Sound classification

The material’s ability to absorb sound is generally presented with absorption coefficients measured in different frequencies. This means in practice that one material has a number of different absorption coefficients based on frequencies.

When specifying a suitable absorption class for each product, a product is first measured according to EN ISO 354. The result from this test is an absorption curve, which declares absorption coefficients for each frequency used in the test.

A weighted sound absorption coefficient (αw) is a result from comparison between the sound absorption coefficient values at standard frequencies and reference curve in accordance with ISO 11654. The αw is communicated by all suspended ceiling suppliers in Europe as it is the method which has been adopted as the norm for CE marking of suspended ceilings.

EN ISO 11654 is also used to classify the sound absorption materials based on the measured absorption curves to categories from A to E. Class A has the best ability to absorb sound, and E has the weakest. The installation method together with material properties has a great impact on the result.

This classification system helps designers to compare and select the suitable absorption material for different purposes. Stone wool-based acoustic slabs are generally the best A class.


Example: When comparing the different products on the basis of their absorption class.

Product 1 has absorption class A and product 2 absorption class C with the same installation method. This means that the absorption coefficient of product 1 is about 50 % better than product 2. The task is to design a kindergarten playroom with a reverberation time of 0.6 seconds. After calculating the amount of needed absorption material we can see that room dimensions are such that product 1 has to cover the whole ceiling of the room, and also a large part of the walls, to achieve the required reverberation time. If we choose product 2, we have to use one and a half times the amount of product compared to product 1 in order to achieve the same reverberation time. In practice, the situation leads to the fact that there is not enough mounting platform available for product 2.


αw  Sound absorption class
1.00–0.95–0.90  A
0.85–0.80  B
0.75–0.70–0.65–0.60  C
0.55–0.50–0.45–0.40–0.35–0.30  D
0.25–0.20–0.15  E
0.10–0.05–0.00  Not classified

Sound absorbing classification