The entire space will function as an acoustic system. Decouple your room from the metal building... Hat channel is good... Resilient isolators and channel is better... Completely isolated free standing room within a room on its own isolated slab... Best.
If you are building the metal structure and pouring a slab, have a slab that is isolated for the studio. It should be physically separate and decoupled. You can use neoprene foam rubber mats in the gaps.
Build the freestanding structure within the space on the isolated slab... Do not attach it to the building.
If you can, use good room ratios... There are many. The Bolt ratios are good... There are tons though. There are calculators on line... One is here.
AcousticsFirst.com - Room Ratio Calculator
You can separate the space into control and live room. 20 x 16 coule be sized down, but if you do that, keep the ratios good for both spaces if you can. Also, may consider using isolates slabs for the two spaces - building 2 separate spaces. This may be overkill for you though.
If you use a larger single space, build some gobos to help isolate for recording and tune the space for mixing.. (block reflective spaces, add diffusion, etc.) Gobos can be absorbers on one side and diffusers on the other for more flexibility.
Consider variable/adjustable acoustic treatments for a single space. A live room and a mix room have two separate room requirements. Live room should have more energy and less absorption. Reverb and reflections in a life room give the recording character, allow the musicians to play "together", and can give acoustic instruments room to breathe. Drums, acoustic guitars, and even vocals can benefit from a little bit of life. This being said... Bass should be controlled. Room ratios are there mostly to control low frequency performance - Modes and nodes. Add traps to increase the effectiveness of those ratios. Traps can be tuned or broadband. Tuned may be better if you want to keep the live feel. Tuned traps generally limit high frequency absorption, and focus on the bass. The remaining treatments should be a mix of diffusers and broadband absorbers to balance the rest of the frequency range... REW will help you track down which frequencies need to have tuned traps, broadband absorption, or reduced flutter.
Corners should be truncated, walls non-parallel is best... There are many different shapes you should consider. Parallel reflections accentuate modes and flutter, as the reflections cause a buildup in the space between them. Turn your corners into traps... Build a slatted 45° angle slotted screen, pack it with fluff in the back and higher density fiberglass in the front. If you can hang a layer of mass loaded vinyl in there as well, you can increase the performance.
Corners are the devil .
The are pressure zones where bass builds up, and reflection sources. If you have even stood in a room with mirrors in the corner (both walls with optical mirrors and joining at a 90° angle) - no matter where you stand in that room, you will see your reflection in that mirror. This is due to reflection geometry and sound does the same thing. The multiple different densities of material, fluff, mlv, and fiberglass board behind a slat wall if very effective. The slats give you Helmholtz cavity which can be tuned by spacing the slats. The mlv is limp mass absorber. And different densities of fiberglass remove more energy as sound looses energy every time it changes mediums.
You should have ceiling treatment... Absorption cloud over the mix positio, absorbers behind monitors and at first reflections points - though some are playing with diffusion at first reflections. Chips Davis has been writing about that recently.
If you can put variable treatment in the main live area, you can adjust the sound... Absorbers which can be covered, or flipped to be reflectors or diffusers, will give you the ability to change the sonic signature of the space for recording, mixing, or just jamming.
The mix position should be free from distructive reflections and flutter, and should be diffuse to the rear. Live space should be free from harsh flutter... It should be diffuse, have controlled early reflections, but maintain a character to the reverb.
And now that my vehicle is warmed up, I should drive to work.
Have a lovely day all. Stay warm. Stay safe.
And good luck.
If I were you, I'd map out (maybe with cardboard) the smallest area you think can be functional for a 4 piece, and then figure out how you'd handle the recording aspect of it. I assume you're going to treat the room?
