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
Theaudiotweak, it looks like you were right on with your suggestion about treating the upper walls. I just got finished building more panels for the front wall behind the speakers. These run along the border between the angled ceiling and the wall, are 18 ft wide, 33" high in the middle, and taper down to nothing as you go left or right. Height is 9-12' above the ground. Great results: this totally killed the rising ringing sound I was hearing with hand claps.

I am now able to hear a little more depth and detail, and some of the sibilance in my favorite recordings seem to strike quickly and not linger on like they used to. Thanks for the advice, everybody!
Bill
Gnobber, Did you mean that your panels are 18 ' wide? Im experiencing some standing waves resonating in my room on certain material and have fixed some of the problem by hanging some plants in the top corners but it hasn't fixed the problem completely. Im trying to follow what you guys did but im still not sure exactly where these panels should go. Look at the pic of my system and see if the ceiling resembles yours.
the panel looks something like a roof truss- 18' wide, because that is the width of the room. 33" high, because that is the distance from the lowest part of the ceiling (9' above ground) to the peak of the cathedral part. I looked at your photo, and your speakers are 90 degrees away from where mine are (i.e. the peak of your ceiling is between your listening spot and the speakers, and my peak runs along the axis between my speakers and my listening spot). So, I might still advise trying this tweak if you have a symmetric cathedral ceiling. This stuff might be hard to visualize, so feel free to write back if you have more questions.
So I would treat the whole legnth from the bottom of the ceiling to the ridge of the vaulted ceiling?