What is a Dipole?
When you look at a conventional loudspeaker, usually there are two main parts. The speaker driver itself, which are the devices that move and directly make sound. They are what you see when you remove the grill. The second main part is ... the box or cabinet that the "backwave" of the speaker driver fires into. The box is nearly always completely enclosed so as to prevent the "backwave" emitted from the rear of each of the speaker drivers from cancelling the sound coming from the front.
In a dipole, the "back" of the conventional speaker box is removed, creating a figure 8 radiation pattern of sound when viewed from the top - sound comes out the front and back of the speaker - one out of phase from the other and spaced out by 12 or more inches. This creates a more "live" feel of sound - since the speaker behaves closer to a real instrument, however, because front and back waves partially cancel, a dipole is far less efficient in producing lower frequency sound - and thus need more amplifier power, equalization, and typically more surface area to produce comparable volumes to a regular speaker.
Why a Dipole?
Many audiosnobs have listened and found dipoles despite their inefficiencies, to sound notably better than a conventional "box" speaker - especially at the bass frequencies. Room boominess, dead spots, and other anoyances are lessened, and produce much clearer, tighter, truer sound. Unfortunately, dipoles aren't very commercially available due to their large size, and sizable amplifier requirements. So if you want a dipole subwoofer, you'll need to make one!
In my case, I used old existing speaker cabinets, measured out to make sure that the "baffle length" would be around what I would need for a suitable response. After doing a bit of modelling (along with the active EQ component) I ended up with a reasonable satisfactory frequency response courtesy of Brian Steele's XLS spreadsheet (local PDF archive of main webpage).
Since I already had the BassAmp EQ completed, it was time to start reconstruction on the speaker cabinets. First the back needed to come off ... and then the insides reinforced with MDF. The goal was to make the cabinet as stiff and heavy as possible so that it would contribute as little to the sound being radiated as is reasonable!
Above is the wood cutting plan as well as the pieces in place and being glued in - as seen from the new now open, back of the cabinet.
New driver holes were cut in the front, and then everything was primed, painted, and refinished. Both Madisound Eclipse 10" drivers were mounted, wired in parallel and then setup for listening in the main room, next to the Magnepan MG12s.
...And decided to do finish the front baffle in red. Not your typical speaker by design, why should it look like one?! Just in case you think I'm totally off my rocker though, the exposed open insides via the back - are good ole matte black throughout!
Now ideally, that rear wall should be further away from the back of the speaker, but sometimes room conditions aren't always controllable. After some minor EQ tweaking, have been very happy with the sound of the design - although certainly for really high volume really deep bass, more drivers or larger ones would be needed. Movies definitely need a bit of volume caution or else all four 10" drivers hit their maximum excursion. Definitely one of the downsides of dipoles, but as long as you aren't totally cranking Kodo drummers or movie soundtrack explosions, their sound is spot on every time.
There are many designs for dipoles, indeed this is one of the less space efficient due to its sheer simplicity, but also one of the easiest to fabricate - especially from old speaker box carcasses. So if you perhaps have the urge to try out a dipole - it might not be as hard as you think!