Fair enough, but I'm willing to give on size to get that last octave or so. Thanks for the mention of MJK's article on dual 15's, I'll have to read about that design.
Sensitivity is very important to me- so much so that padding down the B200 would be criminal.
I'm curious about the side-firing 15s on a U (is that still a U, by the way?) - are you aware of any implementations?
Yes - have implemented; 'can post photos when I get some time. Also have done a variation on the implementation that places woofers at 90 deg on adjacent sides of a cube, with rear opening on the opposite corner. (Picture would be worth 1,000 words here)
The side-firing one functions just like a shallow U-frame, but with vibration reduction, and maximum displacement for a given box size. Call it a T-frame if you like, but it is functionally just a U-frame.
Remember that any (sub)woofers on small baffle/box is omnidirectional until it encounters something that makes it behave any different. In this case the side woofers provide the front hemisphere. and the rear opening -- provides roughly equal-but-opposite displacement on the rear hemisphere behind the box. Remember: The rear radiation would be perfectly omnidirectional too, were it to never encounter the front wave.
All this holds true until wavelengths are short enough to "see" the box/baffle.
The rear dipole hole described previously is optional. If used, it should be off-center a little. It serves to lengthen the front-rear distance, to slightly increase driver loading, and to randomize front-to-rear path lenghts a little more - which, along with stuffing, reduces the dipole peak by a couple db.
Nulls are roughly paralell with the plane of the rear opening (just like a regular U-frame). As is, distance from woofer center to rear opening is short, so functions as a shallow U-frame, easily usable to 200 Hz if 2nd or higher order XO used. If extended and heavily stuffed, can become cardioid Like John K prescribes.
While total box vibration is mitigated, you will still get some vibration from air pressure; Needs same strengh of enclosure as if building sealed box for same drivers. loose stuffing can blast outta there like a canon - may need a net or screen.
When compared to a 'regular' U-frame:
Pros: Double displacement potential for a given frame size; Vibration control; 4 ohm net impedance (assuming 8 ohm drivers); Visual appeal of a plain front panel
Cons: Cannot place flat against sidewall (does work well very close to wall at 45 deg, however); Gotta have more drivers (hey, you always get a price break at qty 4); Displacement could kill any pets that nap inside the box (see my avatar); Not a super-pure symetrical dipole (No U-frame is). May be better or worse for your tastes and room than a flat baffle
And before anyone asks: yes, you chest-thumpers can even do 3 drivers per frame in this configuration (left, front, right), but resulting impedance is too low for most amplifiers, and vibration control is not as good either. I would definitely not use the dipole-hole exit if using 3 drivers.
For reference, Sd (piston area) of a typical 15" driver is ~133 sq"; A 15" x 15" opening is 225 sq in - ~= to two 15" woofers.
A typical 10" driver ~ 50 sq in
A typical 12" driver ~ 75 sq in
Of course, all the above is possible w/18's, but you can get the same bass extension with 15's or 12's.
'Hope this helps,
-- Mark aka Tubamark
