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Author Topic: Resonance problem?  (Read 4596 times)

Bill_Urick

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Resonance problem?
« on: March 02, 2009, 07:35:40 am »

We are treating one end of a room to be used as a tracking space.
The area of concern is 12'2" wide, 12' in length (of a total room length of around 27') and has a suspended grid ceiling of fabric wrapped 1" OC703 about 8' high. 1' of open space, 1' of joists/subfloor above that.

Walls are finished dry-wall. Floor is a composite Bamboo with pad on concrete.

At this point a hand-clap would produce a nasty splattering reverberation-as expected.

I didn’t want to make this room totally dead and opted for a combination of slat-diffusers backed with 703 and a lot of bass trapping. The diffuser design was copied from diffusers installed by George Augspurger at a studio here in Atlanta. (This was done with the permission of Mr. Augspurger and the studio.)

The diffusers are a repeating pattern of white pine 1x2, 1x3 and 1x4 slats, T’s and U’s, 1” 703 behind that. From floor to 2’ up is 4” thick 703 with the diffusers running vertically 5’ above that. This diffuser design covers most of the three walls with 2’ wide/8’ high 4” 703 traps spanning the corners.

If you’ve managed to read all this, thanks and kudos.

Here’s the problem: A hand-clap produces a brief after-ring at around D, 8 steps above middle C. 587 Hz or so.

It has been so far impossible to localize.

Theories:
1) A sympathetic vibration from the floor or the slats.
2) Some sort of resonance from the room itself.
3) It’s a red-herring and I just need to move on.
4) Diffusers are dangerous and I should have just used fiberglass everywhere.

We are continuing to experiment, but any thoughts or suggestions are welcome.

Thanks!
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Thomas Jouanjean

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Re: Resonance problem?
« Reply #1 on: March 02, 2009, 08:38:30 am »

Hi - I have a couple leads, but I need pictures... Hard to be precise with a description only. Can you post a couple?
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
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Bill_Urick

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Re: Resonance problem?
« Reply #2 on: March 02, 2009, 09:50:53 am »

Mr. Jouanjean,

Thanks. Will post some pictures this evening.
At this time we believe it to be the floor, but plan to recheck tonight to verify our results.
Even if it does prove to be the floor, I'd like to know why.
Pieces of 703 laid on the floor and covered with a rug seems to stop it.

Bill
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Good sense is, of all things among men, the most equally distributed; for everyone thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

Thomas Jouanjean

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Re: Resonance problem?
« Reply #3 on: March 02, 2009, 11:03:17 am »

(... Why do I feel this a residual standing wave - flutter like problem...)

If you don't put the rug on the floor but leave the 703, is it still there?
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
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Bruno Gouveia

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Re: Resonance problem?
« Reply #4 on: March 02, 2009, 05:44:31 pm »

I've a very bad experience with drywall, basically to much of it in a room and the midrange will be sacrificed. Thomas, have you this experience as well?

Bill_Urick

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Re: Resonance problem?
« Reply #5 on: March 02, 2009, 08:53:10 pm »

Thomas Jouanjean wrote on Mon, 02 March 2009 11:03

(... Why do I feel this a residual standing wave - flutter like problem...)

If you don't put the rug on the floor but leave the 703, is it still there?


Just checked. Yes, it takes the rug and the 703 to kill it.
Photos coming up.

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Bill_Urick

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Re: Resonance problem?
« Reply #6 on: March 02, 2009, 09:21:39 pm »

index.php/fa/11481/0/
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Good sense is, of all things among men, the most equally distributed; for everyone thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

Bill_Urick

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Re: Resonance problem?
« Reply #7 on: March 02, 2009, 09:25:25 pm »

index.php/fa/11482/0/
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Good sense is, of all things among men, the most equally distributed; for everyone thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

Thomas Jouanjean

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Re: Resonance problem?
« Reply #8 on: March 03, 2009, 03:04:52 am »

Bill_Urick wrote on Mon, 02 March 2009 19:53

Just checked. Yes, it takes the rug and the 703 to kill it.


