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Old 5th September 2007, 02:30 AM   #1
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Default W-frame construction tips sought

Hi,

My understanding from reviewing the archives is that the SL-style W-frame dipole woofer can suffer from panel vibration, as the drivers directly face a panel on either side.

I'd like to address this potential issue during construction, and would be interested in suggestions on how to deal with it.

SL advocates mounting the drivers by the magnets, loosely coupled to a baffle as one technique, but doesn't mention this for use with 12" drivers. Is this approach not feasible?

What might be other effective techniques for bracing and/or damping the panel?

Thanks and regards,

Rob
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Old 7th September 2007, 12:02 AM   #2
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Default this should work well....

lets say that the panel in question is13 inches square. Build it out of two 3/4 inch pieces of birtch ply (void free). Space the two layers using a preferal spacer of 1/2 inch thick ply about 3/4 of an inch wide. In the centre of this position a disk of 3/8 inch thick ply about 1 1/2 inches in diametre. Drill a centre hole through the two outside skins as well as through the centre disk. Mount the 1/2 inch thick X 3/4 wide perimater spacer on to one panel along with the center disk (3/8 X 1 1/2). When set up mount the second 3/4 thick panel with lots of glue and screws to the joining surfaces. Then insert a bolt through the centre hole using flat washers and a nut.
When it is all set up you will have a pretensioned panel with slightly concave panels. This will be as rigid as I can imagine you can achieve for its thickness. You may want to experiment with the thickness on the centre spacer to get the fit right which you can do without glue using clamps. You don't need much of a bow in the panel to prestress it though. You might want to glue the top side of the centre disk after the two panels have been glued and screwed together and have fully set up. You can apply the glue to that surface through the centre hole then pull the two pieces together with the bolt. With a little experimentation you can figure the spacing to give you enough flex on the panel. You can experiment with thinner ply for the outer skins to see what works best (from a flex point of view) building the composite panel. once the whole assembly has set up I doubt that you will need the bolt and the hole can be plugged with a piece of dowel to look nice.
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Old 7th September 2007, 01:08 AM   #3
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I'm having trouble visualizing the solution suggested.

What if I provided a single cross brace at each opening, and lined the inside of the cavities with Whispermat or similar?

Thanks,

Rob
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Old 7th September 2007, 02:01 AM   #4
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Default this is...

two 13 inch panels spaced apart by a 3/4 inch wide spacers all around the outside edge. There is also a disk spacer in the centre of the two plys. The spacer in the centre is a little thinner than is the outside spacers so that when you insert the blot and tighten it up you flex the two paneld toward each other. This places a dished shape on the two panels. The whole thing is then prestressed. To make the panel flex as a whole you would need to have some flex in the panels. The panels can only flex if there is available material in them to stretch and the prestressed dish has removed any possible streatchable material from the two panels so no flex. If you want a thin but really stiff panel that's a really good way to get it. This assembly is probably stiffer than three plys of 3/4 ply laminated together and by a good margin plus it does not weigh a ton. If you really want to go nuts you can then fill the void inbetween the two stressed panels with sand using a hole on the inside surface of the composite panel. Probably overkill as the panel resonance will be well outside the bandwidth of the subs range.
The crossbrace is a good added idea but will not stop the panel from flexing at its centre. The whispermat is just a waste of money and space here as it won't do a thing to damp a panel like this. It might catch some of the cones upper range as it reflects off the panel but will do zip to stopping the panel itself from resonating.
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