This is Brwsaw's room. It was a bit tricky. The images above represent the "ideal" speaker layout considering listener location, reflections, modal response and 2ch\multichannel layout.
The room is a little narrow for a conventional setup, and just a little short to make 2 rows optimal. The dimensions of roughly 21x12x7 make for some challenges in avoiding axial room modes.
Width Modes are 43 86 129 172
Length modes are 24 48 73 97 122
Height modes are 73 147 220
The speaker\listener distance from the front wall was most important to consider, as there is a nasty coincident height\length mode at 73hz. Since the subs are front wall located, we do NOT want them to energize that frequency. The loudspeakers should cross over to the subs at 60hz. The subs are placed at 1/4 width to negate the width null at 43hz and integrate seamlessly with the main loudspeakers for 2ch use. The default seated position height of around 36-38" places the audience out of a height null problem in this room.
The room is narrow for a conventional equilateral setup. Doing so lands the listener close to the center of the room, or forces the loudspeakers way to close to corners, or way to far into the room. With a bit of juggling I gave up ideal width locations to maintain a good width to listener ratio. The proximity to the sidewall is of less concern than to the rear wall. First reflections can be treated if he perceives them as a problem. Preference for a wide soundstage was mentioned, and these will aid in the perception of source width, giving a little image specificity away.
The imaginary toe in convergence point is an estimated 3'behind the seated listener in the MLP. A folded piece of paper can be placed on top of the speaker on-center and looked down from behind like a scope. A piece of string can be taped to the ceiling at room center a few feet behind the listener and aimed at. I used a hockey stick resting against a pool table to do the same thing over Christmas setting up a room. Anything works so long as the sight line from each speaker can be matched up to that reference.
For 2 ch use the L\R first reflections meet the sidewall at just over 5 1/2' (3rd image). These are of a very similar length to the direct sound so should be reduced in amplitude somehow. The second drawing highlights zones I would recommend for treatment. Green is the first priority, orange second, red optional. The room is prone to sidewall flutter echoes and the front 2/3rds of the room should account for some flutter reduction method. This can be as simple as adding irregular shapes to both walls. But they might as well be broadband absorbers or midband diffusers. These can be DIY of course!
The use of a QRD diffuser on the front wall is highly recommended as well as on the sidewalls adjacent the listener (9' from front wall.) This will improve the apparent width of the center channel's presentation, in this case an M80 tower. Diffusing the lateral angles from the listener to the center channel is a very good idea. It will make the source seem more natural. This is also very beneficial for speech intelligibility. The second reflection point is of little concern in most rooms because of the increased path length and reduced amplitude (red on sidewall in drawing.) This area should be treated with diffusion or to mitigate flutter echo if needed to preserve room energy.
36" wide front corner bass traps are already deployed in the room, and it is probably a good idea to add a deep (6") trap in the center of the rear wall. A limp membrane trap or 1/4" plywood resonating panel can be built and tuned to maximize effectiveness. Plans for these online. The rear corners can be treated to reduce the "cue ball" effect from L/R channels. They will also aid in bass absorption.
The overall decay time of the room will be long and uneven if untreated. The sidewalls will dominate the reflection times and mid to high frequencies will linger. To test this out, a balloon pop at either the L\R channel locations is a good test. Listen for a pitched boing or a ringing sound at the tail of the decay. No pitch no worries. The seating will likely contribute to the perception of echo in the room. Plush seating and carpet may have already mitigated any audible issues.
That's all I can think of for now. Need to eat lunch. Feel free to pick it apart and ask questions.
I quickly looked into your old thread. Your tile ceiling is a good news situation for decay times. The modal issues in the room should also be greatly reduced in severity. Good stuff.
