Jam Space in 20X24 Garage. Need help w/ sound isolation

Hi guys. I've been doing some casual research on this project for the past year or so. But now the time has finally come. I have a new house with a 20 X 24 detached garage and my yearly bonus check in my hand. I’m a guitar player and enjoy to crank it up on occasions. I will also have a full band with drummer coming over to jam every now and then. I will probably spend a lot of time in my new space doing the loud things I enjoy doing, for example listening to my vintage HiFi equipment and practicing in the evenings. I want to build a jam space mainly, with one single room for now (may add iso booth as a second phase). My main concern at this time is not pissing off my neighbors. Here are the specs with attached plan and iso views. Garage Dims 20 X 24, 8’ walls, typical pitched metal roof, 1 man door on front right side, 1 overhead door approx. 15’ at front and one window at rear approx. 3’ X 4’ Walls 2 X 4 studs on 16” centers with what I believe is 5/8 plywood outside and vinyl siding on top of that. (no insulation and no Sheetrock inside) Floor Concrete slab that butts up to my driveway My garage is approx. 30’ away from my house and approx. 60’ away my nearest neighbor’s house (which is on the left). I want to have just enough isolation as to not disturb my neighbors while they are inside their house as I'd like to keep my expenses on this project as low as I can within my goal. As far as how loud we will be.. I think about 100db in the room.. Here’s what I’m proposing: A double leaf wall with 2 X 4 studs on 16” centers with standard insulation at both leafs and a double door system for the man door. The overhead door and window will probably be framed in as part of the double leaf walls. I’m not sure what to do with the ceiling at this point. That’s all I have. Any suggestions on the air gap size? thickness of drywall? number of layers of drywall? Should I bother with green glue? Decoupling? What to do with the ceiling? Any other construction advice would be very helpful Thanks! Oh and as far as budget goes, this is a build as I go type of thing. I'm not in a real hurry to get it done. Would like to start out with about 1.5k for framing my walls. Then I'll add as needed

Hi Paul, and welcome! :) So this is basically just a rehearsal room, for now, with the top priority being isolation? In order to answer your questions, you really need to put numbers to the situation. Numbers as in "How loud are you" and "how quiet do you need to be", but in terms of decibels, not words. You can't plug the phrase "not disturb my neighbors while they are inside their house" into the mass law equations and MSM resonance equations, to come up with answers to the amount of air gap and surface density you need. :) You have to have real-world decibels to plug into those equations. Without those numbers, it's all just guesswork. All i can give you are the absolute minimum recommendations for ANY type of isolation wall, but that might or might not be sufficient for your project:

Any suggestions on the air gap size?

Minimum is 4", but more is better. It depends...

thickness of drywall?

Always at least 578", never thinner. However, you might need more than one layer. It depends...

number of layers of drywall?

It depends...

Should I bother with green glue?

Depends on your requirements. If you need high levels of isolation at low frequencies, then yes.

Decoupling?

Always necessary, no question about it. Without decoupling, you are stuck with mass law, and mass law is not your friend.

What to do with the ceiling?

The same as the walls, always. That's about all that can be said, until you figure out your numbers. It would also help if you could post a basic plan of what you have in mind, preferably in SketchUp. - Stuart -