Here is one example. It's called a W frame dipole. In this case, it's also push pull mounted. One driver has the polarity reversed so that while one moves out of the magnet gap, the other moves into it. It's a method to reduce 2nd order harmonic distortion.
A dipole does not contain the rear wave from the driver and since it is 180 degrees out of phase, they both cancel. The result is a null to the sides and much less bass. An open baffle dipole has some unique traits:
- no box spring to limit excursion, so less power is required to reach xmax and there is a high risk of bottoming drivers
- dipole roll-off due to the acoustic cancellation typically adds 6db/octave loss below 120 Hz (frequency depends on baffle size)
- velocity source - the room is not pressurised
- dipoles are a low Q bass source
Many believe that open baffle bass is the best you can get. It's claimed that they are less impacted by room modes and therefore have a smoother in-room result and better sound quality than the alternatives. If this were the case, then it would be worth the extra expense. However, I contend that open baffle bass falls short on both counts.
Smoother in-room bass
The side nulls are said to cause less room interaction. This sounds logical enough, and they do certainly measure differently in a room. However, simulations and measurements have shown that they don't necessarily result in a flatter response in room. It is in fact highly system dependant. You might find a dipole measures better, it might measure worse.
Better sound quality
I have had mixed results here. There are some drivers that sound better in a dipole arrangement, including those not intended for the application. However, in one particular comparison, I found no advantage. My listening comparison used two Rythmik Servo kits, each in a sealed box. Both had EQ to ensure I was comparing the same response. I used a familiar track well suited to subjective comparison of bass. Side by side, trying to pick a winner was like comparing many electronics components. I would have struggled to pick a difference in a blind test. Even testing sighted, expecting the dipole to have an edge, it was neck and neck. At that point, I abandoned open baffle bass. I had one sealed box at the time and one open baffle. When I converted the dipole to sealed, I noticed an immediate increase in low frequency authority. That might sound like a contradiction until you consider that the test was set up to level the playing field. When going back to a range of content, including music with more demanding bass and movies, the improvement was clear.
Still considering open baffle bass?
If you still want open baffle bass, then I suggest doing a test for yourself. You'll need to measure and EQ to match fairly. In many cases you are most likely to find that one modest sealed box will match four drivers in a dipole arrangement.