I think the 703 in the ceiling is too dense. It has a too high surface impedance, which is reflecting mids.

So What *could* be happening is that because of this too high surface impedance you have a standing wave / flutter like problem happening on a very narrow band, caused by the parallel surfaces of the floor and ceiling. which you indentify as a ringing.

Your ceiling is 8', so around 2.40m, which means you'd get full cancellation at this point from ~68.75Hz wich means ~550Hz can be a problem freq too.

Also, you only can kill it with the 703 + the rug which can support the point that the 703 has a too high surface impedance. I have seen 703 only when in the US (we don't have it in Europe) and I felt this thing was not what I'd be using in a studio (indeed way too high surface impedance). Take this with a grain of salt, but that was my feeling about it.

Try this: give an angle to the ceiling tiles (a slope of more than 3" per 1').

If the problem dissapears, then we have identified the problem (hopefully...)
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
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Thomas Jouanjean

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Re: Resonance problem?
« Reply #9 on: March 03, 2009, 03:11:57 am »

Bruno Gouveia wrote on Mon, 02 March 2009 16:44

I've a very bad experience with drywall, basically to much of it in a room and the midrange will be sacrificed. Thomas, have you this experience as well?


No, not really. I don't have a bad experience with gypsum, but we always build resilient wall sandwhiches which contain other products too, not just 2 layers of gypsum. And usually not a lot of gypsum will be visible once the room is finished. So... AFAIC, I have had no problems with it yet.
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
http://www.northwardacoustics.com
http://www.facebook.com/pages/Northward-Acoustics/1062876633 71

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Bill_Urick

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Re: Resonance problem?
« Reply #10 on: March 03, 2009, 07:15:31 am »

Thomas Jouanjean wrote on Tue, 03 March 2009 03:04

I think the 703 in the ceiling is too dense. It has a too high surface impedance, which is reflecting mids.



Being, at this point, heavily invested in 703, this is not something I wanted to hear.

What material do you recommend instead?

We'll try your suggestion and let you know.

Thanks so much!

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Good sense is, of all things among men, the most equally distributed; for everyone thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

Thomas Jouanjean

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Re: Resonance problem?
« Reply #11 on: March 03, 2009, 07:51:16 am »

Recycled paper cellulose bats in density of 65-70kg/m
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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
http://www.northwardacoustics.com
http://www.facebook.com/pages/Northward-Acoustics/1062876633 71

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Bill_Urick

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Re: Resonance problem?
« Reply #12 on: March 03, 2009, 08:58:08 am »

Do you have any brand/model information on these?
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Good sense is, of all things among men, the most equally distributed; for everyone thinks himself so abundantly provided with it, that those even who are the most difficult to satisfy in everything else, do not usually desire a larger measure of this quality than they already possess.

Bruno Gouveia

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Re: Resonance problem?
« Reply #13 on: March 03, 2009, 12:37:05 pm »

Yeah, from my acoustical experience I agree with Thomas. But there's another thing that might be amplifying the flutter. How's the room geometry? Is it a L shaped room?

About gypsum / drywalls, please never leave them directly in contact with sound. Generally speaking, it's remissions, contrary to wood, really destroy the sound.

Thomas Jouanjean

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Re: Resonance problem?
« Reply #14 on: March 03, 2009, 02:54:36 pm »

Bill_Urick wrote on Tue, 03 March 2009 07:58

Do you have any brand/model information on these?

 
Around where I live there is this brand. It's great!

http://www.homatherm.com/

Check the FlexCL model.

I'm sure there is the same in the US (I've seen it) but right now I can't remember the equivalent brand I found...

There is this, but I don't think they have it in bats:

http://www.greenfiber.com/

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Thomas Jouanjean
Northward Acoustics - Engineering and Designs
http://www.northwardacoustics.com
http://www.facebook.com/pages/Northward-Acoustics/1062876633 71

Pro Audio Partners:
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