Since you're putting your ceiling panels on RC and not directly on wood framing, you can just use longer screws for the second layer. The only thing that should touch the wood framing is the RC, which should be fastened to the joists with 30-35mm Type W, for Wood, screws. Then, you would use 1 1/8" (28mm) Bugle Head (type S, for Steel) 1st Sonopan layer to RC, and then 1 5/8" (41mm) or 1 3/4"(45mm) Bugle Head (type S) for fastening the 2nd gypsum layer to the Sonopan layer.
For insulation, the fluffy fiberglas is nearly as good INSIDE a wall or ceiling as the rigid boards we use for acoustic surface treatment, and it's cheaper and easier to find. You should fill at least half of the cavity depth, and more if you can afford it - just don't PACK it tightly and you'll be fine. Let gravity help here - put the backing UP when doing the ceiling, and let the fluffy side rest on the first layer of paneling (the Sonopan in this case) - this will dampen the movement of the panels without tranferring sound directly, for an improvement in isolation of a few dB (as compared to insulation NOT touching the panels)
If you're going to add two layers of wallboard (any kind) to an existing wall, and the frame for that wall already has wallboard on BOTH sides of the frame, you should NOT put RC between layers of paneling. You would be better off just adding the extra layers directly to the existing layer. However, if the frame you're adding layer to is WOOD, then you need to find a source for Type G, (as in Gypsum) screws. These are also known as Laminating screws, and seem to be pretty hard to find. For the way to do this particular construction, I'll refer you to this thread, where I answered this very question earlier today -
http://johnlsayers.com/phpBB2/viewtopic.php?p=5616#5616
Your last question - "And last, for walls, As I said, it's a double framed wall with steel studs.
The sandwich will be like this (starting from the outside to the inside).
Gypsum - SONOPAN - Rc (?) - metal studs - metal studs (second frame) - rc (?) - Sonopan - Gypsum
On which wall should I put RC??? the interior or exterior wall? (interior beeing the one that receives all high SPL). "
Steel stud walls can be either load-bearing (Structural) or non-load bearing - the lighter, 25 gauge non-load bearing studs are flexible enough NOT to need RC for sound control, since the flexible stud keeps one leaf of the wall from having hard physical contact with the other side (leaf) - if you're using heavier steel studs, then RC is a good idea for ONE side only.
(There is more information on walls here - )
http://johnlsayers.com/phpBB2/viewtopic.php?t=598
If you're using a double frame like that, and if the studs are heavy gauge for load bearing walls, then you should use RC ONLY on the side toward the studio. That way, the flex you get from the RC can help your wallboard act as a bass trap. This will save you having to build quite as many traps for inside the room in order to get the sound right.
If you put insulation in both sides of the double framed wall, with the fuzzy side against the wallboard in each case, it will improve isolation by several dB. This is because the insulation damps the vibration of the panel without directly coupling it to the other side of the wall.
Here is a good explanation of some of the things I've covered here -
http://www.domesticsoundproofing.co.uk/tloss.htm
I think that about covers it for now - you've got some good reading to cover (be sure and download the USG manual, it's in one of the top posts in the Construction forum - it's sometimes hard to follow, but still a very valuable reference, and lots of pictures) - When you get stuck, we can probably help... Steve
Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...
