Re: What material to stuff inside drywall cavbity?
Everyone is forgetting the most important item in sound isolation--mechanical isolation. That is, it is absolutely vital to make sure the outside wall is not connected to the inside wall. Normally, both walls are nailed to the same studs. But then, that means when one wall moves, the other moves an equal amount. To resolve that problem, do not nail both walls to the same stud--stagger the stud alignment and attach the walls to alternate studs. That way, the movement of one wall will not be translated to the other wall. Then when you add the sound insulation between the walls you have "relatively complete" sound isolation without affecting the acoustic characteristics of the room. That allows you to fine-tune the room's sound.