sound insulation

I'm planning to sound-isolate an already-converted garage built on concrete slab. I want to modify the walls to have mass-air-mass with two layers of drywall inside and out.

Hi
I'm doing some research on converting my garage/basement (the house is on a hill so the back wall of the garage is essentially underground) into a rehearsal/listening/recording room.

I've been looking at 3 different wall systems from British Gypsum