Topic: Resonance problem? (Read 6239 times)

Bill_Urick · « **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!

Thomas Jouanjean · « **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?

Bill_Urick · « **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

Thomas Jouanjean · « **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?

Bruno Gouveia · « **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 · « **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.

Bill_Urick · « **Reply #6 on:** March 02, 2009, 09:21:39 PM »

Bill_Urick · « **Reply #7 on:** March 02, 2009, 09:25:25 PM »

Thomas Jouanjean · « **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...)

Thomas Jouanjean · « **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.

Bill_Urick · « **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!

Thomas Jouanjean · « **Reply #11 on:** March 03, 2009, 07:51:16 AM »

Recycled paper cellulose bats in density of 65-70kg/m

Bill_Urick · « **Reply #12 on:** March 03, 2009, 08:58:08 AM »

Do you have any brand/model information on these?

Bruno Gouveia · « **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 · « **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/

andrebrito · « **Reply #15 on:** March 03, 2009, 03:11:39 PM »

I also dislike rooms with gypsum they sound horrible ! boinggg !

franman · « **Reply #16 on:** March 03, 2009, 08:59:10 PM »

Bill_Urick wrote on Tue, 03 March 2009 07:15

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!

Guys,
I can't help but butt in here. Thomas, please trust me the OC703 is a staple of all of our studio designs. It's surface impedance has never been a problem. We use it as the first surface in many treatments and sometimes even the more dense 705. Even at a relatively low angle of incidence, the 703 (at 3lbs/ CF) isn't a problem with mid frequency reflections. It would certainly not be the cause of flutter issues at the frequency in question.

There may be a resonance, or you may be hearing (previously) masked flutter at a specific frequency as this area of the room is now well damped. Could it be coming from another area in the overall space??

I don't pretend to have the answer, and to tell you the truth I would never try to diagnose this without an 'in person' visit, but I felt I needed to chime in on the OC703 issue.. Bill, don't get too worried about the 703 being the problem...

FM

Thomas Jouanjean · « **Reply #17 on:** March 04, 2009, 03:01:18 AM »

franman wrote on Tue, 03 March 2009 19:59

Thomas, please trust me the OC703 is a staple of all of our studio designs. It's surface impedance has never been a problem.

I was still very surprised by the product everytime I saw it. The ones I saw used in U.S. studios always had a rigid waffled crust like thin layer on, which I noticed reflected lots of energy. Hence my immediate reaction to Bill's problem...

The rockwool of equivalent density here doesn't have that attribute at all, much smoother surface (We don't have such type of densities in fiberglass here, so I can only compare it to rockwool).

Anyway, you are used to that product and I'm not, so I absolutely take your word for it Smile

(Note to self: go harass a 703 provider in the US next time around)

Ethan Winer · « **Reply #18 on:** March 04, 2009, 02:06:49 PM »

franman wrote on Tue, 03 March 2009 20:59

Even at a relatively low angle of incidence, the 703 (at 3lbs/ CF) isn't a problem with mid frequency reflections. It would certainly not be the cause of flutter issues at the frequency in question.

I'm glad you said that first. Laughing

The measurement shown in the graph below was taken in my living room, and shows the change in response with and without 1-inch thick 703 covering a wood "wall" placed nearby. The angle was about 45 degrees, so clearly 703 works well even at an angle.

--Ethan

http://www.realtraps.com/rfz-response.gif

Thomas Jouanjean · « **Reply #19 on:** March 04, 2009, 03:42:48 PM »

Ethan Winer wrote on Wed, 04 March 2009 13:06

The measurement shown in the graph below was taken in my living room, and shows the change in response with and without 1-inch thick 703 covering a wood "wall" placed nearby. The angle was about 45 degrees, so clearly 703 works well even at an angle.

Thanks for the graph Ethan!

My problem was not a question of incidence though, but of surface impedance which I remember from my US projects to be too high for my tastes (hence I used something else of about similar density, but with a different surface: rockwool).

I did notice though that the core of it had a nice density, which I assumed would make it quite efficient in the lower frequency range.

But then again, this is not a product I am used to as we don't have it in Europe, so I am, as expected, cautious Smile

I will soon have another chance to try it and maybe work with it on a new project on your side of the pond, which will be interesting!

Anyway, back on topic: Bill, any news? Did you try anything?

Bill_Urick · « **Reply #20 on:** March 04, 2009, 07:48:38 PM »

So far, we're planning on not recording any more handclaps, or anything that has a "D" in it.

Edit-No, but seriously...

Not sure, rug on the floor, perhaps some additional non-703 stuff on the ceiling.

I'm feeling reassured re the 703, but there's already a lot of it and it's not killing the ring.

I'm believing it's something about the flooring or just a room resonance.

Could it still be a reflection floor to ceiling? The rug over 703 did kill it.

Ethan Winer · « **Reply #21 on:** March 05, 2009, 05:04:19 PM »

Thomas Jouanjean wrote on Wed, 04 March 2009 15:42

My problem was not a question of incidence though, but of surface impedance

It always seemed to me that the very best mid/high frequency absorber would be fluffy fiberglass. Or a layer of fluffy fiberglass in front of rigid fiberglass, with the fluffy part at least two inches thick. But I never tested that so I'm just guessing. And my test of rigid fiberglass above shows it's fine anyway, even at an angle.

--Ethan

Bill_Urick · « **Reply #22 on:** March 12, 2009, 05:48:45 AM »

Update, FWIW:

Thinking and reading around on this forum, came to the conclusion that trying to make a room this small very live is a mistake.

So far, a rug with a pad underneath is helping a lot.
Next step is to replace some of the slats with absorption, so that we don't have any slat diffusers directly opposite each other.

Letya nough...

rankus · « **Reply #23 on:** March 13, 2009, 02:52:26 PM »

Bill_Urick wrote on Thu, 12 March 2009 02:48

Update, FWIW:

Thinking and reading around on this forum, came to the conclusion that trying to make a room this small very live is a mistake.

As someone with a small room I can agree. I started out as live as possible and kept adding until it got to approx 30% absorbing coverage. Which is where it stayed for the last few years. IE: Broadband traps, clouds, carpet... Then I leave the door open with a mic down the hall for a livelier sound... works decently

R/E/P Community

Author Topic: Resonance problem? (Read 6239 times)