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[Carnac]

Ok friends, I am having some low end resonance in this room that I am trying to futher tame, but with mixed results. Beautiful octagon ceiling that slopes up to about 16ft. in a room 30'x 22'. Except for the octagon insert, ceiling height is 9'6". Well, after the room was enclosed with doors and windows it became apparent there was a honking resonance while standing on the floor smack in the center of the octagon shape. Is it the room shape or the octagon shape that is creating this? Or a combo? At this point we have tamed the low end RT with 705 1.5" baffles as you can see configured in the panned out pic. It was designed by the same acoustic guru(?) that created the space to begin with. Well, it works...kinda. Still have a honking in the center at 142Hz but not so bad that I can't work around it. It's still to heavy in the broadband bass frequencies as well as the resonant one for my liking.
I'm thinking that these octagon angles must be a contributor of bass buildup in the center cause that is the only area that exhibits an acoustic problem. This shell design for some new trap is totally my idea. I see that is the only area that is exposed to some kind of crosstalk between octagon panels.
If this is a resonable approach what do I finish these as? Do I fill with 'chunks' of 703 and frame and fabric. Or a plywood panel on one side with 703 or fluffy stuff and fabric cover. Or something else altogether? The measurements for this shell are about 4' across 2'2" tall, 1'9" deep.
The room now is lively in a good way but I tend to prefer a little less. The walls behind the fabric are most varying 1.5in. wood designs that mix things up. There is some 1.5" 703 on about 25% of the wall. Horizontal ceiling sections are all 1.5" 703. Within the octagon there are 2in. half wood rounds in front of 703. A 2x4 section on each octagon panel has been broken out from the attic side and replaced with a limp membrane (forget what it's called) covering it against the 703.  This was the first thing that was attempted to rid the room of low end buildup.
Again, it has improved but I think my area of concern might be helped further with more trapping.
Thanks for listening.

[Ethan Winer]

Cory,

Carnac wrote on Sat, 02 June 2007 23:17

Is it the room shape or the octagon shape that is creating this?

I'll let Fran give the definitive verdict, but I can tell you that concave shapes are generally to be avoided. All such shapes tend to focus the sound under the center of the circle (or under the peak in a cathedral ceiling). Focusing is bad, and is the opposite of diffusion which is good. Below is a photo showing how I treated under the 12 foot peak in my home studio.

--Ethan

[Carnac]

Hi Ethan,
 Yes, the symetrical concave shape is seemingly a bad thing!!! Had he chose some asymetrical design and not octagon I'm pretty sure it would have avoided what I dislike hearing in the center space. There is a ported octagon opening in the center however, of about 5', that opens to the attic space. This whole octagon shape fits into a dormer facade. Not alot of room up there behind this thing.
  Actually I wanted a design that would have allowed for acoustic flexibility with regards to finish treatments but the designer (Jeff Cooper) was so enthalled with this  ('spiritual' he described it) he had me convinced it would suit my applications with acoustic music ensembles. Well, it's been a nightmare. I resisted the baffle idea fix initially because it just seems odd to have all this stuff flying up there when it could have all been hidden treatments construction-wise from the git go. But I finally installed it and it does help but still leaves me bugged by the remaining bass resonance in the center.
  At this point I cannot afford anything but the DIY approach. Will not a bunch fiberglass in those angled areas help tame bass in the center? When I walk outside the 7'x7' center area on the floor I like what I hear. Inside the 7x7 the low end sounds heavy.
  I've had Steven Klien here for a few days to help with wall treatments underneath the fabric but he was bit unsure about the cause of the peaky middle. George Ausberger came in in for a look see and really had nothing definitive to offer except to keep experimenting covering up various areas and accessing the results.
 I'm needing some DIY solutions here and not so much the 'it's bad to have that design'. Oh God ! don't I know it.

[franman]

If this Octagonal peaked ceiling is a hard construction, then I believe it's the cause.. as Ethan says, these types of shapes are focusing and this is generally bad...

This type of 'architectural appearance' could have been achieved with framed trapping covered in stretched fabric and you could have hidden all the baffles/trapping as you noted..

