Hi there Dave, and welcome!
However, I have recently been told by other (famous) studio designers that the design is majorly flawed.
Looking at some of those diagrams, I'd have to agree at least partially with the "other designers". There's stuff in there that doesn't make a lot of sense. When I see three-leaf walls where none are needed, with no compensation, then I start to question things... Ditto when I see walls "floated" on rubber, but without specifying the type, durometer, thickness, deflection or load... It isn't needed anyway, but even if it was, it would be pointless to mention it without specifying the parameters that would actually make the wall float.
Unfortunately, some of the original text is covered over with the orange scribbles, so it's hard to figure out what the original plan was.
First of all, I decided to float the slab with 1 inch Rigid insulation.
And how will you ensure that it floats? What frequency did you tune it to? And does it even need to float at all? You might find this thread interesting:
http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173
The RC channels shown on the 2nd wall also are not needed (right?).
If you have fully decoupled framing for your inner-leaf, then that's correct; RC would be superfluous, and a waste of money.
We were getting ready to put a layer of 5/8th sheetrock on the outside wall when it was pointed out to me that this would actually make the sound proofing worse.
I assume you mean that you were going to put it on the other side of the studs? The side facing the room? Yes, that would indeed create a 3-leaf system, which would indeed reduce your low frequency isolation.
Because of this now lack of thickness on the outer wall,
The thickness of the materials doesn't matter in the least. What matters is the mass (or surface density) You can have it very thin, provided you use high density materials. If you use lower density, then it needs to be thicker, yes. You have a layer of OSB, plus siding. That's not very high mass, but then again you don't say how much isolation you need, or what frequencies you need it at, so it's hard to say if it is enough or not.
If you do need to add more mass to the outer leaf, then there's a technique called "beefing up between the studs" that would allow you to do that without taking off the siding. You just cut strips of 5/8" drywall to the right size and put it in the stud bays inside the wall, pressed up against the OSB. Use cleats to hols it in place (nailed sideways into the studs) and acoustic caulk to seal the edges. Depending on how much mass you need, you might have to do two layers like that. It's a simple and very effective way of adding mass to an otherwise completed wall, where you still have access to the stud bays in the other side.
we want to make up for it with the inner wall
That will only work to a certain extent: What you are building is technically known as a fully decoupled two-leaf MSM system. It is a tuned system, where the tuning depends on the mass on each leaf, the size of the air gap between the leaves, and the insulation in the air gap. The optimum effectiveness is obtained when the mass is the same on both leaves.
The current designer is suggesting 2 5/8ths, RC Channel and the 1/2 inch Sheetrock.
Bad idea! Really bad! Silly, actually. That would create a very thin air gap between two layers of mass, and also create a 3-leaf wall again, which would damage your low frequency isolation severely, once again. The resonant frequency of the thin air space would be very high, and since the overall response of the wall depends on the higher of the pair of resonant frequencies in a 3-leaf wall, the effect would not be pretty. You can safely stop listening to that "current designer". He doesn't seem to understand the principles of isolation too well...
That doesn't make sense to me and I think we should either do 3 layers of 5/8ths, or one layer of 5/8ths OSB and then 2 5/8ths layers of sheetrock.
Right. Either of those will work, but don't do that until you first beef up the outer leaf.
Is OSB or Sheetrock better for soundproofing?
They both work. What matters is the mass, not so much the material itself. In fact, the equations for figuring out the MSM resonance do not consider the material at all! They only consider the mass, in terms of the total surface density of the leaf.
From that point of view, drywall is a bit heavier: it's around 680 kg/m3, while OSB is around 610 kg/m3. MDF is around 750, so if you wanted to go to MDF, you'd get more mass in the same thickness, but there's not a lot in it.
The advantage of having OSB or plywood as the first layer (on the studs) is that you then have a nailing surface around the entire room for later hanging your acoustic treatment: You don't have to search for studs, as the entire wall can take nails when there is a thick wood backing like that. That might or might not be useful for you, but personally I always recommend OSB or plywood as the first layer.
Is there any benefit for using RC channel on this second wall
None at all. It's waste of money, and would be detrimental to your isolation system.
and a different size of sheetrock?
Only of you can get stuff thicker than 5/8"! There's no benefit at all with going thinner. In fact, multiple layers of thin drywall perform worse then fewer layers of thick drywall at higher frequencies (beyond the region of MSM resonance) even when the total thickness of the leaf is the same. So go with the thickest, highest density drywall you can find, which is usually 5/8" fire-rated drywall.
This is the idea of the interior. Here is the next picture showing the engineered plan of the 2nd wall:
To be honest, I wouldn't do that shape or design either!
There's a LOT of wasted space, the CR is not symmetrical when it could be, the treatment is in all the wrong places, and the geometry is wrong (referring to the relationships between the speakers / walls / listening position). I would re-do that, along more conventional lines. Firstly for acoustic reasons, and secondly to recover all that huge amount of lost space.
We have put lots of conduits under the slab for wiring for the studio and HVAC has been planned,
That can probably be used anyway, as things wont move too far from where they are now. Hopefully....
although I don't know much about the details there as the person who is doing it is another HVAC person who did HVAC for another studio that seems to be silent.
"Silent" isn't the only thing that a studio HVAC system needs to be. It also has to provide the right flow volume of air (room changes per hour / CFM) at the right flow velocity, to the right locations in the room, at the right temperature, and at the right humidity. It also needs to add the right amount of fresh air for the occupancy, and remove the same amount of stale air. It's a lot more complex than it looks at first glance. There's a large number of calculations that go int getting that right: So ask your HVAC guy for those calculations, and ask some key questions, such as what latent heat load and sensible heat load he used, what the flow velocity will be at the registers, and what exchange rate he used. If you get a blank "deer in the headlight" look from him, then it's time to move on to a real HVAC guy who does know about studios....
If you can provide more info about the studio, then you are probably still in time for us to help you salvage what is already there and re-do it correctly. It seems you are not so far advanced that you can't fix things now at now cost (or low cost), but if you go much further then it will get more expensive to fix it later.
It's a shame that this has happened to you, but you aren't the first person to arrive on the forum with this type of issue! Fortunately, you DID arrive, and it is fixable. It's also a shame because you have a nice sized space there, and you are obviously investing a decent amount of money in it, so it could turn out really well, if done properly. Or it could turn out not so well if done the way it looks on the original plans.
- Stuart -
