If your builder only put one layer of drywall, I would suggest adding a second layer of drywall to help limit low frequency transmission through the wall. Is this garage bay next to your house or an outside wall. In either case you probably want to minize sound leaking through the wall.
The usual suggestion for home theater walls is to use two layers, the outer layer is 5/8" and the inner layer is 1/2". Then you could add at least 1" of Insulshield below ear level on the sides and back and one the screen/TV wall.
You also need to think about the floor. While you could put carpet and padding directly on the concrete, carpet installers don't like installing on concrete because it is a pain to attach the carpet tack strips to concrete. If you live in a cold climate you may want to build up a subfloor so you can put insulation beneath the floor. To build a subfloor you probably want a few layers to add weight and limit low frequency sound getting through the floor and reverberating in the hollow spaces between joists.
You definitely want thick padding for carpeting, or perhaps a double layer of padding.
For the floor you could sandwich sheetrock between Â¾ â€œ plywood tongue and groove, or 5/8â€ particle board or MDF. MDF or particle board are preferred because they are cheaper and heavier. You could add layers of roofing felt to the sandwich, but in your case that may be overkill.
For better sound isolation, you could float the floor. That means it is not attached to the structure below, but instead the floor joists rest inside U shaped holders. The U shaped holders could be made of MDF, particle board, or rubber. Auralex sells rubber ones. Again, floating the floor may be overkill for you case.
Look for carpeting prices in Georgia. Some people use commerical carpet which wears well. I donâ€™t think the type of carpet affects the acoustics much.
Where you have a 2 - 3" overlap you could use 2 x 3's or 2 x 4's as necessary to bring the wall out even with the edge of the sillwall. You could notch the ends to fit over the sill wall down to the floor. If you do that, I would put some felt or rubber between them and the sillwall and the use liquid nails subfloor clue to hold that part in place so it doesn't vibrate.
ps. Since you mention damn codes on your web site, you do realize that building a wall just inside your garage door will likely be in violation of building codes. I only point this out because any construction that isn't up to codes can give you insurance company an easy out should something nasty happen. Also your house will be very difficult to sell removing everything that is in violation.
I'm surprised the builder did not have the option of converting the garage into a bonus room.
I had the exact same scenario in my basement on the walkout wall where it met the floor. They poored about a 8" wide concrete sill that's about 4" tall, and then sat 2x6" wood sill plate on top of the 8" concrete sill for the framing. Since the 2x6"s were about 2.5" inset to the sill, I did the following.
Ripped 2x4" treated for a true 1"x. Nailed to the concrete floor in front of the sill, using a 22cal nail gun. Ran a 2x4" header on the ceiling so that the outside edge (to the room) was square to the outside edge of the 1"x treated. Notched 2x4" studs to go over the concrete sill and nail into the 1"x treated. Make your notch a 1/4" wider than the sill, so you don't have to worry about rubber/felt isolation (it won't touch). Nailed the other end of the 2x4" stud to the header. This brought out the wall a couple of inches but made everything square.
I think you could put polyester/cotton batting over Insulshield. Both materials create lots of air pockets which scatter and abosrb the sound. That tends to reduce sound reflected back into the room. I think most of the sound is reflected off the hard gypsum board beneath the batting or Insulshield. The thicker the layer of batting/Insulshield on the wall, the less sound should be reflected back into the room. The Insulshield does a better job of limiting sound reflections.
On other issues, fireproofing may be a big concern for insurance and building codes. Polyester/cotton batting burns easily. You want a fire restantant wall between the HT room and the garage, and between the HT room and the house. I think 5/8" gypsum board is acceptable. If you cut holes in the fire resistant wall, I think they need to be plugged with cement or other fire resistant materials.
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