Oh well, that was then and this is now... Not being able to 'hear' the room, I would recommend doing some serious depth of broadband trapping right over the center area (peak).. maybe 2' deep with 4" of 703 in the flat framed surface and some hanging baffles, like you've done, or some loose batt insulation in the cavity above...

Think of it as an opportunity... not a problem (a great quote from a contractor I know!!)... it's a chance to do some significant broadband bass trapping which it sounds like you need, based on the description of the other treatments..

that's my 10 cents without hearing the room of course..

[Carnac]

Thank you Fran, that sounds like something I am going to consider seriously. I had envisioned awhile ago dropping a false ceiling down about halfway (not sure really) into the octagon space. Frame 2x4 and cover with fabric (that I still have enough left). In the cavity above loop (accordian style) batt insulation to varying depth loops. Use the sloping part of the ceiling to staple to as well as span over the center area (at 2' depth?) and staple it to that. How much sq. footage would be needed? I could do some mock ups before commiting to the structure
Dropping a suspended structure would have to look 'purty' as opposed to framing off the existing structure and hiding the batting behind the fabric. Then I could put four can lights in and maybe create a chute for a simpler skylight.
Also the center opening spans about 5' and is a skylight but with thin cloth. I guess it still is considered to narrow too qualify as an 'unpeaked' structure.
I could take down the baffles (hooray! and clean up the look of the space. It's just a shame I could not have that 'architectural appearance' with the bass trapping hidden.
Am I on the right track?
10 cents indeed!
Thank you!

[Carnac]

This is working, even at this stage of developement, better than the 705 baffles. Think I'll go ahead and frame the ceiling height just under the blue tape, redo the trapping better, etc. It'll take a few weeks, going on a vacation soon and will knock it out after that.
Thanks for the advice, it helped push me to look at the 'oppourtunity' to do some serious bass trapping.

[franman]

That looks like a nice versions of what I was suggesting... The only thing I would point out is to be careful about how you deal with the opening for the skylight...

I might suggest a framed opening covered with stretched fabric that creates a channel up to the skylite opening... I wouldn't want to have hard surfaces on the side around this channel as it would again provide a cavity to resonate and send nasty stuff down into the middle of the room.

Good work so far!!

[Carnac]

I'll use either the white ceiling fabric or Mylar (2mils thick) to create the chute down to the new skylight. Should be pretty transparent in those lower frequencies. Again thanks for the encouragement to try this, it's really working!

[franman]

it's only because the answer was sealed in a mayonaise jar on funk and wagnels porch since noon Sunday... that's the real ticket!!  

[Carnac]

You are correct sir!  

[Carnac]

Thought I would post the work I finished this morning. Carpenter framed... I stuffed. Looped the batting in two different areas within each bay. I call them bays cause I used the original 705 hanging baffles and placed one on top of each 2x4 thereby creating something like a separate space. They snuggled up real nice cause of the angle on the backside was already sloped to the correct angle. Very cool, almost like I planned using it there.
And, as you can see I used Auralex space couplers in between each bay and also attached Minifusors to each baffle side. Very nice sound now. Lots of life on the floor and no bad acoustic spots anymore. Very musical sound. Balanced.
 Thanks for the input Fran and Ethan. Good to move forward now smiling with what happened.

[franman]

Glad it worked out well... I'm still not sure what the acoustic couplers do in this type of application, but if you had them already???? anyway... glad it was a succesful operation without having to rip things apart!!

[franman]

Glad it worked out well... I'm still not sure what the acoustic couplers do in this type of application, but if you had them already???? anyway... glad it was a succesful operation without having to rip things apart!!

[Carnac]

franman wrote on Fri, 22 June 2007 17:08

I'm still not sure what the acoustic couplers do in this type of application, but if you had them already????

Back from a vacation...I wanted to show the use of the couplers previous to my adding them to the new ceiling. The last acoustic 'expert' in here suggested using them in this fashion to break up the bass resonance peak. Well, it only seem to add to the problem. In retrospect now, from your earlier suggestion, if I had hoisted this contraption up to about 2ft. from the peak and layed 6in. of 704 on top of it I bet we woulda solved it in a more elegant fashion and kept some of the spaciousness to the room.
But I'm OK with the new ceiling. Certainly the couplers add 'something' back into the new space, some kind of diffuse reflections perhaps? Hell if I know.