AudioNervosa
Systemic Development => Psycho-Acoustics => Topic started by: richidoo on August 30, 2009, 01:03:03 PM
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I want to use Green Glue (http://www.greengluecompany.com/) between two layers of 3/4 MDF to make non-resonant speaker cabinet walls. The largest parts are 12" x 48" with open air on all four edges. Some questions:
1. Is the MDF porous enough to absorb the moisture to let it dry? Is the green glue wet enough to cause significant swelling of the MDF?
2. Is the green glue a strong enough adhesive so the panels don't have to be fastened mechanically? I am thinking that staples will short circuit the damping effect. Is this true?
3. To get maximum damping effect, can I trowel the green glue to total coverage (thin coating) on both sheets to be joined, or should I use the recommended "shoot the silly string" method. Seems like a solid layer would work better, damping every cm^2. Will it still dry well?
4. I assume that the thinnest layer of damping will have stonger effect than thicker layers because sheer force will be higher in proportion to the difference in thickness. Unless Green glue damping is optimized to work best in a thicker layer.
I will try gluing up some samples, but I am interested in everyone's opinion.
Thanks!!
Rich
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1. Shouldn't be a problem in either case.
2. Definitely not.
3. You can trowel it with a notched trowel - just be prepared to throw it away.
4. It's designed to work best at the optimal thickness - 2 tubes per 4'x8' sheet. More will work SLIGHTLY better at 50% more cost. For a speaker cabinet, might work worse.
Bryan
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Rich,
Have you considered a sambitch of MDF -> Green Glue -> Hardibacker? I think that is what Bob Brines and a few others do in their speakers for CLD. Just place some squares of the Hardi on the inside walls, attached with the glue. I'm thinking of trying that on subs since I have some of it laying around after a big tile job.
Tom
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Tom, the speaker has a precise volume coupling chamber which leads to precise horn shape, so I can't stick things on the inside. But's that's a cool idea. Different materials should resist vibration better than all the same.
Bry, on #2, do you mean Green glue is not strong enough, or that staples will not short circuit the attenuation, or both? Or neither? :-k
Thanks guys
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If you're going to use GG, use FULL layers of all materials. Do not leave gaps. Those will cause resonances.
The glue will not be sufficient. I'd recommend building a box, green glueing the whole thing, then adding another layer all around. If possible attach them where the sides connect and nowhere else. We want the layers to be able to move together AND separately at the same time in this appllicaiton.
Bryan
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I still have a few tubes of it out in the garage if you want to experiment... call me and I'll share my experience with it...
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Thanks Bryan, I think I got it. Inner strong box floats inside an outer strong box separated by a layer of GG, with no mechanical connection between inner and outer boxes.
What is this green glue like when it dries? It is just soft mush like those slime frogs my kids throw against the walls? Does it stick like glue or is it just held in place by friction?
The speaker has two horn openings on the front, so I can't do a full wrap. It will be more like a speaker in a bathtub, or wearing a helmet. What's gonna happen at the edges where inner and outer layers have a GG gap exposed? Will the GG leak out like slime or is it solid enough to stay put when dry?
Thanks 'doc, I think I have only one tube donated by Carlman. This will need a little less than 2 tubes.
The good thing is I played the speakers today. They sound really good but they do ring a little. Sounds like everything has midrange reverb added.
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sticky, gooey stuff will leak that will never fully dry... it's worse than getting Liquid Nails all over your hands.
I'm not sure you want to do the "bathtub" idea....
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I can't think of a way to fully encapsulate the GG inside the inner and outer sarcophagi without them touching each other. I can glue thin wood faces over the GG edges to prevent leaking, but that will couple the layers at the edges. It should still damp the vibration right? What if I seal the edge wounds with silicone caulk? It will act as a spring with no damping, but it will stop the bleeding.
Thanks
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What if you made a front baffle with lots of big holes, would that work? That way it'd wrap around all edges and leave all the important parts on the front open..
I don't see the problem with the mechanical connection personally.. It's going to be sitting on the same base anyway.. It's not free of Newton's laws. ;) In standard applcation, you use sheetrock screws when you use this on your walls.. If you don't have a mechanical connection, I think it'll be tough to move the speakers around... or the GG will shift and get weird.
Just some thoughts...
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What if you made a front baffle with lots of big holes, would that work? That way it'd wrap around all edges and leave all the important parts on the front open..
Yes, I think that would work. Good idea. I might have to rebuild the core so I end up with the right dimensions, like horn length, etc. Worth a try. But with the big holes on the front baffle, sealed against the original baffle with green glue, I still have weeping wounds around those front baffle holes for horns and driver. I think I have to develop a way to seal the cracks.
What about silicone? Or is there another more rubbery caulk that completely dries and sticks good?
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The Green Glue takes about 30 days to fully set up. Even then, it's kind of like the consistency of a super ball.
You can fasten the layers together at the perimeters with no problem. Where you fasten them and how often will determine the resonant frequency of the assembly.
For the holes, you could just seal them with a cheap veneer that wouldn't transfer much between layers - or - you could even just do something like a gasket of some sort.
Bryan
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OK, that sounds good. Maybe vinyl contact paper, like woodtone. Like modern chinese Wharfdales.
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How about a short horn in the port. Only attached to the outer layer, not touching the inner layer?
That would keep the layers mechanically isolated.
Bob
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Good idea.... thanks Bob.
Mine are Big Vent Reflex design by Scottmoose and Dave Dlugos at planet10, similar to this:
(http://homepage.mac.com/tlinespeakers/FH/images/curveChang-tn.gif) I'll take pics next time I have them playing.
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I tried it, and it worked! Thanks for the help.
The green glue knocked down the ringing of the cabinet nicely. But there are still small panels that I didn't treat which still ring a little. I think the best thing is to build it from scratch with panels made ahead with layers of thin board glued together with GG. It only adds about 1/16" layer so 3 layers of 1/4" MDF with 2 layers of GG would be very very dead, about 7/8" thick.
I used blue masking tape to mask off a 1" clean zone at the edges of the board, then trowelled on the GG with small sawtooth notches, then peeled off the masking, and layed it on the speaker carcass, then tacked it with 1-1/4" staples at point where there is crossbracing in the speaker where it doesn't vibrate anyway. There was no leakage.
The damping effect increased noticeably over 3-4 days as the goo dried. Even after 6 months it's still like soft gum so it never really dries. Adhesion inside the thin gap will prevent any oozing.
The advantage to damping versus bracing with dowels or internal panels is that they do not reduce energy, they just change the resonant frequency, usually upward into the midrange. Adding mass lowers the frequency but does not attenuate. Damping is the only way to quiet it. Green glue is good stuff!
Thanks to Carl for 3 tubes. I used 1 1/2 tubes on this.