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Originally Posted by Roscal
Some sound absorption sheets are made of a sandwich of different density material to work on a wider spectrum.
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I've had best luck with "limp wall" sandwich foams. A layer of foam (sometimes with composite or tar to go against the metal wall and lower resonance frequency), a flexible membrane, then sound absorbtive foam. A good portion of what makes it to the membrane converts to mechanical energy and is then dissipated by the "lower" foam (and what little makes it past that has to deal with the lower resonant point of the sheet steel + asphalt). The sound that reflects off the membrane is then absorbed by the foam, if the wavelength is short enough. The problem is that, to be effective, this stuff is usually 2"+ thick - which is inconvenient for the inside of a PC case.
I used to use something called "Black Hole" acoustical foam (limp wall layered) but it seems to have disappeared off the market...(?)