Guidelines for treating cathedral ceilings


My listening room is 18' wide x 18.5' deep, with a cathedral ceiling symmetrically over the room.
I have done the 'standard' room treatments (absorption behind the speakers and listening seats, and absorption at the first reflection points on the side walls). Absorption is 2" thick Owens Corning 703 fiberglass.

However, I am still hearing a sharp slap echo when I clap my hands in the room. I am beginning to suspect some sort of ceiling or upper corner interaction. Is there any rule of thumb for where to first start with a treating a cathedral ceiling? I am a little bit surprised, since the ceiling is angled and does not present two parallel surfaces for sound to bounce off of.

The practical concern is some sibillance when playing certain CDs at high volumes (possibly exciting something in the room and exaggerating the treble?).

thanks in advance,
Bill

(I hope this drawing comes through OK- the A is the top of the ceiling, and the two S are the speakers).

.......................A
.................... /....\
................./.............\
............../...................\
.........../..........................\
...........|...........................|
...........|...........................|
...........|...........................|
...........|.....S...............S.....|
............ --------------------------
gnobber
Chris I am not kidding..They helped my Dunlavys and all that I placed them on..Tom
I know you're not. I just thought your alternate name for them was kind of funny. I'll probably be trying them out in the near future. In the meantime , I wanted to experiment with some room treatment and couldnt figure out If you guys were treating the ceiling or the walls
I've read all your suggestions and I'm impressed- very good advice. My husband is about to shoot himself b/c the echo in our room is horrible. It's a 19' room with an assymetrical cathedral/vaulted ceiling. Compounding the problem is an entry hall, open staircase and hallway that are all connected- one big open floorplan. We're about to buy foam to make panels to put high on the walls but it seems there may be better ideas. Should we use foam panels, continuous fiberglass following the ceiling line, wool?? Any suggestions would be helpful.
My advice would be to start with some inexpensive foam panels, just to experiment. You can get 4 x 8 sheets at Home Depot, 1.5" thick, for around $10. This would allow you to temporarily hold or nail the panels in different areas and test the results. You might cut some of the panels into 2x4 and 4x4 pieces, depending on your room's corners, etc.

Overall there are better materials (some fiberglass, etc.) but I've found that you won't know what you need or where you need it until you've done a fair amount of experimentation.
I have left over carded sheeps wool that I used as part of an acoustic fix for a school recital hall.This wool is very linear in absorption unlike most foams or fiberglass. This material would work well in your home..Tom