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,

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

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Michael Greene of roomtune fame has products which fix this problem the original corner tunes or the even better newer PZC controllers
Dumb question time - Where are you standing when you slap your hands? If your not at your listening position it probably doesn't make a difference. Have someone stand where you have placed your speakers and clap their hands while you are seated in the listening position and see what happens.
I don't agree with Newbee. Flutter echo makes a difference anywhere in the room, since sound reflects from all of the surfaces and combines into what reaches your ears.

The source of flutter echo is usually easy to locate. If you walk carefully around clapping, you may pinpoint the exact place where it begins and ends. Open hallways, or reflections between ceiling edges and untreated hardwood floors are typical sources. You can check floor-ceiling bounces by putting thick towels or rugs down on the floor and see if that kills it.

In my setup, I've found that foams and absorbers on the ceiling can do too much absorbing. Diffusors at either reflection point work equally well and, in fact, better than absorbers. So you might experiment with RPG or other diffusors, as well as rugs, bookcases and the like when you identify the source.
One other possibility for your room shape might be reflections between the back and front walls (behind the speakers vs. behind the listener) up near the ceiling. You can tape towels or cloth on the walls to check.
To answer Newbee's question, I get the flutter echo pretty much everywhere in the room. I did have my wife clap at the speakers while I sat in the listening chair- pretty much the same results as anywhere else.

I have a plush carpet with 1/2" pad througout, so no hardwood floor interactions.

I haven't been able to pinpoint the source. I have some 1" styrofoam sheets (4'x4') that I can hold up in various places to do a 'clapping' experiment. So far no smoking gun, although I used this technique in another room and was able to tame it almost completely. I guess my next move is to try climbing a ladder and blocking various spots up near the peak, or maybe the high corners . . .
I'm not sure I have a solution, but I have an experience to share which might bare some light - I worked in an office with a high pyramid ceiling. The sound all along the outside walls (the executives offices) followed the roof line and focused at the cented and beamed down in to the center of the office (the secretarial pool). Boy was that fun!. So its possible your hand clap is bouncing off the ceiling and due to the distance travelled it would be clearly heard. Try the hand clap experiment with a piece of deadening materiel held between the clapper and the ceiling. If you don't have an echo you've found your problem and can focus on what you can do to fix it....keep us posted.
I recently have fixed a room such as the one you describe.You probably have a foam pad under your carpet. Replace the foam pad with a natural fiber pad. The panels can be made to look like large eyebrows high up on the wall behind the speakers. They can be designed to follow the angle of the roof line and can be covered with cloth to match the painted wall. You can do much the same for the rear panels as well. These panels need to be about 6ft in height and 24 to 36 inches in width, the bottom of which should be placed slightly above the television that is compounding your problem. Better than fiberglass is natural longhair carded wool. The wool is much more linear in its absorption rate than fiberglass..Wool has less suckout and will give you smoother overall response. The problem is on your walls and not your ceiling. The problem is above 4 ft and beyond. Tom
its the upper corners of the room where the walls meet the ceiling causing the majority of the echo you are hearing trust me get the MICHAEL GREEN DESIGNS PZC corner controllers which are easily adjusted will totally alleviate the echo. Believe their is a review on Audio asylum
Pzc controllers do not have enough surface area to even budge the problem. The angled ceiling is reflecting energy back into the listening room which is also a good thing. The problem lies between the front flat wall surface and the rear flat wall surface. Other than designing a multi angular cantilevered attachment to both front and rear walls the dampening panels offer the most cost effective approach..Tom
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!
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?
Mr.Promo you need the sonic surfboards..Tom
hehe,Thats good. Dont say the s word. you might get into another heated debate. The room corner treatment did help though. Thought a little more might help.
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