αw and NRC are single-number ratings of how much sound a material absorbs, both running from 0 (fully reflective) to 1 (fully absorptive). αw is the UK/European rating (BS EN ISO 11654), derived from ISO 354 lab tests; NRC is the North-American average of absorption at 250, 500, 1000 and 2000 Hz.
What the numbers actually measure
A material's real behaviour is a curve, not a number: it absorbs different amounts at different pitches, usually much more at high frequencies than low. The full picture comes from a lab test to ISO 354, which measures the absorption coefficient (α) in each one-third-octave band, typically from 125 Hz to 4000 Hz.
Because a whole curve is hard to compare across products, two single-number summaries are used. Both compress that curve into one figure between 0 and 1 — but they do it differently, which is why a product can quote both and the values won't be identical.
αw — the UK and European rating
αw (the weighted sound absorption coefficient) is defined by BS EN ISO 11654. A reference curve is shifted against the measured values until it fits, and the point it lands on gives αw, reported in steps of 0.05. The same standard also assigns a sound absorption class from A to E — Class A is αw ≥ 0.90, the most absorptive.
αw is frequency-weighted toward the speech range, so it reflects how a material performs where intelligibility matters. It is the figure most UK and European acoustic products lead with.
NRC and SAA — the North-American ratings
NRC (Noise Reduction Coefficient) is the average of the absorption coefficients at just four frequencies — 250, 500, 1000 and 2000 Hz — rounded to the nearest 0.05, under ASTM C423. SAA (Sound Absorption Average) is its more detailed successor, averaging twelve bands from 200 to 2500 Hz.
NRC and αw usually land close to each other, but they are calculated from different bands and rounding rules, so treat them as related — not interchangeable. If a specification calls for one, quote that one from the test report rather than converting.
How to read a panel's figures
A credible absorption figure traces back to a named ISO 354 test report for that specific construction — including the mounting method and any air gap behind the panel, both of which change the result significantly. A number with no report, or one copied from a different build-up, is not evidence.
For a room, the single number is only a starting point. What controls the result is the total absorption you add relative to the room's volume — which is what a reverberation calculator estimates. For a regulated space such as a school under BB93, the per-frequency data and a qualified acoustician matter, not the headline figure alone.
Frequently asked questions
Is a higher αw always better?
For reducing reverberation, yes — a higher αw means more absorption. But acoustic design is about hitting a target reverberation time, not maximising absorption; an over-damped room can feel dull and make speech sound weak. The aim is the right amount in the right places.
Can I convert NRC to αw?
Not reliably. They are calculated from different frequency bands and rounding rules, so any conversion is an approximation. If a specification asks for αw, quote the αw from the ISO 354 report; if it asks for NRC, quote that.
Why does the same panel have different figures?
Because the mounting changes the result. The same panel tested directly against a wall and tested with an air gap or insulation behind it will absorb differently, especially at low frequencies. A test report always states the mounting it applies to.
What does an αw of 0.65 mean in practice?
It means the material absorbs, on the weighted average, about 65% of the sound energy that reaches it — a solidly absorptive panel. Class-wise it sits around Class C to D under BS EN ISO 11654. Whether it is enough depends on the room's size and target reverberation.