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Looking for info on injecting acoustical foam into an existing wall cavity.
Tx.
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There is basically two type of spray insulation. Open cell and closed cell.
If I were to do it again..and could afford it (my listening room/home theater is in my basement) I would use spray foam,especially in the floor joists. If you are using it for acoustical properties you want to use open cell. For construction in areas like rim joists and the such you would use closed cell.
It downside is that it is kind of a messy project and the process doesn't lend itself to retrofits. You want open wall/joists, all wires and plumbing prerun. Basically new or remodel construction. The full cavities make fishing for wires later almost impossible.
It is more expensive then batt insulation but insulating areas above your head with fiberglass is downright hateful. Even if you insulated your bays you will still need to acoustically treat your room. I do have to disagree with Jon on the effectiveness of the spray foam. If you decide to utilize spray foam (especially open cell) it will be extremely effective in isolating the room. I would of loved to used it on all the walls and ceiling but it was cost prohibitive. Good luck!
Any type of foam that you spray (or inject) is almost certainly going to be a closed cell foam. Closed cell foam does not absorb mid and HF sound very well, if at all.
Acoustic foam is open cell foam, and the foam cell size needs to be within a certain range, or it does not absorb as well as it could.
If you are trying to help block sound from one room to another, injecting polyurethane foam meant for heating and cooling insulation into the wall spaces WILL NOT provide that much sound blockage.
Sound isolation efforts are different than sound absorption efforts.
Sound isolation involves such things as air-sealing the room, so that sound can not get out under the door, or out through a window, or easily through a heating or cooing vent (in the case of an air vent, you create a sound absorbing barrier, so the sound waves have to go around a barrier that is lined with absorption material, sort of like a muffler).
In terms of isolating sound from one room to another that has a common wall, decoupling, mass loading, and structural isolation are the techniques used.
Trying to "sound proof" an existing wall is often a futile exercise, as it just does not have the proper decoupling and mass, etc. These normally need to be built-in to the wall from the beginning.
About the only thing you can do, is to add sheer mass that is decoupled from the existing wall, and this would involve using special wall studs, additional sheets of drywall and/or plywood, along with foam strips at the top and bottom to air-seal the new wall surface from the existing wall.
Even then, after all of the expense and labor, it still might not reduce the sound as much as you would want or need.
Jon Risch
All of the acoustical foam I've heard about comes in sheets or blocks, they aren't a spray product.
I think the foams that are sprayed into wall cavities are intended for thermal insulation. They may have sound absorbing properties, after all the fibreglass insulation used for most DIY acoustic treatments is also intended for thermal insulation, but you'd probably need to approach the manufacturers to find out what the sound absorbing properties of the various products are.
I assume that if you want to put the foam into wall cavities your aim is sound proofing, ie stopping sound travelling from one room to another, rather than acoustic treatment of the listening space. You probably should read something like Everest's "Master Handbook of Acoustics" which has a good treatment of soundproofing strategies. Treating walls only may well not be enough for good soundproofing because sound can still travel to adjoining rooms via floor and ceiling cavities. Foam may also be rather ineffective at reducing the transmission of bass frequencies which can be quite difficult to block.
David Aiken
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