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In Reply to: RE: Help needed: Tympani IVa midrange driver problem posted by mikebarney on April 07, 2021 at 12:50:52
Progress Report: I've confirmed that reducing the bow of the midrange back panel clearly increases the output, so thats the problem that needs to be solved.
Methodology: I built a stand to hold the driver vertically, and measured the frequency response with the driver free-standing (and the back bowed out as I received it). I then clamped a couple of aluminum bars across the back, touching the middle of the back plate, but spaced out about 1/8" on each side, which reduced the bow a small amount, but did not bring it flat, so it still has some bow.
As you'll see in the attached FR graph, the output level increases significantly with just a small reduction in the bow (shown in blue).
These graphs were made with no baffle, and with a patch cord approximation of the original crossover, so don't draw any conclusions from the absolute frequency response, just the relative measurements. And those measurements confirm what I and several contributors here thought: the output level problem is a function of the bow in the backplate, which is significantly greater in this midrange than my other channel.
Next Steps: I've designed a mechanism to route the MDF panel to mount a number of crossbars as well as clamps to held the driver in place in the MDF baffle. Once that's done, I can adjust both the number of crossbars and the tension on the crossbars to match the output level of my other channel before buttoning it back up. That will take a week or so...
Mike
PS: I was checking the woofer panels for bowing, and noticed they in fact have metal cross bars, very similar to what I plan to use on the midrange, to keep those wide panels flat.
Edits: 04/11/21Follow Ups:
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Side clamp design proposal.
This so NO HOLES are drilled in the panel, which I've that others do. Panel should be floated in the frame and I'd use a special 3M Teflon Tape (not plumbers tape) to allow some easy movement.
2nd shot is a variation using a 'bowed' piece of wood as a spring. I've been expermenting with steam bending, so this is possible.
Too much is never enough
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