A friend of mine is building a cinema room. He's going to put a pair of huge (and I mean HUGE) speakers in a rather modestly sized ~15X15ft room. All the walls as well as the ceiling are stuffed with rock wool, but I told him he'll probably still have huge bass issues. Other than having some type of external traps in the corners, he's left (in a rare moment of clarity) a big recess in the ceiling, a rectangle about 6X9, which is about 2ft deep.
Question is - what's the best way to utilize this recess. Build a bass trap? What kind? Or maybe use some of the space for diffusing or absorbing elements?
Guidance will be appreciated.
Thanks all,
-T