Hello frequency. Say, you know I'm no expert, but personally, since this is a very small room, except for the floor, I'd suggest making this as dead as possible and utilize "superchunks" of mineral wool or rigid fiberglass in ALL corners, even those at WALL/CEILING intersections. Perhaps use some various half cylinders( hi freq/diffusers) of various diameters in FRONT of broadband absorbers on the walls as well. On the ceiling, I would completely cover it in "suspended" broadband absorption panels, or use a combination of "skyline" type diffuser designs as well as absorbers. This will eliminate and or diffuse hi freq comb filter effects at the overhead mics, as they are usually quite high. Of course, this will also give you the option of dialing in the "fx" to taste via electronics. Trying to capture room "ambience" for drums in a small room is a lesson in modal "boomiiness". Here was my design for a drum booth approximately the same size as yours, although this was planned to include "non parallel" walls as well. The tubes were going to be cut from lenths of various diameter "abs" plastic drain pipe, and used as the fronts of slats on a John Sayer inspired "inside out" wall with a "slat absorber" surface. Here is an overall view, althought the drawing doesn't show all, it pretty much illustrates the idea. Contrary to what I've usually suggested by using "tested" concepts, I've kinda garnished different ideas from pictures of Johns various studio designs, modified and incorporated them here. Mainly, the use of "corrugated" metal roofing panels to make a curved wall surface(polycylindrical diffuser) iitself also haveing a an uneven surface by virtue of the "corrugations" in the metal. I simply substitued various diameter plastic pipes cut lengthwise for the metal corragated panels, although I think his idea is great. I've seen this idea used in other "booths" as well. Anyway, here was my plan, although I've yet to build it and at this point, I may never. Oh well.
It may not be "scientifical", but.......well, it looks good.
