[quote name='leschirons' timestamp='1366616600' post='2054772']
Proper acoustic insulation is expensive. Over the last few years here, I've been involved with quite a lot of dry lining work including two rehearsal studios (mine and a friend's) I used the real stuff for mine from a local company called Eurocoustic but only because I got some B stock with damaged boxes. Rocwool is not really dense enough to make a difference as it's more designed to have good thermal protection qualities. The good stuff comes in panels and is much more dense and heavy.
As BigRedX points out, the secret is layers. I'm assuming that you'll be partitioning off say, half of your garage and that some of the walls will be brick. Best to put up some stud work, fill that with the cheap stuff, board it (10mm board will do) and then another layer of stud on top, fill that, and then board with 13mm, compacting each layer of insulation as much as possible each time.
Of course, if you're leaving any windows in the studio section, they would ideally need to be covered too but you'll never really be able to completely isolate 100% of the sound.
[/quote]
Part of the secret is the layers. It's important to decouple the plasterboard from the brickwork though, ie mount it resiliently, or the lower frequencies just go straight through. Soundstop.co.UK have loads of information (and numbers)