Now my reply:
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The first wall is the garage, .... then the second wall (2x4 construction) with another layer of R13 insulation. Within the 2nd 2x4 wall .... I'm planning on using 5/8" sheetrock, then put 1/2" spacing strips (furrier strips of wood) to create another dead air space before attaching 1/2" soundboard drywall.
That would be a REALLY bad idea! That would create a three-leaf system with a very high resonant frequency on the third leaf! Putting thin light-weight panels over a thin air gap is a MAJOR bad idea. The isolation would be pretty bad.
Just stick to a standard fully decoupled 2-leaf MSM structure. Two leaves, and only two. Fully decoupled. Each leaf should be two layers of 5/8" drywall on its own set of studs. Seal it all airtight, and fill the air gap with 48 kg/m3 insulation. Simple!
which will have R13 insulation added to it, then a dead air space of 1.5",
In an MSM wall system, your air gap (spring) is not just the empty space between the studs: it is the entire space between the two leaves, including the insulation. In your case, the air gap will be something like 8 1/2", which is good. Or at least, it will be good provided that you do everything else right!
:)
The 9x12 and 17x23 foot room are the ones that need soundproofing, so they'll both have double walls, insulation, etc.
Don't forget the ceilings! Just doing walls won't help at all. Room-in-a-room with MSM means the entire structure, not just the walls. It means keeping the mass on both leaves roughly the same, all around all five sides of the entire room.
1) Should I go with R13 insulation twice, or something thicker (say R30) once or twice (two layers as described in each 2x4 wall with the R13)? Is two layers of R13 opposing each other with a dead space of 1.5" thick effective at sound deadening or only insulation of heat and cold?
The purpose of the insulation inside an MSM wall is NOT to "deaden" the sound. Its purpose is to act as a damper on the "air spring", to reduce resonance in the cavity, to lower the resonant frequency, and to broaden and flatten the Q, thus maximizing isolation. Leave out the insulation and it will cost you many decibels of isolation! So you certainly do want it in there, and you want as much as possible. You can use ordinary pink fluffy fiberglass, or mineral wool. You need something with a density of around 48 kg/m3, give or take a few kg (less for fiberglass), and you should make it as thick as your building code allows. If you can fill the entire cavity, then do so! If not, then fill as much as you are allowed to fill.
2) Since the 5/8 sheetrock is roughly $6 per 4x8ft piece, and the 1/2" soundboard is roughly $9 per piece (Home Depot), why not spend the extra money on two layers of 1/2 soundboard for better sound control? Or is there a good reason to have the 5/8 standard sheetrock (gypsum), then the 1/2" soundboard?
Yes, there are a number of very good reasons why you do not want 1/2" soundboard in your build! Forget the 1/2" stuff, and go with the 5/8". One reason is mass (you want as much mass as you possibly can get), but other reasons include the flexibility and resonant frequency of the board itself. 1/2" is too thin, too light, to flexible, and has too high a resonant frequency for doing what you need for your application. Stick with 5/8" for everything. Do at least two layers of 5/8" for each leaf, with staggered joints, and use Green Glue between the layers. Forget the 1/2": it is not your friend.
I would think two layers of 1/2" soundboard would be more effective, but maybe I'm nuts.
It would be much
less effective: Like many things about acoustics, this is not intuitive at all, so don't feel like you are nuts! This isn't about what "sounds like a good idea", but rather about what actually works, based on the laws of physics that govern sound waves and acoustics. The 1/2" stuff has much less mass than 5/8" stuff, is more flexible, and has the wrong resonant frequency for what you need. That all means that it does not isolate well at low frequencies, when used as part of an MSM structure. Cranking up a large amp with big speakers can potentially produce a lot of low frequency energy, which is really hard to isolate. You cannot isolate low frequencies with highly flexible low mass light weigh drywall. It wont work. And stacking up more layers of 1/2" on top of each other won't work either: that adds mass, but does nothing about the other problems. So just stick to 5/8. 5/8 is your friend.
The low ceiling (7ft 8in tall) in the 17x23 ft room is made of 3/4 pine (solid), with 6x6 beams across it. It has insulation on top already,
So what is above the insulation? That's the real question. You don't want to create a three-leaf system above you, any more than you want a three-leaf system in the walls. You need to get up there and see what you have above the pine and the insulation: How big is the air gap where the insulation is, and what is above it?
but I'm planning on framing in the ceiling like the walls (16" centers), and then doing the same two layers of sheet/soundboard scenario with more R13 in between the beams.
Once again, go with 2 layers of 5/8" with Green Glue in between, hung from your new joists. And don't forget that the new ceiling CANNOT rest on the existing walls, and CANNOT be attached to the existing ceiling in any way. It must rest ONLY on the new inner-leaf walls, or all your effort will be wasted. 16" OC might or might not work, depending on how much you need to span and what type of wood you use for the joists. 24" OC would be better, but you'd need larger joists.
Am I missing something with this plan,
Yes, quite a few things:
#1: You are forgetting to seal and beef up the existing garage doors! Since you say you want to use the existing shell as your outer leaf, that implies that the existing doors are going to need some major treatment. Your best bet if local regulations allow it would be to simply remove those doors and brick up the hole. Second best would be to leave them in place and brick up behind them. Third best would be to beef them up in place somehow.
#2: Another thing you are forgetting is HVAC and electrical: you need to ventilate and cool those rooms, and you need to get power in them. That all has to be planned properly in advance, just like the walls, ceiling, doors and windows (if any).
#3: You didn't mention it, but I assume that you are aware that you have to seal everything air-tight. Each leaf of your walls and ceiling must be absolutely hermetically sealed. So must your doors.
I'm getting the insulation tomorrow, ordering the sheetrock and soundboard tomorrow afternoon if at all possible.
Whooooaaa!!!! It sounds like you are getting WAY ahead of yourself here! You don't even have a workable plan in place yet, you haven't considered some major issues that are extremely important, and you seem to be heeding some of the common myths and rumors about acoustics that are still floating around the internet, instead of basing your build on science! I'd say it's a little premature to be ordering materials at this stage! I'd suggest that you slow down, throw out the myths you have learned, get your head around how sound and acoustics REALLY work, get your plans sorted out properly in full detail, and THEN think about ordering materials.
For example, you don't even know how much isolation you need! You have given no measurements here of how loud you intend to be inside your room (in terms of decibels), or how quiet you need be outside to avoid lawsuits and cops banging on the door (also in terms of decibels). Without those very basic numbers you cannot possibly know how much isolation you need, and therefore you cannot possibly know what kind of construction you need.
Isolating a room acoustically isn't guesswork and hope: it is science. You start with the numbers, and set a goal, then figure out what you need to do to reach your goal. The numbers will dictate how much isolation you need, which in turn will give you a couple of choices on how to build your room: You then select one of those choices based on your budget, and design the room with that, taking into account a whole bunch of factors that you haven't even mentioned so far (such as room geometry and ratios for example). Then you look for building materials that will do what you need to do. And only then do you go out and buy the materials.
So my advice would be to take several steps back, and start planning your build properly before you do anything else. The chances of it working out perfectly without it being designed properly in advance, are somewhere around zero.
- Stuart -