Hi Matt: I’ve just finished my first DIY lean diffusor designed by Tim Perry (Arqen). I was going to make a more finished version and take final room measurements before posting on his site and AV NIRVANA but since you asked.
There doesn’t appear to be a consensus as to whether the First Reflection Point (FRP) should be treated with absorption or diffusion. The “Master Handbook of Acoustics” recommends absorption but I agree with Nyal Mellor and others that absorption is not the automatic default option - especially in a passive listening environment.
The location of this diffusor is in a living room listening space and not a studio control room. Therefore, it has some constraints regarding decor and size. The use of a diffusor was in part to deal with asymmetric speaker placement and differing FRP surfaces - windows vs. dry wall. The window side could benefit more from a diffusor. I was considering a clear SMT winged version but the architectural committee nixed that.
Does it change the sound? Yes, a little. It shifts the sound stage to the center some and gives the vocals a little more clarity. In terms of REW measurements, I see a difference in the Distortion Graph and the Waterfall Graph. However, almost anything changes those graphs, like moving furniture around, so its not clear to me if those measurements are significant.
Tim Perry says its technically daunting to try to measure the coefficient performance of a diffusor but it seems like we should be able to measure the over all results. What is your view?
regards,
Mike W
Hey Mike,
Thanks for sharing, those look great. Out of curiosity, how long did it take you to build these? I have the plans for them downloaded and I've read the articles pretty thoroughly. Seems like a great design, very space efficient. My take has been that it works its magic when used in large groups by extending the LF diffusion down some, but still works quite effectively in singles as well. The ideal minimum group is 5 which is hard, I don't have room for that, and looks like you don't either.
As for measuring, no I don't think you can measure the results of a diffuser in a room. I've read the articles by most of the topic experts on how to measure diffusion and what it is you are actually measuring. From that, I gather that it simply would not be possible to measure the effect of diffusers in a normal room with any certainty. There is little reason for it to reduce distortion, so if it is, I would question what that is. If the distortion reduction is all near or at the noise floor of the room or measurement, all the more reason not to trust it. As for decay, I'm of two minds. On one hand a pure diffuser should not have any effect on RT60 which means it should have no effect on decay. However, diffusion means taking a large soundwave and reflecting it in smaller components in many random directions. The total energy doesn't change, in theory. In practice air and existing absorption will absorb the sound and the diffused waves would be more easily dissipated, so I think there is something to be said for a diffuser allowing existing absorption to more effectively dissipate the energy (by spreading it across more of the absorption in the room in any given moment) and so I do think that would improve decay.
However, as a whole, I don't expect it to be possible to measure diffusion in a room. The way in which diffusers are measured is to take a diffuser and place it in a controlled ultra low noise space. Preferably an anechoic space to avoid other reflections from corrupting the measurement (but a reflective room could be used if large enough). Then placing many mics in an arc around the diffuser at a given space (Say every 2.5 degrees). In line with the center mic (roughly) is a sound source. That sound is reflected off the test specimen and each mic picks up sound waves. This is done with and without the diffuser in place. A calculation is used that creates a kind of difference score which reflects the diffusion at a given frequency. This is then averaged for the overall specimen diffusion coefficient.
I know a lot of folks have taken the view that if it can't be measured in their room then it isn't audible. This is really not true, mic's don't measure like our ears and their noise and distortion, their frequency response, and the way they pick up sound is both different and worse than our own hearing. There are lots of things that average joe cannot measure that we can readily hear. Diffusion is one of them. Diffraction is another (in all fairness average joe can measure this if he knows how and has the right environment to do it).
