Slap echo from hell

I'm an AV tech for a senior citizen lecture hall of 500 seats. When the room is filled, it sounds fantastic (aren't bodies so great at absorbing sound?) As you may have predicted.... when its empty, the slap echo is so bad and produces very distinct echo (although the reverb time isn't too bad at all).



Every speaker is mic'd and the speech goes through 16 JBL ceiling speakers dispersed evenly about the room. The audio from movie/youtube clips goes through two speakers at the front of the hall.



I've been commissioned to redesign the acoustics of the hall. I want about a 70/30 ratio of diffusion to absorption. My boss and I don't want to completely kill the hall, just get rid of the slap echo that has been raging hell on the microphones with feedback.



I'm prepared to order QRD diffusers and 2x4 absorbers. My question is, how to arrange them? Due to columns sticking out of the side walls, I'll have to arrange them in a diamond pattern to

A. Look alright and B. hang high enough as to not smack the patrons of the hall in the head (the floor is tilted at a fairly aggressive angle)



So, if I placed two diffusers and two absorbers in a diamond shape... would that be alright? My math and thought process checks out, but I just want to be sure.



Also, would I put the diffusers on the outside or inside? I'm leaning inside so that the diffusers can diffuse the sound into the absorbers more efficiently.





Please let me know your thoughts and I apologize for the long read!





p.s. we don't really have any more feedback because I was able to graphic EQ notch out the nasty frequencies, but thats just a bandaid on a broken leg.

Slap echo from hell