Posted by KlausR. (A) on February 19, 2013 at 00:50:54

In Reply to: RE: What is your question? posted by Ethan Winer on February 15, 2013 at 12:49:43

When you look at the results from round robin tests, e.g. fig. 3.2 in D'Antionio's paper, it becomes clear that reverb room methods are not reliable at any frequency.

http://www.rpginc.com/docs%5CTechnology%5CWhite%20Papers%5CState%20of%20the%20Art%20in%20Measuring%20Acoustical%20Coefficients.pdf

Everest does not mention these large uncertainties and discrepancies between labs, neither in the 4th nor in the 5th edition.

Impedance tube methods are valid for perpendicular incidence, whereas in rooms you also have incidence at an angle and random incidence. From the few stuff I’ve read I understood that measurements with incidence at an angle is done in the free field.

You are right when saying that reverb room measurements (probably) do not mimic real use conditions. But I suppose that you cannot do measurements for each and every possible mounting configuration at reasonable costs.

Everest shows a graph to demonstrate the effect of airspace behind the absorber, so maybe there is a way to simply convert the absorption data from surface mounted to spaced away condition.

Your measurements at low frequencies using waterfall plots: I suppose that the results are very much room dependant, meaning that you’d get very different plots/decay times in an empty room and in a fully furnished room, in rooms with hard walls and few openings and in rooms with flexible walls and many openings. Did you check whether or not the data are consistent in different rooms? For comparison purposes your method might work well, but does it also provide absolute data?

In 1996 the Fraunhofer Institute of constructional physics proposed a method similar to yours, i.e. in the frequency range with less than 5 eigenfrequencies/3rd octave band these individual frequencies are driven by a speaker in the room’s corner and the decay time is determined. Sabine’s equation is then used for computing an effective absorption coefficient (which is actually not correct because the sound field is not diffuse).

Klaus