Does a flat ceiling provide the best sound?

It's not so much the space, but the reflection trajectory. Sound can come up, reflect, reflect, and come back down.

However, good acoustic treatment is essential regardless, so if you are that serious, I would talk to GIK Acoustics and treat the ceiling appropriately.

Best,

E

Yes. In general ceilings between 8 to 12 feet high work best. The problem with very high ceilings is that it usually results in the room being more of a cube and much worse for room modes and slap echo.

Concert hall shape is best - rectangular shape layout with speakers on short wall and greater length than width and a lower ceiling at the stage and rising modestly to the rear.

If height to width, width to length are in golden ratio then that is close to ideal

Speakers are designed with an 8ft flat ceiling in mind.  Anything else requires adjustments and compromises

good info above - flat ceilings are much easier to deal with in terms of calculations and empirical treatments; the same is true for rectangular walls vs. trapezoids 

read more in the Master Handbook of Acoustics

no matter what, I predict you will wind up with some pads up in the corners of the ceiling & 2 walls

I agree with Erik. I have a 9' ceiling and  treated the corners with ACS tube traps, back and front walls with various diffusers, 1st reflection points with smaller tube traps and GIK diffisers. Room, soundstage, imaging, music is superb. But the reflection from ceiling is very noticeable to me now.
Had back surgery and am just waiting to get my balance fully recovered so I can install the ceiling panels recommended by Nick at GIK. Very good people and laser recommended by "Jim Smith; Get Better Sound" was/is an indispensable tool in a 107yr old house. Or for Speaker setup Period!!! 
I am an field engineer for 40yrs. Have plenty of steel tapes and am used to accurately measuring. The laser will put the tape to "shame" and make it (3) times easier.

Best to All on this Journey 

I have 16 ft ceilings (though flat) and it was a nightmare to adjust for good sound.

Great advice--interesting to hear that speakers are designed for ceiling heights of around eight feet--will have to keep that in mind.    

Albert Porters ceiling is not an eight foot flat ceiling. As you look at the speakers the right wall rises up considerably higher than the left

stick some tmts up high in the 3way room corners

It’s not only the shape of the ceiling and walls but also the color of the ceiling and walls that affects the sound. Whaaaat?! Anyone care to hazard a guess what colors are best for the sound - one color for the ceiling and another color for the walls?

Geoffkait, you made a great point a comfortable pleasing room can set a mood one could only think would positively affect listening.
I wish you would start a topic in that regard.
24X14X8 with acoustic resonators sounds great but the room is 1960 cheap dark fake wood paneling acoustic ceiling panels.

What I had in mind with that post about colors of wall and ceiling wasn't so much the idea of making the room more comfortable per se, but affecting the way the mind perceives sound. More subconscious than conscious. More akin to mind matter interaction. The great sound was in the room but you can't perceive it, hear it, as such until certain things are done. It's all about mind matter interaction. 😐