I am in the process of finishing a sound isolated room, it is a double wall set-up. (box inside a box) From what I understand, any noise not blocked by the first wall will enter the cavity and bounce around between both hard surfaces so some means of absorption within the cavity is necessary. A few questions, the first is why. Why is it so important to not have sound bounce around, since I am not recording in that space how would it affect my recording? The second, if I actually need it, what possible material could I use? I live in a third world country and we don't have typical building supplies and ratings about sound absorption that most are familiar with. The general idea is to fill the space with fiberglass insulation but I cannot buy any here, even if I could find it, it would be too expensive to use. I've considered using styrofoam, because I can buy it, but others have said it isn't good because it too reflects sound. Can someone shed some light on this area for me?