Diffuser decision for ceiling

Here's what I think you should do: Treat the ceiling

What I think you should avoid however is diffusion there.  Your ceiling is too low.  You want pure absorption. 

Further, please keep in mind that treating 1st reflections ALONE is usually not very useful.  You need a minimum critical mass of room control before that really helps.  In other words, yes, treat the 1st reflection points, but plan to have 2-4 x more treatment than that. 

I like to think of the room as a cross, with the arms of the cross the speaker locations.  You want diffusion at the 4 points of the cross, absorption elsewhere.   That is, directly between the speakers, to the sides of the speakers and behind the listener are good places for diffusion.  Absorbers elsewhere, and possibly bass traps in the corners based on room response. 

Thanks @erik_squires 

I like this info:

I like to think of the room as a cross, with the arms of the cross the speaker locations.  You want diffusion at the 4 points of the cross, absorption elsewhere.   That is, directly between the speakers, to the sides of the speakers and behind the listener are good places for diffusion.  Absorbers elsewhere, and possibly bass traps in the corners based on room response. 

I think what I will do is wait for the ceiling, treat the front and back walls first. Try to learn REW before that , take measurements and go step by step.

The side of speakers, at the very end, i would need to remove the 1 panel on each side and replace it with diffusers.

My front wall (behind the speakers in the middle) I was told to use QRD diffusers. 

My front wall (behind the speakers in the middle) I was told to use QRD diffusers. 

With that short ceiling, I’d stick to horizontal diffusers vs. any that do vertical and horizontal.  Not just behind the speakers but anywhere else you plan on using them. 

The long slats in a QRD are ideal for this. 

Thanks and what type of diffuser should I use on the sides of the speakers?

On my back wall, I was told to use a 2D QRD Diffuser (the wooden one with squares)

I had Vicoustic do an analysis of my 12 x 22 room. On the ceiling, starting at the front wall, I have four cinema rounds, followed by 2 DC two at the first reflection point, followed by three wave woods, followed by two cinema rounds. As far as the Wavewoods, I found that the Vic Pattern Ultra exacerbated a suckout between 100 and 200 hz, so I only used one, flanked by two Wavewood Ultra Diffusers MK IIs. Very positive results. I used REW to run curves before treatment, then I ran a curve after installing each row, experimenting with quantity and placement so that I knew what each component was doing.