Definition
Sound absorption is described by a frequency-dependent coefficient α (see Sound Absorption Coefficient). Because a full curve is inconvenient to compare, three different single-number ratings are derived from it: NRC, SAA and αw. They are not the same thing and should not be used interchangeably.
α — The Sound Absorption Coefficient
α is the frequency-dependent ratio of absorbed to incident sound energy at a single frequency. It is not one fixed number — it is a value at each frequency band (for example, α = 0.30 at 250 Hz and α = 0.85 at 2000 Hz). All single-number ratings are computed from this curve.
NRC — Noise Reduction Coefficient
NRC is the arithmetic average of α at the four octave bands 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05. It is defined in ASTM C423. NRC is not a percentage of sound absorbed — it is a simplified average over four specific frequencies.
SAA — Sound Absorption Average
SAA is the arithmetic average of α over the twelve one-third-octave bands from 200 to 2500 Hz, also defined in ASTM C423. Because it averages more, narrower bands, SAA is a finer summary than NRC — but it is not equal to NRC; the two can differ by a few hundredths for the same material.
αw — Weighted Sound Absorption Coefficient
αw is the weighted single-number rating defined in ISO 11654. It is obtained by fitting a reference curve to the measured α spectrum (125–4000 Hz) and reading a single value, which is then assigned an absorption class (A–E). αw is a weighted rating — not a plain average, and not the same as NRC.
Comparison Table
| Parameter | Meaning | Unit | Frequency basis | Main standard / use |
|---|---|---|---|---|
| α | Absorption coefficient (ratio of absorbed to incident energy) | Dimensionless | Single frequency / each band | ISO 354 / ASTM C423 — measured |
| NRC | Average of α at 250/500/1000/2000 Hz | Dimensionless | 4 octave bands | ASTM C423 — rating |
| SAA | Average of α at 12 one-third-octave bands (200–2500 Hz) | Dimensionless | 12 one-third-octave bands | ASTM C423 — rating |
| αw | Weighted single number with absorption class (A–E) | Dimensionless | 125–4000 Hz (reference curve) | ISO 11654 — rating |
Frequency Dependence
All of these ratings reduce a frequency-dependent curve to a single number, so they lose frequency detail. Two materials with the same NRC can behave very differently at low frequencies. When low-frequency performance matters, review the full α curve, not just the rating.
Measurement
α is measured in a reverberation room per ISO 354 or ASTM C423, producing the frequency-dependent curve. NRC and SAA are then computed from that curve (ASTM C423), and αw is derived by the ISO 11654 reference-curve procedure. The ratings themselves are not measured directly — they are calculated from the measured α.
Relevant Standards
- ISO 354 — measurement of sound absorption in a reverberation room.
- ASTM C423 — sound absorption and absorption coefficients; defines NRC and SAA.
- ISO 11654 — rating of sound absorption; defines αw and the A–E classes.
Examples (illustrative only)
These are illustrative examples, not product values: a material might measure α = 0.85 at 1000 Hz, giving an NRC of 0.80, an SAA of 0.82, and an αw of 0.75 (class C). The three numbers are close but not identical — this is expected and normal.
Common Mistakes
- "NRC is the percentage of sound absorbed" — incorrect; it is an average over four bands.
- "NRC = αw" — incorrect; NRC is a simple ASTM average, αw is an ISO weighted rating.
- "SAA = NRC" — incorrect; different band sets and averaging.
- "αw is measured in dB" — incorrect; it is dimensionless.
- Converting "αw 0.60 → NRC 0.60" or "NRC 0.90 → αw 0.90" — incorrect; there is no direct conversion.
- "α > 1.0 means more than 100% absorption" — incorrect; it is a measurement effect of edge and diffraction in the reverberation room.
Limitations
- Single-number ratings hide frequency detail.
- Ratings depend on thickness, mounting, air gap and test configuration.
- α can exceed 1.0 in laboratory tests without meaning >100% physical absorption.
Frequently Asked Questions
Which rating should I use? NRC is the most common in North America; αw is standard in Europe (and often in the Gulf) with its A–E class; SAA is a finer ASTM alternative. Choose the one your project specification references.
Can I convert NRC to αw? No, not with a fixed formula — they use different procedures.
Why can α exceed 1.0? Edge and diffraction effects in the reverberation room can make the measured value slightly exceed 1.0; it does not mean the material absorbs more than 100% of energy.
Related Yamamah Solutions
For help interpreting absorption data and selecting materials, contact us for a free acoustic consultation.
Read more: Sound Absorption Coefficient (α) · What Is NRC? · What Is RT60? · Sound Absorption vs Sound Insulation
Verified Sources
- ISO 354 — Acoustics — Measurement of sound absorption in a reverberation room.
- ASTM C423 — Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method.
- ISO 11654 — Sound absorbers for use in buildings — Rating of sound absorption.