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Constrained layer damping adhesive
Constrained layer damping adhesive
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Old 7th May 2014, 10:58 PM   #1
Richidoo is offline Richidoo  United States
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Default Constrained layer damping adhesive

I built Jeff Bagby's Kairos in Baltic Birch. It is a great speaker, btw. But I did not follow his window bracing advice on cabinet construction and instead, I have a lot of corner braces of 3/4 x 3/4" BB sticks inside and some cross braces, making the max distance between braces <3", but the brace points are just a single point, not a beam. Above 300 it is pretty dead, the normal 400Hz BB ring is gone. But the LF is vibrating and distorting the sound. It must be some complex bending to get around all those braces, but there it is. I can feel some vibration on the sides when playing music. The audible resonance peak is at 220Hz. Below the resonance in the low bass it sounds a little cleaner.

I bought some heavy 18" true porcelain floor tiles to use as exterior stiffening or dampers. Each tile weighs 10+ pounds and rings high and long like a crystal goblet, very stiff, high Q. I intend to glue this onto the exterior surfaces of the speaker box on sides top and rear. I am hoping that the different resonant frequencies of the materials (BB and tile) will damp each other and the added stiffness of the tile will stop the LF ringing.

My question is about the adhesive to join them. Should I use PL polyurethane construction adhesive, which is super strong and rigid? Or should I use Green Glue which is soft but it's a true engineered damping material?

I have used green glue in this way before, laminating layers of MDF together on a planet10 Maiko2 where it worked very well with a couple staples at the edges to prevent sliding. Green Glue Company advised me that the BB will be able to absorb enough moisture to dry the green glue, but it might take a little longer than if it was 2 sheets of wood.

My question is should I think of this as a constrained layer damping situation to damp LF vibration, or should I look at it as a stiffening mod to raise the resonant freq of the cabinet to stop the LF vibes?
Thanks for any advice.
Rich
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Old 7th May 2014, 11:35 PM   #2
speaker dave is offline speaker dave  United States
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Definitely go the constrained layer route. Lowering the Q of panel resonances is always more successful than trying to push them up and out of band. The panels you add will increase wall mass, and that is what gives isolation ( prevents sound leakage).

David S
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Old 8th May 2014, 12:37 AM   #3
OllBoll is offline OllBoll  Sweden
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If you haven't yet bought your drivers I suggest getting dual drivers in the bass in an opposed PP fashion. It would cancel almost all mechanical forces which is nice. If you are going sealed then you still have to worry about stiffening the box but at least vibration isn't a problem anymore =)

If you oppose one of the drivers you also get a massive distortion reduction too, which is even more nice.
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Old 8th May 2014, 01:14 AM   #4
puppet is offline puppet  United States
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Constrained layer damping adhesive
I'm not sure that the green glue will cure enough to hold the tile to the substrate. It's not the intent of the adhesive. You'd want an adhesive that would cure out and stay flexible .. like silicone.
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Old 8th May 2014, 03:49 AM   #5
Richidoo is offline Richidoo  United States
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It's a good point about drying. With solid wood or MDF or sheetrock there is a lot of absorbent material to absorb the moisture. With BB ply, there is the surface wood layer 1.5mm thick then a glue layer which is probably waterproof, so all the moisture has to migrate through that thin top birch layer. Hmmm, thanks for the insight puppet. Anybody ever use GG with BB?

I don't want to invest a lot of money in this box, like buying special damping sheet material. I'd rather start from scratch with a vibration aware box design if GG won't work. on this one. This was the easy and cheap solution, but maybe not doable. The nice thing about PL is it cures with moisture, so no problem curing under the tile. But it isn't a true damper like GG.
Other ideas?
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Old 8th May 2014, 04:35 AM   #6
sippy is offline sippy  United Kingdom
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How about a contact contact glue?
Evo-Stick or some-such.
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Old 8th May 2014, 10:31 AM   #7
system7 is offline system7  United Kingdom
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Adding bracing willy-nilly is a recipe for disaster, IMO.

You don't say if this is closed box or reflex which affects your options for dealing with standing waves which are quite distinct from cabinet resonances, but the 12mm thin birch plywood approach is really to avoid bracing and battening. You might get away with a plywood window brace, but really the trick is to LET the ply walls flex below 100Hz here. Then the rubberised/bitumenised sidepanels control everything cabinet related above that.

You are evidently using a thicker grade of ply than is strictly BBC, but that just means it resonates higher, perhaps to 200Hz. You can't make a decent cabinet by later adding mass to it, because you really aren't improving rigidity. Where the BBC used light beech battens, it was actually for strength, not rigidity.

Your cabinet is just plain muddled between two concepts here. The BBC concept is that the vibration can freely travel around the cabinet, and get damped everytime it passes the rubberised side, top and bottom panels. The cabinet is meant to flex. The removeable front panel in BBC designs is also a deliberate feature designed to be a "Cracked Bell" that doesn't ring.

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Old 8th May 2014, 11:08 AM   #8
speaker dave is offline speaker dave  United States
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Forget the nay sayers. Go with the green glue. I have experimented extensively with constrained layer damping and it is the best way to get really dead wall panels, I.e. to kill your resonances.

Although your tiles are heavy, you will have huge surface area for gluing and they won't fall off. At this point it is also an easy mod for you to do, so give it a try.

David
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Old 8th May 2014, 05:59 PM   #9
Richidoo is offline Richidoo  United States
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Thanks for the replies gents!

The speakers are sealed, Q~.65. The walls are vibrating, so it's not just cone retransmission. Porcelain tile is extremely stiff, gluing it to almost anything would add stiffness, but only with hard glue. Soft glue would not move the resonant freq, methinks.

I can tack 2 corners of each tile with a small dab of silicone to keep them from moving. I'll probably wrap the tile edges in wood anyway for decoration.

I'm mostly concerned whether the GG would actually dry with BB, or stay wet under the tile. I'll do some testing on scrap BB to see how water moves through it.
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Old 8th May 2014, 06:17 PM   #10
pooge is offline pooge  United States
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Try Liquid Nails for Subfloors. It's cheap and damps well. Test to see if it sticks to tile first.
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