# Back wall treatment



## atledreier (Mar 2, 2007)

Hello again!

I have some challenges in my room, and been reading up on alot of stuff.

First of all, I came across the Porous Absorber Calculator by Chris Whealy. Sat fiddlign with it to try to see what configurations would be effective for the materials I have. Turns out that my 4" (10cm) 2'x4' panels should give pretty good and linear absorbtion quite low with a sizeable air gap. Hardly news, I know, but seeing the graph change on my screen was revealing. So, thinking about how to get that air gap most efficiently and without interfering with my room too much.

The back wall has the subs (sonotubes) and projector is ceiling mounted as far back as I can get it. I have corner chunks floor to (almost) ceiling. I have curtains hung about 2' from the rear wall across the room to hide electronics, projector and treatments. So, between the subs and up to the level of the projector I have an empty area of back wall that is covered by thin velour fabric. If I stack 4" panels about 15"-18" from the back wall, I could effectively get an absorber that was 8' wide and 4' tall with a 15" air gap. The calculator says this configuration should have an absorbtion coefficinent of 0,5 at 50Hz, and tapering fairly linear to 0,28 at 20hz. Thsi is at normal incident. At an angle it gets even more efficient, I guess because the effective thickness is increased. At 45 degrees the calculator estimates abs. at 0,65 at 50hz, and 0,4 at 20Hz. Is this just wishful thinking? There would be open sides and top, would I need to close that off with some sort of sheet material?

Continuing the playing around, I though "what happens if I double up the panels?". Well, I tried 11" gap and 8" panels, and the efficiency plummeted! I have no idea why, but I have a hunch that it's just too much mass in the material to be efficient or something? I guess it has to do with the mystery parameter 'Flow resistivity'? In my mind that is basically how dense the material is, right? So a lighter material will do better in a very thick absorber.
So, by that reckoning, a panel straddling a corner should actually be MORE efficient than a corner chunk, because you have the less thick material and the airgap, instead of maybe 15"-20" of material at the deepest part of the trap.

Edit:
Playing around some more, trying to find an effective use of my thinner panels, I wasn't expecting much. I tried them in my back-wall experiment, and WOHA! 0.83 coefficient at 80Hz! The tradeoff is much less linearity to the absorbtion, so it would have to be combined with other thicknesses and gaps around the room to take care of that. The thicker panels are much more linear.


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## Dale Rasco (Apr 11, 2009)

Have you been taking readings that you can post as far as frequency response goes?


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## atledreier (Mar 2, 2007)

Not really. I have some measurements, but Bryan and a few of the wizards over at gearslutz thought there had to be something wrong with them, and I haven't got around to redoing them.


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