Acoustic absorption - ISO 11654


Sound absorption classes according to ISO 11654L = low frequenciesM = middle frequenciesA = high frequenciesαp = practical acoustic absorption coefficientαs = frequency-related sound absorption coefficientIn the case of the example, the weighted sound absorption coefficient αw = 0.70 (MH) indicates that the material reaches its maximum efficiency in the medium and high frequencies.
Reverberation room according to ISO 354Type G mountingIt's mainly used for testing fabric curtains.The test specimen is hung parallel to the room surface.The numeric suffix (e.g. Type G-100) is the distance in mm from the test specimen to the room surface.The test specimen can be:- flat- with fullness (with different percentages)C. Room surfaceT. Test specimenG. Mounting Type G - FlatA. Airspace behind the specimenS. Dodecahedral sound sourceD. DiffuserM. Microphone
A wall in a common room is normally reverberant, that is, it does not have acoustic absorption properties and its weighted acoustic absorption coefficient is practically αw 0.
On the contrary, a 100% absorbing material has a coefficient αw 1.00.
The coefficients are measured by means of a reverberation room.

αs measures the frequency-dependent sound absorption in a reverberation room, it is used to measure the practical sound absorption coefficient αp.
αp is the practical acoustic absorption coefficient and it is frequency-related.
αw measures the weighted sound absorption regardless of frequency with classes going from A (maximum absorption) to F (reflection) to which the following indicators in brackets are also attributed:
- (L) if the material is efficient in the low frequency range, e.g. αw = 0.60 (L)
- (M) if the material is efficient in the mid-frequency range, e.g. αw = 0.70 (M)
- (H) if the material is efficient in the high frequency range, e.g. αw = 0.85 (H)
Combinations are possible, e.g. αw = 0.70 (MH) or αw = 0.60 (H).