Less "board-y" or "cabinet door-y" than just the plain hunks of 1/2" plywood I was using in the original mockup. They're not perfect, but certainly better. I can record the difference and post a link sometime. Hopefully an "at the listening position" recording also. The panels are 20 X 20", I found a pile of 8 pieces 12 X 12 X 1/4" cork which I glued up to 4 cut sections of a large sheet of 1/4" ply I had on hand. I used the 3M spray adhesive for attaching laminate to kitchen countertops to build the layup.
The local farm supply sells 1/4" rubber sheet, intended for footing in animal stalls. I think that stuff would do even better than the cork.
Here's a picture of the backside; the hard plastic traffic cone ($1.29 at the dollar store) is stuffed with absorbent padding. I got the idea from reading XRK's posts on the dagger enclosure. The driver inside is wired in series with, out of phase with, the main driver on the opposite panel. A parallel 8 ohm resistor decreases output somewhat, making the SDA effect more subtle, vs in your face. These drivers run maybe 5 ohms for a lot of their range above resonance, my amp is small and I dont think it'd like a 3 ohm load as much as ~9 - so series, versus parallel.
Instead of pinning the panel at the bottom edge to stand, I decided to make a 2.5" wide spine of MDF shelving; there's a 5 deg tip back of the panel also. I had to cut 5 individual reliefs for the backside of the drivers because...I glued on the spine before realizing I had to route out the reliefs on the backside of the driver cutouts! Hand drill - drum sander.
It sounds so much better with the SDA drivers enclosed - that's what makes the idea "work" for me, anyway.
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Did they work? I mean, the claim I thought I'd read is it's similar loading to the driver as OB - which is what I'd want; same as open, but no sound from that side.
The single version seems to work really well, used with a driver mounted magnet out ( as an omni ) the tiny 40mm driver fills the room remarkably well - with help of a sub. The square quad version is yet to be tested.
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That is the sealed half-wave midTL just like i was first shown in the late 70s.
X's dagger is typically shorter, 3 sided and goes to a single point.
dave