do you have some feed back on what you would do with the 2 leaf system , use existing wall or just build 2 more walls 2x4?
Any way you look at it, the existing building wall IS your outer leaf. There's no getting around that.
The question is: How good of a leaf is it?
If it isn't all that good yet, then as eightamrock mentioned, your best bet is to "beef it up" by adding additional mass to it, which can indeed be done by inserting panels of OSB, MDF, plywood, drywall or fiber-cement board between the studs, pressed up against the existing "leaf" materials (with no air gaps trapped in between). Once you have enough mass on that outer leaf, then you need only ONE more leaf for each room, which is the inner-leaf. So you erect stud framing for that, then put enough layers of OSB/MDF/Plywwod/drywall/fiber-cement board on just ONE side of those studs, to get the mass you need for the second leaf.
So, you end up with just two leaves: the outer leaf, which is the existing shell of the building on the outer side of that stud frame with NOTHING else on the inner side of those studs, plus the inner leaf, which is a separate stud frame that has your materials also on only ONE side of it.
Two leaves, only two leaves, and nothing but two leaves. If you add yet another leaf to that system, then you DECREASE your isolation, especially in the low frequency end of the spectrum. With 3 leaves (or more) isolation might get better for the highs (which don't really matter that much), but will very likely get worse in the lows, such as kick drums, snares, toms, bass guitars, the low end of keyboards, etc.
In the reference section of the forum there's quite a bit of info on why 2-leaf is the only way to go, and why 3-leaf is a bad idea. Sometimes you can't avoid 3-leaf, and in that case there are ways you can compensate for the lost isolation, but in your case you have the option to do it right: you have a brand new stand-alone building that is purposed especially and only for your studio, so you can do it right: do a 2-leaf system, to get maximum isolation for minimum cost.
From your PDF layout, it looks to me like you have a 3-leaf system, and with a very small 1" air gap between the outer and middle leaf. That's not a good situation at all. You also have a note saying '2" air gap' on that wall detail, and although that might seem right from a construction point of view, it actually isn'0t correct from the acoustic isolation point of view. What you mean is that you have a 2" gap between those two stud frames, but since the framing seems to be 2x6 (which actually measures 1 1/2 x 5 1/2"), the actual "air gap" there is 5.5 +2 +5.5, which is 13". That's excellent.... for a 2-leaf system! But adding the 3rd leaf outside that, with just a 1" gap, is NOT doing you any favors. I didn't do the math, but I suspect you'd be losing a bit of isolation in the low end from that. My suggestion would be to re-design so that you only have a 2-leaf system, not a 3-leaf system. In other words, get rid of the framing and mass in between the actual building shell, and your rue inner leaf. That would increase your air gap by another couple of inches, and thus improve isolation even more. If you put that same mass (the two layers of drywall) on the inner leaf, you'd be increasing yet again, probably getting beyond what you need.
Your slab is your limiting factor, as Glenn pointed out. But since it is a stand-alone slab-on-grade (and therefore very well damped), plus the building is relatively far from places you might annoy, it's unlikely to be an issue. If your slab limits you to (for example) 55 dB isolation, that's already pretty darn good. You should then concentrate on maximizing isolation for the rest of the building. Your weak points will pretty much be doors, windows, and the HVAC system, as well as possibly the roof. Getting 55 dB isolation from HVAC isn't easy. Getting 55 dB from doors isn't easy. Ditto windows. It can be done, of course, but it requires attention to detail and careful planning.
So, with the above in mind, your current plans reveal a few issues that would need fixing in order to get you where you want to be.
Firstly, your control room is not isolated. It is built as a coupled leaf system, so you will only get maybe 35 dB from that. The slab simply doesn't play into this at all, since the walls and doors will give isolation that is far worse than the flanking limit of the slab. I'd suggest re-designing that as a proper 2-leaf system.
Second, you are missing a door from the bathroom/lobby area into the live room. Right now, you show only one door there, on the leaf that faces the live room itself, but the leaf that faces the bathroom/lobby has no door in it. It must have a door.
Third, you aren't showing any provisions for your HVAC system. I understand that this is just a first rough sketch of the overall isolation, but HVAC takes up a lot of space, and optimizing that should start early. It would be a good idea to start right now with planning for the exterior air inlet and outlet vents, the duct paths, the locations of the silencer boxes, the location of the air handler, and the locations of the registers that supply air into and remove air from the rooms. You probably don't need to do that in great detail just yet, but at least make some provisions for it now so it doesn't become a major headache later. And oversize all of that right now! Make the ducts and silencers way bigger than you think they need to be, to give you some "wiggle room" when it comes time to do the details.
HVAC is probably the single biggest "gotcha!" in studio design. Start roughing it in now, so it won't be such a huge problem when you get to detailing it later.
One more thing: You should take some time planning your control room layout now too, before you finalize the positions and sizes of windows and doors. It seems to me that the door from the control room to the lobby/bathroom area is too far forward (to close to the front wall of the control room). I'd suggest sliding it back as far as you can, towards the bathroom, so you have enough space for the front corner acoustic treatment in the control room. That can take up a lot of space, especially if you plan to flush.mount (a.k.a. "soffit mount"!) your speakers (which is highly recommendable!). The control room also doesn't seem symmetrical, which is vitally important. It looks like the window isn't centered in the front wall. I'd suggest that you get i better centered, and start planning (roughing out) control room acoustic treatment and interior, to ensure that you don't run into unexpected issues later. It's a nice size control room, so acoustic treatment shouldn't be as complicated as if it were a typical small home studio control room, but it will still need good planning and design to make sure that the acoustic response is good. As the saying goes; "The devil is in the details".
- Stuart -