Grannyring,
I've done quite a lot of diffusion studying, communicating with Prof Trevor Cox and Dr Sean Olive, and built several DIY Skylines and Hemi-Cylindrical diffusers, so I hope my words of wisdom will be considered by you.
CAUTION: www.mioculture.com/store/pc/vi... This stuff is recycled paper, is only 2.25" deep, and its installation instructions say it will warp with humidity. RED FLAG, RED FLAG, RED FLAG. Paper is too thin to be considered a diffuser, except for the highest of frequencies (which is great if you're a bat). It's depth needs to be a miminum of 8" so 2.25" is clearly inadequate. So, I'd recommend you pass on it. (I guess you get what you pay for in this instance.)
Why do you want diffusion (or anything) on the ceiling? Is it to raise the apparent ceiling height? I can tell you from 1st hand experience that the RPG Skylines on my 7ft ceiling had little effect on raising the apparent stage height. Dissapointed - yes, a bit, but it could be due to any number of things like my speaker's vertical off-axis response being poor, too sharp an angle to 1st reflection point based on sitting distance from speakers etc etc.
Have you considered using say 2"*6" or larger planks of wood and angling them? Start with the 2" part touching your ceiling and hanging down perpendicular to the ceiling and running across the room width of your ceiling to cover the left and right 1st reflection points. Then begin to move the bottom of the wood up and towards your speakers; any sound hitting the ceiling will become trapped inside the area bound by the ceiling and plank and will bounce around before coming out delayed and attenuated. Hang several of these and you can elect to put some fabric over them and put fibreglass inside the fabric to partially absorb some of the sound.
Another effective and yet reasonably priced product is GIK's D1 diffuser which follows a modified QRD pattern. Hang them so the well orientation aligns with the room's length so sound hitting it will be diffused to the sides.
You will want diffusion that latterally affects the sound so QRD (with wells oriented vertically) or hemi-diffusers (convex and length oriented vertically) should do the trick. Latteral sounds help envelop the listener and widen the sound source, both positive traits.
With any diffusion product, it is the depth of the QRD wells, Skyline cells, or hemi-cylindrical radius that determines the low frequency that it will effectively work to. You want one that will work down to 300Hz (which is a good estimate for a residential room's Schroeder/transition frequency) for which Dr Floyd Toole recommends a minimum of an 8" depth for QRD and Skyines and 12" depth for hemi-diffusers. The material needs to be firm enough - no paper products please - to be effective at diffusing high and medium frequencies.
PLACEMENT: I would suggest that you put diffusion on the wall behind your listening chair before you try the front wall. Firstly, you're sitting closer to the back wall so it's effects should be more pronounced and will help deal with many 1st reflection paths from the speakers. Secondly, unless your speakers have rear-firing tweeters that're turned on, the front wall is likely the least important because sound has hit several surfaces before making its way to the front and each surface 'hit' attenuates it, and the sound signal experiences propogation loss (6dB down per doubling of distance). My experience with absorption on the front wall - I'm excluding bass traps in the corners now - is that the absorption will deepen the sound stage while diffusion will bring it forward, clearly a personal preference thing.
I would suggest that you also take down the side wall absorption - especially if it's thinner than 4" - and listen with just the bare wall as Toole's studies have shown the side wall reflections to be GOOD things in widening the apparent sound source when enjoying music. He saying that side wall 1st reflection points should be absorbed when you're mixing music or evaluating listening components.
In my room I had a wicked slap echo between the side walls so my hemi-diffuser (i)killed the spap echos, (ii)had a hard exterior (i.e.red oak veneer) so that medium/high frequencies weren't absorbed and (iii) used OC 701 fibreglass inside it for addional bass trapping. I experimented with moving the hemi-diffuser along the side wall and found the best sound came when the 1st reflection point on a side wall from the closest speaker was left bare (i.e. reflective) while the reflection point from the speaker across the room was diffused. This seems to align with Toole's research that says the most significant side wall reflection comes from 60degrees (looking straight ahead = 0degrees and over your shoulder would be 90degrees). You can use trigonometry to figure out where the 60degree point is and mark it - it may be very close to the 1st reflection point of the closest speaker when you use the mirror tecnique.
