resonating wood behind the foam?

[bozmillar]

Has any one tried this? I'm going to be building some acoustic panels in the next few weeks. Looking around here, it seems that putting a thin sheet of wood will help to absorb low frequencies better because the wood resonates with the low frequencies. The only issue is that the high frequencies reflect right off the wood.

What if I put the wood sheet behind the absorptive material? Any high frequencies are absorbed when they hit the insulation, and the low frequencies that pass right through can resonate the wood and lose some energy.

Has anyone tried this? Any reason why it shouldn't work the way I'm thinking?

[garageband]

Sounds valid to me.

[Ethan Winer]

Originally Posted by bozmillar

Any reason why it shouldn't work the way I'm thinking?

Yes, several.

For porous absorbers (foam, fiberglass), the material needs to be thick enough to absorb down to a low enough frequency, and the material works better when spaced off the walls or straddling corners. This is the standard arrangement, and in that case adding a wood backing only reduces the absorption.

There is a type of trap that uses a wood membrane:

Build a Better Bass Trap

But that type of trap needs to be sealed airtight and damped internally to work. Just sticking a sheet of plywood out in the air does not absorb very much.

--Ethan

[bozmillar]

ok, I think I get it. But what I'm thinking is that if you add a porous absorber in front of the membrane, the low frequency junk would pass through the porous layer then onto the membrane. What if the membrane was airtight like you explained in your article? (I'm looking at this picture: http://www.ethanwiner.com/basstrap.gif just backwards.)

I guess I'm not taking into account the role that the foam in between the wall and the membrane has.

So what I would really need is foam->membrane->foam->wall, with sufficient space in between each layer for it to work effectively? Or does the foam actually have to be between the wall and the membrane? Isn't the foam just there to dampen the vibrating membrane? As long as it's sealed, would it serve the same purpose (plus absorbing high frequencies)?

I'm just sort of spatting stuff out here.

[bozmillar]

wait wait wait. I was just reading through this: Acoustic Treatment and Design for Recording Studios and Listening Rooms and you mentioned that panel absorbers don't need to be distanced from the wall to be effective. I guess there is still the question of how to seal it.

[Bigduggieface]

Hey boz,

Be sure to keep us informed on what you choose to do with your absorbers and why.

I just bought some rigid fiberglass and I'm about to construct some absorbers of my own. I'll be watching this thread like an empathetic hawk, and learning from your mistakes.

Maybe I'll start a new thread with my own specifics and we can compare notes.

[bozmillar]

I will. It will be a few weeks off. I'll be moving into the new house, starting a new job and all that good stuff, so I'm not sure how long it will be until everything settles and I can put the time into doing it.

I'm still in early planning stages, but I think I have a pretty good idea of what I want to do. As always, I'll be spending as little as humanly possible.

[Ethan Winer]

Originally Posted by bozmillar

I guess there is still the question of how to seal it.

The complete plans are in the article I linked above. However, for most rooms I suggest conventional porous absorbers rather than the system of low/mid/high traps which work better in larger rooms.

--Ethan

[bozmillar]

Hey, cool, thanks.

Here's a picture of the room I'll be using: http://www.bozrecords.com/otherFiles/IMG_2985.JPG The carpets will be ripped out and replaced with wood floors. I liked the idea of not being in a closed room, especially if I'm going to be recording drums. I'm a little worried that with wood and tile floors the room will be too live sounding, but I can at least lay some rugs out to dampen that a little.

I don't plan on adding any acoustic treatment to the larger room, just the dining room where I will be mixing.

In your experience, do you have to add more or less low frequency absorption when you have a small room that opens up into a large room? My thought is less, but then again, I've never set up a large room before.

[garww]

Much of this is like designing a speaker enclosure - well that is what a room becomes : ) For reference, the box is rigid, and, with the old Wharfdale boxes - filled with sand. That's is not directly translated, but be careful panels.

I've actually use a lot of foam peanuts since the '70s, but I have no direct comparison to know if they are worth using .