diffusion on the rear wall of CR - above listening position

intro> In my new place I have 4.5x4.7x2.7m room (i know i know, it is almost square, but what can I do about it). I designed drop ceiling with perforated (~5%) gypsum cover. It is half tilted (the tilt begins in the middle of the room lenght and it is 12°) stuffed with rockwool 5 to 20cm thick and air gap 5 to 30cm high. I hope this ceiling will take a lot of the bass boom, however I don't want to stop there... more rockwooling to come. question 1> I want the rear wall to have absorbtive bottom half up to 150cm (10cm rockwool over 20cm airgap), this should absorb the reflections from the monitors. And above I'd like to put diffuzors, to broke the room reflections a bit...mostly for the recording (voiceovers etc) that can take place this high. I know there is a lot of disrecommendation for diffusion in CR and the back is only around 3.0m away from listening position, but I think this can work. Or am I terribly wrong? ;-))) One thing to add... I have plenty of old par parquets available and I want to use as the diffusor material, so it will be pretty cheap in the end. question 2> One thing bugs me, the workers putting up the drop ceiling used quite a thick foil under the rockwool... and I wasn't there to stop them. I saw a lots of studios putting rockwool in thin foils... it seems ok, but this thick one makes me worry about the efficiency of the treatment as an absorber. In the worst scenario I will pierce through the holes and penetrate the foil... tell me I don't have to ;-))

why not put the diffussor in front of you instead of behind you. The foil will just keep your high end a bit brighter - shouldn't cause any problems IMO in fact it will be a benefit. cheers john

diffusion in front? that sounds strange to me... you mean like heavy absorbtion in the height of speakers/listening position and diffusion above?

oh and thank you ;-)

Well the diffusor is only going to effect the highs and all your highs are directed behind you and you don't want them coming back to you as it will muddy your imaging - because of the directivity of your speakers the front diffusor won't effect them but it will effect the highs in the room for your vocals etc. cheers john

good point! but it would be vise to put rockwool behind the diffuzor or somewhere near to tame the lo-mid freq difracted at the edge of (free standing in my case ADAM S3A) speakers. Sorry I didn't mentioned I'm not going to soffit mount...

well personally I wouldn't put a diffusor into a room that size anyway :) I'd go more for slots to tame the low mids and to break up the parrallels.. cheers john

If you're going to stand-mount the speakers, then having the front end of the room (behind the speakers) absorb lows but reflect highs and mids will most likely serve you best. If you find you're getting some weird reflections off that wall when you record vocals or whatever in the room, a diffusor will break that up, but as far as sound coming out of the speakers, very little mid or high freq sound will be hitting that wall directly. In a room that size, the back wall should absorb as much as you can get it to.