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Old 4th November 2010
Lives for gear
Jens Eklund's Avatar

When looking at data derived from reverberation chambers, it’s more or less pointless to care about absorption coefficients below approx 100 Hz for two reasons; if not a huge room was used, the limit of reliable data will be around 100 Hz. Even if the data is “accurate” at 80 Hz, the absorption coefficient is valid only for random incident and a normal control room is not a diffuse field below approx 150-300 Hz depending on volume. Room modes, that totally dominate the “modal region” (and that’s the reason for the name), are not random incident. Another issue with reverberation chambers is the edge effect, causing absorption coefficients above 1.0. If “true” absorption coefficients is needed for normal incident, the most reliable test procedure is the impedance tube. Unfortunately, the testing of pressure absorbers is a bit difficult to do in these since the tuning sometimes is related to the dimension. Testing porous material is on the other hand, very simple in an impedance tube but the values will naturally not look as “good” compared to the reverberation room. I wonder why some manufacturers of porous “bass traps” frequently use the latter method even though it’s a lot more work and gives less reliable results …