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In Reply to: RE: DIY acoustic panels questions posted by temporarymadness@aol.com on February 02, 2008 at 22:04:03
I tend to be against hybrids if you don't understand what you're doing. In this case you're actually combining 3 very different ideas.
The Jon Risch trap uses the paper backing of the fibreglass as a light membrane that's directly damped and Ethan uses the wood panel as a membrane that isn't directly damped but which is acoustically damped by the insulation which is spaced away from the membrane and is not in contact with it. By putting the backed fibreglass right up against the plywood, you're ensuring that plywood and the paper backing membrane mutually damp each other and that has to lose efficiency because you're going against the idea behind both of them as far as I can see.
I don't know what the idea behind the pegboard is in the third panel. The instructions I've seen for use of perforated boards also space it away from the insulation in a similar manner to what Ethan does with the plywood, and the perforations and space behind then act as a Helmholtz resonator with the insulation helping to give a more broad band response. While the insulation will damp the pegboard in this case so it won't act as a diaphragm like Ethan's plywood, the holes will allow sound to travel into the insulation where it will be absorbed. I suspect the real absorption, however, is being done from the other side and the pegboard may just be providing a stiff backboard for compression to increase the density of the insulation and allow it to absorb to lower frequencies.
Unfortunately a simple fact of life with these designs is that while they all may work, they don't all work in quite the same way. If you mix approaches you may certainly come up with something that combines the benefits of the different approaches, but you can just as easily—probably more easily—come up with something that negates the benefits of each individual approach and that is not what you want. Following a single set of instructions is a lot safer, is probably going to be more effective, and if you run into problems the person who provided the instructions will be in a better position to provide assistance.
I'd go with either Ethan's instructions or Jon Risch's, and build according to the instructions. Don't mix the plans. I wouldn't try the pegboard approach because I have no idea what the person who designed it intended to do with that approach but it isn't like the instructions I've seen in Everest's Master Handbook of Acoustics. Ethan, Jon, and Everest are all professionals working in the sound area with a sound knowledge of acoustics and at various stages I've learned useful things from all 3 of them.
David Aiken
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