Go to audioasylum.com. Do a search for Jon Risch. Find his website. He has all kinds of DIY acoustic treatment projects that are cheap to do. I can't remember specifically if he has this, but his site is worth a look.
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If you have a table saw, I have the answer for you, in the form of detailed DIY instructions for making your own Skyline-type two-dimensional primitive root diffusors. Mine are based on prime 31 instead of prime 157, which makes them simpler, less tower heights, but in head to head comparison with Skylines work just as well. (I have four Skylines on a back wall and two DIY's on a front wall, but did comparisons before arriving at that final setup.)
Email me for my DIY stuff if you could use it. Anyone else too. I've sent it to dozens of people and a few have used it with reported good results. DIY Argent Room Lens clones come with it, and they could be a good place to start.
Just to reafirm, you probably, yes, do want to stay with "diffusion" on a ceiling vs. "absorbtion" in most cases indeed! most people have carpet on the floor as well, which having absorbtion on the ceiling and floor would probably yield much too much aborbtion in the upper frequency reigions, giving a lifeless or "dead" room sound. The diffusion if going to be the best, yes.
You can do some elaborate stuff(i.e, Coffering,Poly Cylindrical diffusors, etc), which you can find by searching the net for ideas on. But the former might be a bit costly depending. You might have to get creative to do this one, as the ceiling is a large area. Your main treatment area needs to be the area between your main front speakers and your seating area however! That said, there's a number of factors to consider ideally, as different materials will have different effects on aspects of the sound. Ideally you just want to "diffuse" the reflected sound into a random, even Phase cancelling pattern...giving the room more life and helping immaging and soundstaging.
One idea I've NOT TRIED, but think would work exceptionally well(might have to fill the tubes with insullation, or various), is to use different diameters(mostly larger ones of up to a foot in diameter) of long PVC tubing running across the ceiling, cut in half pipe shapes!!!!(i.e, cut down the middle into two long half pipes). You could figure out how to fasten them to the celling joists by screwing something to the inside of the pipe, and then fastening that to the ceiling joists!!! This would, if you took caution to secure the plastic pipes to the ceiling firmly, creat a "differing" yet uneven surface that would diffuse reflections off the ceiling, greatly improving the sound in the room! You could indeed be resourceful and use other ideas. But this one I just mentioned I just mentioned would act more like "coffering" on a ceiling, which has been proven to work exceptionally well for diffusion and reflection treatment...giving the impression of a much more solid immage, much better perceived "size" to the room, and more "life to the room"!
Another idea, similar to what I recommended trying, is to build "poly cylindrical diffusors" on your ceiling. This has been done effectively in a number of high end audio installations, as well as sound rooms.(Again, F. Alton Everest mentions this technique in his "master handbook of Acoustics"). You can go to the Depot(or Lowes) and pick up materials to build this with.
Be forewarned however, that different materials and construstion techniques will cause other acoustic varriables to come into play here. You can easily over absorb at certain bass and even midrange frequencies if you're not careful, so read up first! This is mostly why I recommend the more "solid" PVC suggestion.
You could go through F Alton Everests book, and check out back issues of the audio mags, research the Audio Engeneering societies writings, and just generally check the web for ceiling acoustics, or acoustics and such.
One think you might want to consider (which I think makes the lightweight, yet solid construction of the varying PVC tubing idea more interesting).
I've built other diffusors myself, as well as many absorbers, but often elaborate diffusors made from more commonly available materials can get HEAVY!!!...not the best solution for most ceiling applications I think.
Another suggestion would be to go to www.rivesaudio.com, and perhaps you could get some assistance from Christopher or Richard on some consulting in that area.
Getting really into it, there's some more effectively perfect diffusion calculations and quadratic diffusor stuff to possibly look into, but you might have to weight it all out. BAsically, you want to just really "break" up the sound into random patterns for the above mentioned reasons.
Hope this helps