Sound Absorption classes are designated A to E, class A has the highest sound absorption. and E the Lowest. In addition should also show the specified overall dept of system (o.d.s) this must always be stated for a given sound absorption class.
The ASTM standard C 423 specifies two different single values of sound absorption, the NRC (Noise Reduction Coefficient) and the SAA (Sound Absorption Average). Both values are calculated as an average over the frequency ranges (250-2000 Hz and 200-2500 Hz respectively)
But the NRC is not the whole story it just gives an overall indication to the materials absorption properties. In the case of acoustically treating a room you may need to be more selective than this. The document where the NRC data is derived from will also show the materials absorption at many different frequency responses. This is where you can see what the material will do at the frequencies you need to tackle.
In the case of acoustically treating your room you may not always want a highly absorbing foam tile. For example if you want to take some reverberation out of the room yet still keep some liveliness a thin foam tile with a class D rating might be just what is needed to take the highs out. A general rule of thumb is the thicker the tile the better it will deal with mid’s and lows. however highs will be dealt with just as well using a thin tile.
Below are the detailed independent NRC test results for our acoustic foam tiles and bass traps just click on the text to download the document