low-frequency excess, large enough to potentially induce perceptible vibration in lightweight building elements like windows or suspended ceiling panels, receives an RV flag. If the high-frequency bands (4000 to 8000 Hz) exceed the reference, the spectrum receives an H for Hiss. A spectrum that stays close to the neutral reference throughout receives an N for Neutral. This means two rooms can share the identical numeric RC rating, say RC 35, but one might be labelled RC 35(N) with no issues, while the other is RC 35(R), indicating an underlying rumble problem that the number alone does not reveal.
Why RC Often Catches Problems NC Misses
Because NC curves flatten out at low frequencies, a spectrum with substantial low-frequency energy can pass an NC rating comfortably while still producing a noticeably rumbling background sound. This is a common source of post-occupancy complaints on projects that were designed and verified only against NC targets. RC's explicit quality assessment surfaces this condition directly, giving the design team clear direction that low-frequency-specific noise control, such as a heavier duct silencer tuned for low-frequency attenuation rather than a standard mid-frequency splitter, may be required. Run the same octave band data through the NC curve calculator to see both perspectives side by side, and use the octave band calculator first if multiple sources need to be combined before rating the resulting spectrum here.
Addressing a Rumble or Hiss Flag
If a spectrum is flagged for rumble, the corrective action typically focuses on the fan and duct system: verify fan selection against actual system static pressure to avoid an oversized, high-slip fan, and consider a duct silencer with dedicated low-frequency performance using the attenuator sizing calculator. If flagged for hiss, the source is usually excessive air velocity at diffusers, VAV boxes, or dampers; check velocity against recommended limits using the duct self-noise calculator and reduce velocity or add flow-control devices designed for low self-noise.
Frequently Asked Questions
The RC number is the arithmetic average of the sound pressure levels in three mid-frequency octave bands, typically 500, 1000, and 2000 Hz, rounded to the nearest whole number. This differs fundamentally from the NC method, which is set by the single worst-case band compared against a family of curves. The RC method then separately compares every octave band, including low-frequency bands not used in the number itself, against a neutral reference spectrum shape to determine the quality assessment letter. See the NC curve calculator to compare the same data using the worst-case method.
The letters describe the spectrum shape relative to a neutral reference curve, not the loudness itself. N means Neutral, closely following the balanced reference shape with no significant rumble or hiss. R indicates Rumble, meaning low-frequency bands exceed the neutral reference enough to be perceptible as objectionable low-frequency noise. H indicates Hiss, meaning high-frequency bands exceed the reference. RV indicates the low-frequency excess is severe enough to potentially cause perceptible vibration in lightweight building elements such as windows or ceiling panels, not just an audible rumble.
The RC system's explicit quality assessment addresses a known weakness of NC, which can pass a spectrum with an acceptable overall rating even when it contains perceptible low-frequency rumble, since NC curves are relatively permissive at low frequencies. A room can have an acceptable NC rating and still generate persistent complaints about a rumbling or throbbing quality. RC explicitly flags this through its quality letters, giving engineers a clear signal that low-frequency-specific noise control, such as the attenuator sizing calculator can help specify, may be needed even when the numeric rating looks fine.
Recommended RC ranges are similar in magnitude to NC ranges for the same space types, but always review the quality letter alongside the number. Private offices and conference rooms typically target RC 25 to RC 35(N). Open-plan offices are often designed to RC 35 to RC 40(N). Classrooms typically target RC 25 to RC 30(N). A rating with an R or H suffix, even at an otherwise acceptable number, signals a spectral quality issue worth investigating. Use the noise criteria selector for a fuller lookup by specific space type.
Related Noise & Acoustics Tools
Worst-case Noise Criteria rating
International Noise Rating curve
Sum spectral sound data
Target levels by space type
Low-frequency silencer sizing
Velocity-generated hiss check