A nice deal came up on a Peavey Black Widow 18". The new version on Parts Express costs about $570 for a pair shipped to Australia. As I bought these second hand, the cost was considerably less.
SPL: 96 dB 1W1m
Le= 2.073 mH
These will be powered with a Behringer Europower EP2500 which can put out 450w into their 8 ohm load. Turns out that much power is "just right" for this driver.
The nearfield response shown above was measured on an open baffle. You can see the response is quite smooth, but high inductance rolls of the top end early. At 200 Hz the sensitivity has dropped by 2 dB to 94 dB and at 300 Hz it is only 90 dB.
The numbers work quite well for a 90L sealed box or 170L vented. The design that I have in mind will be more like 150L. The reduction is not significant and saves buying more sheets.
It will feature a composite of 18mm MDF and 17mm form ply, with a thicker front baffle. The sides and rear will be constructed out of one long continuous curve similar to the JL Audio Gotham.
These will be placed in the corners, installed flush inside huge bass traps so their size is less critical than usual. In that position SBIR is reduced and the overall room response is quite good. I will need to EQ out a large 15 dB peak centred at 44 Hz that is room related. This room mode has caused me to consider a sealed box, since the mode actually behaves like a vent port already. I will be experimenting with a sealed and vented version in the prototype. The bottom end will be augmented by a pair of Rythmik Servo subs. One of the goals with this project is to increase bass headroom to 40 Hz. Like most subs, the Rythmiks are thermally limited. It might seem crazy to have 18" woofers down to 40 Hz then 12" sealed subs below, but my future plans include tapped horn subs. Initial simulations indicate a pair of Rythmik 12" subs in a tapped horn will achieve about 98 dB 1W1m down to 20 Hz. Two of them (4 x 12") installed under the floor have an estimated max SPL of 126 dB. With the current pair I expect to achieve THX reference (115 dB but with greater extension). With a sealed box and the driver running at 36mm excursion, the output is 101 dB @ 20 Hz. A vented box offers around 106 dB.
The design has evloved, here is the bracing in 3D:
You can see the slot port at the bottom needs some work. The bracing has a H core where the curved sides aren't braced to each other. The idea with the bracing is to tie the flat surfaces together while still being open enough to be able to apply damping to the inside surfaces of the box.