Ceiling diffusion

When facing my speakers from my listening chair my ceiling slants up from right to left from 7.8ft to a little over 10 feet. Should the diffusion/absorbtion panels be mounted flush on the ceiling or hung so they are parallel to the floor?

Ore layer,
You mention ahaving a flutter echo problem. If you have a carpeted floor between the speakers and listening chair, then the flutter echoes may not be ceiling related. Do you have a carpet?

Personally, I have ceiling diffusion from the front of the speakers to the listening chair and 8 feet in width. Above the listening chair is a bass trap to reduce a height modal null. Of course your room is different but first start with covering 1st reflection points and them test out directly above your head.

I would be surprised if having the ceiling diffusion at different heights on your sloping ceiling makes an audible difference as the sitting distance is not near field.

Kevinzoe,
I have Audio art 2" panels on the ceiling taking care of 1st reflection points and 4" panels on the 1st reflection points on the walls. I installed 4 DR6 (MSR Acoustics) diffusion panels above and slightly behind my listening position. You are correct, I did not hear any difference.
Yes I have carpet. After some careful listening I think the flutter is coming from two adjecent rooms that are open to the listening room. Maybe I should try treating those two rooms as well.

Orelayer
acoustic treatment

First try these foams. Best we have ever tried.
very light weight, easy to use and very effective
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http://www.acousticsfirst.com/sound-absorbers.htm

http://www.acousticsfirst.com/
acoustical-foam-cloudscape-ceiling-tiles.htm

We have no interest in this company, but to use them for
our own audio projects.

Not knowing your dimensions, the best we would say is to

place them at the first third distance of your room from
the source.

Add two more the next third and soon on.

Cjl,
You are on a roll today for Acoustic panel suggestions!!