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RE: Progress report: Tympani IVa midrange driver problem

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...
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: 04/11/21

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