Some progress made recently... pics:
Photo descriptions
1. Strut bracing connects adjacent walls together, triangulating the corners which are the mechanical "ground" in a box structure. I have found this to work very well, without eating up a lot of volume. These are spaced so that 6 inches is the maximum unbraced span. There are also 3 angled, side to side braces per box. All surfaces are braced. They could have been smaller with 4" spacing, but it would be hard to get the stuffing under them.
2. The braces to the front baffle are installed before the baffle is glued on, with one end coplanar to the coming baffle. While installing the stuffing I elbowed a couple of these off by accident, due to MDF being so weak. But most subs are made of all MDF and a lot more internal stress than these, so it should be strong enough.
3. Woofer box upright as it will stand in use. It can be turned upside down to look better with woofer close to the mid driver, but that risks floor bounce cancellation, so I made it invertable. The speaker wire outlet is a simple hole drilled same size as the hookup wire in the height and width centers of the rear panel.
4. With the mighty Eton 11-580 woofer resting loosely in the hole. These are the same drivers used in the Usher Be-10 and Be-20, which have the best bass I have heard. In early Be-20s the woofer to mid crossover was 640hz! necessary because of the tiny midrange driver, but still, even with 2nd order crossover at 640hz, trumpet reference tracks sounded perfect. Amazing performance in speed and non resonance for a big woofer. they give excellent spatial detail in low freq reverb. Wicked scary slam on transients with strong amps, and they can even do organ music with low Fs of 23Hz although not too loud. I like these woofers. Found a demo pair on closeout recently, never used.
5. Closeup of the corner roundovers. The box has a 1.5" radius roundover on the front side edges to reduce diffraction from sharp corner. Actually this speaker will never play high enough for 1.5" radius to come into effect, these would need 12" radius to have an effect. So it's just for looks, to match the radius that will be on the top boxes for the mid/tweets. The other edges are 1/2" radius made with a router bit. The 1.5" radius roundovers were made by cutting off extra wood with table saw, making a 45 degree bevel edge, then further paring it down with a couple 22.5 degree cuts on either side. Then rounding off the four 22.5 edges with a 80 grit sanding disk in a drill. Feeling the roundness by hand, it wasn't too hard to make a pretty good smooth roundover. A 1.5" roundover bit is expensive and I don't own a router table, so it's not really safe to run such a big bit by hand. Moving on...
6. Closeup of the edge of the Eton hexacone material. This is very stiff and light, with mild resonance compared to metal. I don't like the cone cry and 3rd harmonic distortion that paper drivers produce as they flex under high air pressure at high spl. I want to play loud symphonies without the woofer distorting. I also like shallow crossover filters which makes using metal woofers difficult as they breakup too harshly for a shallow simple crossover. This solves both problems. Stiff, light, and relatively mild breakup. Low weight of the cone allows 91dB sensitivity.
7-8. Top and bottom of Eton.
9. The internals of the box, with pink FG stuffing on 3 walls.
The combination of plywood and MDF laminated with green glue, the corner bracing and FG stuffing makes the boxes very quiet. Knock test is very dead. More than adequate for this speaker to play no higher than 500Hz, probably more like 250-300. I think baffle step for the woofer box will be around 300, so I will try to put the passive crossover near there. But I will be doing active through JRiver filters first.
Looking at House of Kolor Urethane candy paints. It's an expensive, complicated process that requires considerable skill so I'm not yet commited to that, but looking for similar alternatives. I did find a Behr wall paint color that I like.
This means, YES, these speakers will actually be painted!