This method tests the ability of an element to absorb the sound energy emitted by the source. The evaluation index expressed is the weighted sound absorption coefficient according to UNI EN ISO 11654:1998 is αw, which is assigned a class as follows:

Class: A / αw – Values 0.90; 0.95; 1.00 / Absorption class: highly absorbent
Class: B / αw – Values 0.80; 0.85 / Absorption class: highly absorbent
Class: C / αw – Values 0.60; 0.65; 0.70; 0.75 / Absorption class: highly absorbent
Class: D / αw – Values 0.30; 0.35; 0.40; 0.45; 0.50; 0.55 / Absorbency class: absorbent
Class: E / αw – Values 0.15; 0.20; 0.25 / Absorbency class: poorly absorbent
Class: unclassified / αw – Values 0.05; 0.10 / Absorption class: reflective

The test is carried out in a reverberation chamber, emitting and switching off the source. The reverberation time, i.e. how long it takes to go from emission to detected background noise, of the empty reverberation chamber is assessed. Then the material is installed and the test is carried out again, verifying the variation.

For example, in a material that has an αw of 0.7, at 100 per cent of the emitted sound energy the material absorbs 70 per cent of it.