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I made a small 3" diameter x 1.75" long PVC chamber for my new (but slightly used) D75MX-21 midranges (thanks again, Dan!). Below are a few construction pics with comments. Qtc with this test chamber came out to 1.2 (see DATS V2 screenshot). Is this an acceptable Qtc for a midrange of this size? Should I make the chamber bigger or smaller?
When I removed the green felt along the perimeter to glue the PVC in place, I discovered a large number of vent holes.
The PVC tube blocked off these vent holes, so I ground clearance chamfers using my dremel tool.
I made a cap out of 1/4" masonite and then glued a white rubber bumper to the center of it to spread out internal resonances.
DATS V2 measured Qtc with test chamber sealed in place with caulking cord:
FR measurement at 31", midrange placed on floor flush with thick blanket: