Sound absorption classes A to E: what they mean

The rating follows fixed αw limits from Annex B of the standard: class A covers αw 0.90 to 1.00, class B 0.80 and 0.85, class C 0.60 to 0.75, class D 0.30 to 0.55, class E 0.15 to 0.25. Values of 0.10 and below remain without a class. Because αw is always rounded to steps of 0.05, the boundaries are sharp: a product with αw 0.85 is class B, one with 0.90 is class A.

The slider below shows the mapping interactively: set αw and read off which class a product lands in and how wide the classes are. The large span of classes C and D stands out; two products of the same class can differ noticeably.

It starts with a reverberation room measurement to ISO 354, which delivers the absorption coefficient αs per one-third-octave band. From these, practical absorption coefficients αp per octave band (250 to 4000 Hz) are formed. A standardised reference curve is then shifted in steps of 0.05 until the sum of the unfavourable deviations is at most 0.10; the value of the shifted curve at 500 Hz is αw.

Important for comparison: the value applies to the measured build-up including the mounting type. A ceiling tile measured with a 200 mm cavity (mounting type E-200) can perform much worse when glued directly. Class and αw without the build-up are therefore only half the information.

The classes look abstract, their effect on area is not. The equivalent absorption area of an absorber is area times absorption coefficient. A class A product with αw 0.90 delivers one and a half times the effect of a class C product with αw 0.60 per square metre. Put the other way round: for the same reverberation time, the C product needs fifty percent more covered area.

This often shifts cost calculations unexpectedly: a more expensive A product can be cheaper per square metre of effect than a nominally cheaper C product once mounting and limited surfaces are counted in. On ceilings in particular, whose area is already cut down by luminaires and services, the effect per square metre counts twice.

For most rooms dominated by communication (office, meeting, school, nursery) highly absorbent ceilings are the standard: products of classes A and B are usually used there, because the ceiling carries the main share of the absorption and the requirements of DIN 18041 are otherwise hard to reach economically.

Classes C and D have their place as supplementary surfaces: wall absorbers against flutter echoes, design elements, robust claddings in corridors and halls. What matters is the sum of the equivalent absorption area in the room, not the class of every single element. A room can be better off with a strong A ceiling and a few C wall elements than with B material throughout.

αw and class rate the spectrum with a single number weighted towards the speech range. Low frequencies are systematically lost in the process: a porous absorber can reach class A and still do little at 125 Hz. For rooms with low-frequency problems (music, conferencing with media technology, studios) a look at the band values αp or αs is therefore mandatory.

The standard has shape indicators for this: if the measured curve exceeds the shifted reference curve clearly in individual ranges (by 0.25 or more), an L, M or H is appended to the αw, for example αw 0.60 (H). That signals additional performance at low, mid or high frequencies that the single number does not show. The product data on Acoustic Index always contains the full frequency values, so products can be compared beyond the class.

In specifications the combination has proven itself: required absorption class or minimum αw, plus the mounting type (with cavity depth for ceilings) and, for acoustically critical rooms, minimum values per octave band. That keeps the competition open without letting a weak product slip in through a favourable measurement set-up.

The test report of the ISO 354 measurement serves as evidence. Reputable manufacturers provide it; in the Acoustic Index catalogue αw, class, mounting type and band values are stored and filterable per product, so candidates for a specification can be narrowed down in minutes.