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I have no idea how to soundproof a big double bay door. Has anyone here ever done this?
Yes! Several times...
There are two ways of dealing with that. The most drastic and best way is to take the doors out and brick up the opening, taking care to seal the sides and top to the existing rough opening. Drastic? Yes. Effective? Yes. But not always possible. Some areas do not allow you to change the facade or appearance of your house, especially gated communities or where all the houses on the street are designed to look similar. Or you might not want to do that, for your own appreciation of aesthetics, or because you might want to sell the house at some point, and return the garage to being a garage. For those cases, you need "plan 'B'": build a new stud-framed wall just inside of the garage door. First you fix the door in place with some form of clamps, bolts, locks, steel plates, etc., but ones that can be removed in the future, then you remove all the hardware from the lifting mechanism, so that all that is left is the door by itself. Then you do your best to seal around the edges of that door, so that water, wind, dust, bugs, etc. can't get into the cavity that you are about to create. The build your stud frame in front of it, taking care to seal that to the existing walls and floor. Put insulation in the cavity, and put two layers of 5/8" drywall on the studs, joints staggered. Caulk around the edges. Done!
Yes, when you then build your inner-leaf studio inside the garage, that will create a 3-leaf system, but it is usually nothing to worry about. The thin metal of most garage doors is not going to be much of a leaf. But to be safe, you should still check the calculations for F+ and F- to make sure they are low enough, and compensate with more mass on the middle and inner leaves if necessary, and/or by increasing the cavity depths.
As I said, I've done a few of those, both ways, and both work. The first way is better, of course, because it does not create a 3-leaf system, does not take up extra space, and has a lot more mass, but "plan B" works well too, if you do it carefully.
- Stuart -