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Old 11th November 2004, 02:47 PM   #21
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I have used two part polyurethane resin with about 60% by weight 320 mesh silica filler. It was very dense. The filler keeps the shrink down also. The fine mesh filler keeps the surface nice.
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Old 11th November 2004, 03:33 PM   #22
rdf is offline rdf  Canada
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Quote:
Originally posted by trans192

8. If you play your cards right you can create a pair of skins of another material and simply use the plastic to fill the void. If done right you can side step the mould, but then you need to worry about finishing.
Thanks, that's very useful and exactly the thought I'm toying with for a pair of Jordan JX92s. Minus the details: two thin aluminum skins formed into a cylindrical JR149-type shape, the void between filled with a resin. Possibly a third sheet in the middle to create a psuedo constrained-layer damped cabinet. The baffle can be 1/4" aluminum between the skins, drilled and tapped for speaker screws.
I've had the opportunity to compare two wildly different cabinet materials - 1/4" carbon steel and 3/4" pine - and though both are ridiculous the difference through the bulk of the lower midrange and down is amazing. I'm really keen to avoid wood for the Jordans and a resin sandwich construction seems appealing and relatively easy to make.
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Old 11th November 2004, 04:11 PM   #23
Nuuk is offline Nuuk  United Kingdom
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If you have two 'skins' you can also fill the void with expanded polystyrene using the aerosols that they sell at DIY stores.
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Old 12th November 2004, 04:23 AM   #24
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I like aluminum for the skins. I thought I was being smart and used sonotube. The tubes worked fine to make the cabinet and there were no structural issues from that stand point. The problem I encountered was in finishing. The thick layers of paper were murder to flatten out for a fine high gloss finish.

If you decide to go with the expanding foam nuuk mentions above, you should be careful not to use to thin a material for the skin, as the foam may cause it to buldge out.

For my baffle I used 2 inch think oak. It was a bear to cut the baffle holes in and the finished product, although strong, was not resonance free. the thin area between the tweeter hole and the woofer holes vibrated quite noticeably.

I would suspect that quarter inch aluminum is too thin for you baffles. If you investigate the Wilson Benesch line of speakers, they are using composite cabinets and metal baffles. I think depending on sizes of your drivers and baffles, you should be looking at something thicker than quarter inch. The WB act2 white paper suggests they use an 18mm steel/aluminum sandwich (constrained layer maybe) for their baffles.
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Old 12th November 2004, 05:10 AM   #25
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I work for a company that builds sunrooms and screenrooms. We have this board made from a sandwich of plastic, superdense foam, and plastic. It is extremely light weight (a 4x8 sheet weighs less than 30lbs) with a thickness of about 2.5 inches. The board seems to be very stiff and might be a neat experiment for a subwoofer enclosure. I was thinking how nice it would be not to have a 200 pound subwoofer.
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Old 12th November 2004, 05:37 AM   #26
rdf is offline rdf  Canada
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For the front baffle my thinking was more along the lines of skin-damping-1/4" alu- damping-skin. I don't want too much overhang at the back of the driver. Total thickness would be closer to 1/2" of five different layers and the baffle itself wouldn't be much wider than the Jordan, 8" max. It would be trivial to cut the two surface skins in such a way as to create a non-uniform wall thickness in the resin sections, say 1/2" at the baffle, quickly expanding to 2" by half way towards the back and taperig to 1" at the rear. It would be extrodinarily stiff. In fact, if someone wanted to go to the trouble they could deform the internal skin into any desired arbitrary shape to optimize stiffness or control internal reflections.
I'm not sold on using a light material between the skins though. The cabinet walls must be stiff to avoid resonating - either material could do that - but it must also contain bass waves. I'm not convinced a light foam will do that.
BTW, one of the thoughts I had was bonding an aluminum wrap around Sonotube with polyurethane glue. Instead I think I'll use Sonotube as a jig for shaping the walls and go high mass for the resin.
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Old 12th November 2004, 03:05 PM   #27
VvvvvV is offline VvvvvV  United Kingdom
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Great advice for the moulding resin and polyurethane and thing, especially with the preparation and hazard instructions. Installation foam isn't very springy though, I wouldn't recommend it for filling any void, in fact I would suggest to get some expanded polystyrene from any place and put it in a blender until a desirable grain size is achieved and then use this in the void. It would probably have better soundproofing properties. But you probably aren't as insane as me.

To mould a perfectly round 12 inch pipe, do you think it might be possible to make a 12 inch tube from some suitable cardboard, and to link it to a motor so it turned around like a wheel, and then pour moulding resin on it so that gravity assures an even coat.? How viscous is it? If this method works wouldn't it allow you to sandwiches many layers as you want just like skins, but in a composite tube instead ?
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Old 12th November 2004, 04:40 PM   #28
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Quote:
Originally posted by VvvvvV
and put it in a blender until a desirable grain size is achieved. -snip- But you probably aren't as insane as me.
I'm guessing this isn't the blender your wife uses for margarita's.

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Old 12th November 2004, 05:25 PM   #29
Variac is offline Variac  United States
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Good sailboats are made from rigid foam (1-2") or balsa sandwiched between outer layers of polyester or epoxy resin infused fibereglass, kevlar, and carbon fiber. The whole shebang is put in a (big!) vacuum bag to squueze out excess resin.

This makes a VERY stiff hull that also has some damping It apears. Although rigidity is a major factor on a boat, they also need some toughness to take impacts and overloads, which is why they don't use only carbon fiber. In a speaker I would think that carbon fiber is the BEST. I believe that W. Benesh agrees.
I would think that even one layer of carbon separated by an inch of foam would be impressive..
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Old 12th November 2004, 05:51 PM   #30
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I'm surprised no one has mentioned concrete yet. It's a very effective speaker material and you can make cabinets by pouring liquid concrete into molds.
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