Hi,
Here is a link to a calculator for this kind of case (edit: direct link doesn't work, so go to site then choose 'porous absorber calculator'):
Acoustic Modelling - Home Page
You'll have to know flow resistivity of porous material you plan to use though.
And a graph to have an idea of what flow resistivity the material may have.
Building Acoustics - Tor Erik Vigran - Google Books
Here is a link to a calculator for this kind of case (edit: direct link doesn't work, so go to site then choose 'porous absorber calculator'):
Acoustic Modelling - Home Page
You'll have to know flow resistivity of porous material you plan to use though.
And a graph to have an idea of what flow resistivity the material may have.
Building Acoustics - Tor Erik Vigran - Google Books
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Bentoronto -
The links krivium posted are an excellent first step, but they don't really explain and I think there's a small misunderstanding based upon your title.
When you put air space behind a fiberglass panel, it doesn't have a higher absorption value. The air isn't doing more or anything like that. Instead, you're really just moving the fiberglass panels to where they are effective.
Fiberglass panels are velocity based absorbers (as opposed to pressure absorbers). The velocity of a soundwave decreases as you move closer to the wall, while the pressure increases. Putting the air behind the panels has nothing to do with the air but rather that the absorber is now where there is a higher velocity for it to interact with.
Make sense? You do make a point though, if you have space in the room, use it to your advantage.
The links krivium posted are an excellent first step, but they don't really explain and I think there's a small misunderstanding based upon your title.
When you put air space behind a fiberglass panel, it doesn't have a higher absorption value. The air isn't doing more or anything like that. Instead, you're really just moving the fiberglass panels to where they are effective.
Fiberglass panels are velocity based absorbers (as opposed to pressure absorbers). The velocity of a soundwave decreases as you move closer to the wall, while the pressure increases. Putting the air behind the panels has nothing to do with the air but rather that the absorber is now where there is a higher velocity for it to interact with.
Make sense? You do make a point though, if you have space in the room, use it to your advantage.
Good point mushroommunk. Another way of thinking of it is that the material does less and less as it gets closer to the wall or boundary, so that making it thicker isn't a lot better than just moving the thinner piece away from the wall with a space behind it. Good way to save costs and weight of things hanging on the wall. Best is a sheet hanging from the ceiling away from everything, but that's not very nice decorator-wise.
Mushroommunk, Bwaslo thank you for your insight, very interesting way yo put all that into perspective. This explain in part why i'm not always sure the corrner are the best place to place absorption. There is place around all the boundary where treatments can be as or more effective given you know which freq you want to treat.
Pano,
Yes the calculator are fairly instructive about what effect have space behind a panel.
However be careful with high Q absorbtion: they can have side effect and give artefacts more annoying than the issue you try to fix ( in the lows they can give non natural behavior, dual decay,...).
Sometimes lower Q will be more effective as it can deal with multiple problematics frequency relative to mode( if the room doesn't have a 'golden ratio' you will probably have closely related mode and dealing with lower Q unit can make treatment easier and less noticeable than an high Q one. E.G. play with a high Q notch eq and a low Q one you'll hear the effect i mean ).
Pano,
Yes the calculator are fairly instructive about what effect have space behind a panel.
However be careful with high Q absorbtion: they can have side effect and give artefacts more annoying than the issue you try to fix ( in the lows they can give non natural behavior, dual decay,...).
Sometimes lower Q will be more effective as it can deal with multiple problematics frequency relative to mode( if the room doesn't have a 'golden ratio' you will probably have closely related mode and dealing with lower Q unit can make treatment easier and less noticeable than an high Q one. E.G. play with a high Q notch eq and a low Q one you'll hear the effect i mean ).
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