Help Building an Audio Room

Thank you all so much! I really am overwhelmed by the community, ideas, and support on here. You all are awesome!

I’m going to go with 13x15x9 and mobile in-room sound treatment. It’s not a forever home. If it turns out to be a forever home, when the kids launch, I’ll have bigger rooms to chose from at that point. I hope :) ....WAF. 

Thank again so very much!. I got Harley’s book... thick!

Thank you both for the level-headed input.  
I love the idea of a bookshelf/built-in wall to effectively build one wall out and provide some trap and diffusion in doing so. I think that will go a long way, and I can see in the pic they did something similar. 
If I did that on a side wall, I might explore building the front or back wall with a bit of angle; top edge of wall built out 6-10” more into the room than the bottom or vice versa. I’m thinking that may help and be barely noticeable. 
The cost is essentially the same to the builders/me, but as I socialized the idea with them a few weeks ago, they said, “That’s a first. Build it out of square?.. We can do that.” 
I’ll check out the book, too. 
Thank you again. 

Just ordered the book.  Thanks for the recommendation. 

Thanks @mesch and @dlcockrum 

I really appreciate the thoughtful input and sharing.  

I'm stuck with the 15x15' max size, and of course I don't want to turn a medium/small room into a small/tiny room. 😁  I'm trying to balance it.  I'll play with the tool and hope i can find something more in the 14.5 x 13.5' space that will work.  Ideally, I don't want to lower the ceiling, but maybe 6-12".  I'll look at what that might do. 

Good to hear about regular insulation.  I will build all walls off the (3) concrete walls w/ 2x6 and add the insulation.  Although, I'm considering building one of the side walls maybe off further, like 12".  Something not equal to the room isn't square.  Also, thinking of tilting the front or rear wall a few inches...likely not noticeable if you're not looking for it.

I can't do anything toooo drastic or the WAF will trump it.  And I get her point.  I don't want to end up with a 5x7' fun-house room.  

This could be good; ASC’s ISO-Wall resilient channel system.  My audio room is below the master en suite.  So, pretty close.  

Thanks again to all!

Per the tool, looks like 13x15 x 9H is fairly decent and (barely) within the Bolt Area.  

That might a a good starting place.  To create the 15' wall, just leave as normal.  The 13' wall could be built with bookshelves in most of it. E.g. like the top picture Mr_Slate posted.  

There will be thick pad and carpet on the floor.  Maybe that's a decent start and then, work with sound treatment and furniture and speaker/listening placement.  

I figured I would check in on this to share some 'results'. 

I ended up having the builders frame the 15x15' room 14x13.5x10h' room and added the sound/fire insulation between the drywall and the poured concrete walls.  Also, there is low-pile carpet (higher than Berber) with a thick pad. The back corner of the room is open about 4' wide into a hallway. Added two medium upholstered chairs. I sat down in the area where I would be listening.  The echo of my voice was very apparent. 

The results: I gave up quickly.  The (slap, I think) echo was so bad, I saw many more dollar signs and an ugly room full of sound treatment.  I've moved my system back up to the main living area.  Much better and no further cost.