Building a Practice room
what jeffrey did will work great. *i helped a friend and he didn't want to do all that to his spare room. *we basically loaded it with foam. *we bought tons of it from a home store...where they sell bed sheets and stuff. *we tacked foam bedding, about 2" thick around the perimeter of the room after applying foam sheets to the walls... about 4" thick, that we got from home depot. *we cut squares in the wall for the cold air return. *so there is about 6" insulation all around the room. *then we just took S-hooks to slightly hang a couple king-sized bed comforters from the ceiling. *it does a good job. *you can hear the drums from the outside but it isn't bad at all and it helps with the recording. *cost us about $180.
when i get my studio i would like to do it right with auralex.... $$$$.... and the floating room idea. *
i would do what jeffrey did and leave at least a 2" space between the real wall and the false wall. *and insulate that space with 1" foam throughout. *so sound would hit the auralex in the sound room, and whatever got through the false wall would hit another barrier of foam before escaping. *not to mention, after going through the first foam wall, it was dissipate more once it hit that 1" space, and then hitting that 2nd foam barrier. *
it's fun to do this stuff but it's $$$