Jim, first of all you didn't mention what the 2x6 joists will be sitting on - is there going to be a perimeter foundation, or maybe some concrete blocks with the 2x6.s resting on those, or what?
From what you've said so far, nothing short of 3-4 FEET of concrete would help you with soundproofing the floor, if you are only using one solid layer of material between you and the outside world - you would need TWO centers of mass separated by only ONE air space. Anything else is going to be a waste of time.
Case in point - I built a room into my barn almost 20 years ago - 2x6 joists resting on concrete blocks, with 6 mil black poly vapor barrier on the ground under it - one layer of 3.4 ply, two layers of 30# roofing felt (thick tar paper, 2 layers = 2/3 of one layer of 90# roll roofing) then, ANOTHER layer of 3/4" particle board (better than plywood, more mass) - Bottom line (pun intended) the floor lets just as much sound in/out as the standard 2x4 walls with an extra layer of 3/4 particle board inside. (All that was before I even knew that egg crates were almost worthless for ANYTHING other than holding eggs)
I know this will raise the cost, but if you want your floor to be as good as even basic staggered-stud walls, you'll need a double floor.
From the ground up, that would be something like this - black poly vapor barrier on ground, concrete blocks, 2x6 joists (need to be supported at least every 7 feet, if you're using #2 and better douglas fir 2x6.s on 24" centers, you can go 8 feet if using 16" centers. Then, a layer of OSB (cheaper than plywood, use 3/4" at least) then a couple layers of the 90# roll roofing (butt joints and caulk, second layer with seams staggered, preferably at right angles to first layer) - then, a layer of MDF or particle board (more mass, better isolation)
That would be your FIRST center of mass - now you need an air space, and a SECOND center of mass.
The wider the air space, the better the isolation - the problem here, is if you build the double floor first, you have a path thru the upper mass into the inner joist cavity, and through the outer frame members - that path doesn't have a 2-mass, 1 air space barrier, so will leak sound a lot more.
The way to get around this, is to build the soundproof walls on TOP of the first platform (just like regular house construction, only with doubled flooring as I explained above, and with 1/4" neoprene between the floor and the wall frame - caulk everything thoroughly with REAL acoustic caulk, usually only available at commercial drywall places, insulation contractors or online at twice the price - finally, after the rest of the room is built, put a perimeter strip of Celotex or other soft "sound board" around the base of the walls (kind of like wide molding) that is maybe 1/2" wider than the total height of the INNER floor, which should be built as follows -
1/2 to 1" neoprene blocks supporting a second 2x6 joist frame, 3 pound/cu ft Rockwool or mineral wool (same thing) between the joists, then 3/4 OSB, at least 5/8 particle board (caulk seams, second layer at right angles to first) then vinyl or whatever (not carpet) floor covering.
Be sure to caulk well all around the perimeter. The "soundboard" perimeter should keep the floor from having a hard physical connection to the wall, the caulk should hermetically SEAL ANY POSSIBLE air leaks, and you will have a fully isolated, floated floor that will keep up with pretty much ANY wall you can build. It will also kick ass on any door you're likely to put in, so the door should be your weakest link if all your other construction is done right.
The ceiling should be done with the same 2-mass, 1 air space construction. If you violate this rule by having more masses with more air spaces, you'll be just wasting time and money. Period.
If you're planning on rehearsing with LIVE drums instead of an electronic kit, and you DON't build the way I outlined, I GUARANTEE that any neightbors within 200 feet will hear your drums/bass too much. Been there Done that - my double-layer, single mass floor room let my wife hear nearly everything we played live in that room from INSIDE the HOUSE, and they are 100 feet apart!!!
Short of building a room in a cave half a mile underground, there is no substitute for proper design.
Here's a link to some further discussion of wall construction - please note in particular John's wall chart posted on Mar 01, and my second post on Mar 17, for a better understanding of what I recommended above -
http://www.homer.com.au/phpBB2/viewtopic.php?t=86&start=0
Hope that helped; as I said, I know you're not gonna like spending the extra - but I know you'll like the results even less if you DON'T do it that way... Steve
