Welcome!
Would the audio setup be along the 11ft or 18ft wall? (11ft seems a bit narrow for audio and very narrow for HT.) Bigger is better, as long as a proper shape is maintained. Will the room be enjoyed by more than just you (all but impossible to achieve proper multi-loudspeaker balance in a room that narrow for multiple listeners if set up along the 11ft wall).
I have an 8ft x 13ft x 21ft dedicated audio (first priority)/office (second priority) that was included when we built the house 15 years ago. It was done on a budget with lots of research. Suggest reading Floyd Toole's "Sound Reproduction" 3rd edition (
https://www.amazon.com/s?k=floyd+toole+sound+reproduction&ref=nb_sb_noss) the consummate audiophile guide for how loudspeaker/subwoofers behave in-room.
Highlights of the room:
1.) Shape follows Fibonacci ratios (
http://www.cardas.com/room_setup_main.php).
2.) Insulated staggered stud interior walls (even though they adjoin storage spaces).
3.) Insulated fiberglass exterior door with weather seal.
4.) Insulated flexible ductwork (can barely hear the furnace).
5.) Setup is mid-field along 13ft wall with loudspeakers 5.5ft from front wall.
6.) Use three carefully located subwoofers to reduce bass peaks/dips (see Toole).
7.) Use ten GIK (see them here at Audio Circle) 2ft x 4ft 244 absorption panels (they have many decorative options).
8.) Use three tall randomly filled bookcases on side walls as crude diffusion.
9.) Topped off with <500 Hz Dirac room correction.
10.) Ran separate 20 amp circuits to each of three 20 amp cryo'd hospital grade receptacles and grounded them together, separate from the house.
Floor is thin wall-to-wall carpet on concrete slab (highly recommended). Walls/ceiling painted drywall. My downfall was the ceiling. Builder refused to install floating drywall ceiling and I'd specified recessed can light fixtures (air tight, rated to be in contact with insulation). Even with 12 inches of insulation batts sound transmits up and down. But the initial impression was that the room was spooky quiet (have gotten used to it now).
Recommend wall sconces and floating the ceiling if below living spaces.
Dirac, or REW (Room Equalization Wizard) can measure then adjust overall response. No need for outside audio consultants. Most suggest limiting adjustments to under 500 Hz and only apply after use of multiple subwoofers and room treatments.
Can't recommend DIY loudspeakers. Modern state of the science loudspeakers only come from a few companies, those with deep R&D and in-house manufacturing resources like Focal, JBL, or Dynaudio that are active designs (one channel of amplification per driver). Passive designs from some guy working out of his shed are crude, way under performing, and antique in comparison.
