What is the difference between αw and NRC?

NRC is an American metric: a plain arithmetic average of four bands — 250, 500, 1000 and 2000 Hz — rounded to 0.05. It doesn’t use a reference curve, ignores 4000 Hz, and has no classes. It can come out numerically close to αw, but that’s coincidence, not a conversion: two data sheets, one quoting NRC and the other αw, cannot be compared directly.

Why is αs on a data sheet sometimes greater than 1.00?

Because the sample in the reverberation chamber under PN-EN ISO 354 also absorbs through its edges, while the result is expressed relative to its plan area. The material doesn’t absorb more than 100% of the energy — it simply acts over a larger area than its outline suggests. In classification, values above 1.00 are capped at 1.00, which is exactly why class A ends there.

What do the letters in αw = 0.90 (H) mean?

It’s a shape indicator. It is added when the actual curve exceeds the shifted reference curve by 0.25 or more: L for 250 Hz, M for 500 and 1000 Hz, H for 2000 and 4000 Hz. Read it as a warning that absorption is uneven — a material marked (H) damps high tones more than the rest, which in a room is heard as muffling with the low-end rumble still present.

Is class A always the best choice?

No. The goal is the target reverberation time, not the highest class — in a rehearsal room, an assembly hall or a sacred interior , too much absorption kills the sound just as effectively as too little. There’s also the arithmetic: sometimes it is cheaper and better to cover a larger area with class C than a smaller one with class A. That is settled by the acoustic design , not the class table.

For designersFor clients

Absorption class A doesn’t mean it will be quiet

What the A–E symbol and the αw coefficient on a product data sheet actually tell you, and what they don’t show — including a frequency band the standard doesn’t take into account at all.

Opublikowano 3 July 2026

For designers and specifiers: how to read a declared absorption class and what to add so the figure on the data sheet is comparable.

Every acoustic ceiling data sheet carries one number everyone looks at: the absorption class. Convenient, because it is unambiguous — A beats C, end of story. The trouble is that this number is produced by deliberately flattening a dozen-plus measurement results into a single symbol, and the flattening loses exactly what decides the outcome in a difficult room.

The mechanism is simple and worth knowing, because it explains every surprise that follows. The material is tested in a reverberation chamber under PN-EN ISO 354, results are averaged into octave bands from 250 to 4000 Hz, then a reference curve from PN-EN ISO 11654 is overlaid and shifted in 0.05 steps until the sum of the unfavourable deviations fits within 0.10. The value of the shifted curve at 500 Hz is αw — and only that value sets the class.

In other words: one point at one frequency, set so the rest of the curve doesn’t stray too far from it. Below: the class table, three things the class doesn’t show, and a paragraph to paste into a specification.

Five classes and one unnamed category

Sound absorption classes under PN-EN ISO 11654 — αw range and what it means in a room
Class	Coefficient αw	What it means in practice
A	0,90–1,00	Almost complete absorption in the speech band. The standard choice wherever reverberation needs to be extinguished, not just shortened: classrooms, open-plan offices, sports halls.
B	0,80–0,85	Close to A, but 15–20% of the energy comes back. In a small room you won’t hear the difference, in a large volume you will.
C	0,60–0,75	About a third of the energy is reflected back. A sensible choice where reverberation should be shortened rather than silenced — and where area can make up for the class.
D	0,30–0,55	A finishing material that happens to absorb something along the way. As the sole treatment in a room, it won’t solve the problem.
E	0,15–0,25	The lower limit at which the standard still declares anything at all. Treat it like a hard surface with a small margin.
—	poniżej 0,15	An unclassified product. The sales sheet may call it “acoustic” — the standard doesn’t call it anything at all.

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Three things the class doesn’t show

The class describes a square metre, not the room
Equivalent absorption is a product: area times coefficient. A hundred square metres of class A ceiling (αw 0.90) gives 90 units, but two hundred square metres of class C (αw 0.65) gives 130 — and that second room will ring for less time, despite the “worse” class. A premium island covering only part of the soffit regularly loses to a cheaper ceiling covering the whole area. Class is one of two factors, and not the bigger one.
αw doesn’t see the 125 Hz band
The rating covers the 250–4000 Hz bands. Everything below that — a ball hitting the parquet floor, bass from a sound system, ventilation rumble, music in a fitness club — plays no part in the class. In a sports hall that happens to be exactly the band left over once everything else is damped, and a thin build-up close to the soffit copes with it worst of all. If low frequencies matter, ask for αp at 125 Hz separately: it is on the data sheet, it just doesn’t feed into the symbol.
The same tile has several different αw values
The coefficient depends on the space above the ceiling — the same cladding suspended 200 mm below the soffit absorbs differently from the same product glued at 25 mm, because the cavity works together with the material. That is why a class without a stated cavity depth (o.d.s.) isn’t comparable to another class; system manufacturers say so explicitly, because this is the most common source of mismatch between quotes. When comparing two data sheets, first check that they describe the same installation method.

To paste into a specification

Three sentences that settle the argument over whether quotes are comparable.

Suspended ceiling with sound absorption class ____ (A–E) under PN-EN ISO 11654, confirmed by a data sheet with αp values in the 250–4000 Hz octave bands.

The declared class applies to a cavity depth (o.d.s.) of ____ mm. Solutions with a different cavity depth require the class to be reconfirmed.

Required absorber area: ____ m², derived from a reverberation-time calculation under PN-B-02151-4. For rooms where the band below 250 Hz matters (sports halls, sound-reinforced venues, fitness clubs), also state αp at 125 Hz.

Najczęściej zadawane pytania

What is the difference between αw and NRC?
NRC is an American metric: a plain arithmetic average of four bands — 250, 500, 1000 and 2000 Hz — rounded to 0.05. It doesn’t use a reference curve, ignores 4000 Hz, and has no classes. It can come out numerically close to αw, but that’s coincidence, not a conversion: two data sheets, one quoting NRC and the other αw, cannot be compared directly.
Why is αs on a data sheet sometimes greater than 1.00?
Because the sample in the reverberation chamber under PN-EN ISO 354 also absorbs through its edges, while the result is expressed relative to its plan area. The material doesn’t absorb more than 100% of the energy — it simply acts over a larger area than its outline suggests. In classification, values above 1.00 are capped at 1.00, which is exactly why class A ends there.
What do the letters in αw = 0.90 (H) mean?
It’s a shape indicator. It is added when the actual curve exceeds the shifted reference curve by 0.25 or more: L for 250 Hz, M for 500 and 1000 Hz, H for 2000 and 4000 Hz. Read it as a warning that absorption is uneven — a material marked (H) damps high tones more than the rest, which in a room is heard as muffling with the low-end rumble still present.
Is class A always the best choice?
No. The goal is the target reverberation time, not the highest class — in a rehearsal room, an assembly hall or a sacred interior, too much absorption kills the sound just as effectively as too little. There’s also the arithmetic: sometimes it is cheaper and better to cover a larger area with class C than a smaller one with class A. That is settled by the acoustic design, not the class table.

Sources

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