Some notes on the room size...
A "square" is an acoustical nightmare. 1 Foot difference in length or width will make a difference (pun intended) in the acoustical performance.

Staggered Stud Wall is what Catbrat is mentioning with the 2x6 top and bottom plates. I did this. You will need to be a little creative with a door, but this will help with a lot of the sound proofing. You will still need to do something with your ceiling (search Whisper Clips and Hat Channel). Floors are REALLY tough to do.

Do NOT "fill" the wall cavities with insulation. Fill 1/2 of the walls with regular pink/fluffy normal insulation, but leave the other side with an air gap. You want sound isolation, and stuffing the wall cavities recouples the inside and outside of the room.

Here are some pages with GREAT information (and pictures) on the subject. Look at the first link and note that it talks about filling in just some (most) of the air gap between the two walls with insulation in point #2

4 Elements of Soundproofing

Here is the one graphic that I like. Think of the "his side" as the theater side. This has 2 layers of drywall on both the inside and outside of the staggered stud wall, with green glue applied between the layers. Now, I opted NOT to do the 2nd layer on the outside of the theater because the performance improvement really goes down at that point, but if you can swing it, do it.




Better yet, just go here and read all of the links within the link below. If you have questions, give Ted or John a call. They will give you sound advice without trying to "up sell" you on things that you don't need. You can certainly get products like Green Glue and the Whisper Clips elsewhere, but their prices are good, and they really helped me out.

***************************************************************
Soundproofing 101

***************************************************************

Lots of good stuff right there...

Or you can ask me. I've gone through so much of their material and others. Their website and Ted and John both give real and true assistance and are considered one of the best and most helpful soundproofing companies out there.

Farewell - June 4, 2020