Wow !
I've only just been able to view your pictures, Mark
Which model sub have you stacked there?
Those would be our F18+ and F18-Slave modular subwoofers (
more info here). One F18+ contains a 4000W, US made amplifier which has the capability of driving up to 3 additional slave units (4 18s per 4000W amplifier). They don't have to be stacked, but obviously work well when you want to minimize footprint and don't need additional locations. We used 2 of the same stacks in
this theater I detailed here on WBF. As you can see in my avatar, three stack up nicely as well.
I have had other customers put two stacked pairs in all 4 corners with a one powered unit up front and one powered unit in the rear. The latter is probably the most flexible solution for home theater use and multiple rows in a more typical, enclosed, rectangular room. Forum member audioguy has 4 in the front of his theater and is planning swapping an earlier design of mine for a matching set at the rear of his theater.
For 230V/overseas operation the F18i+ version is required which does afford even more amplifier headroom and power line regulation in the 6,000W version of the same amplifier design (it delivers less power at 120V and costs more which is why we don't recommend it stateside).
While I understand where the scale of the stack and 18" woofers and common demonstration in home theater use make some question if they are truly hi-fi quality, the F18 is exceptionally low distortion, provides 7-12Hz extension in most rooms, and the headroom allows the low distortion motor to always remain in control of the glass-fiber honeycomb cones. Effortless and bottomless is the best description of what they sound like when you assemble enough for ample headroom and have them blended well with capable main speakers.
OK now please file this under 'dumb things punters have asked'...
But could there be any merit in turning one pair in each stack the opposite direction, to achieve dipole - like omni directional bass?
There would be almost zero acoustic benefit. For these wavelengths moving 2 of the 4 to the back simply moves those woofers ~18" further, which is about the same as simply moving the stack 9" back (summed result is the midpoint). At the frequencies these will be used, the 18" woofer s and 23.5" wide cabinet don't create any forward directivity, so they really are already omnidirectional.
If we locked them all together with 2 firing opposite directions it would create a force cancelling assembly where the drivers would physically push against each other, similar to my dual opposed SubMersive subwoofer, but with 3-4 stacked units, simply putting spikes on the bottom unit is enough to keep them quite still. An interesting benefit of the column of woofers is the way in which they all push on the column evenly top to bottom, such that the woofer moving fore-aft doesn't create a desire for the tower to rock, but rather it has a subtle effect for the whole column to quiver forward and back when we really push the excursion of the woofers. Even this is really only noticeable when we have the 460 lbs on the 4 rubber feet. If rigid spikes are inserted in place of the M6 feet, there is almost no perceivable movement.