Q: Bipole, dipole, North Pole: What's the difference and why do you need them?
Bipole and dipole speakers are primarily used for the surround channels in a home theater. Both types contain two sets of speaker drivers that direct sound to both the front and back of the room, thereby dispersing the sound.
This is what surround speakers should do, after all. They are intended to reproduce ambient effects like crickets chirping in the night or the hum of a spaceship so they sound like they're coming from everywhere in the room-not just where your speakers are located. That's why bipole and dipole speakers have two sets of speaker drivers pointing in opposite directions. And that's why they are often triangular in shape, with baffles sticking out slightly from an in-wall or in-ceiling installation. That, at least, is the extent of a bipole speaker.
A dipole goes one step further, making the sound from the two sets of drivers "out of phase." What happens is this: As a speaker driver on one side or baffle moves outward to fire sound, its companion on the opposite side moves inward. Any opposing bass waves cancel themselves out at the sides of the speaker (similar to what happens in noise-cancellation headphones), and the higher and midrange sounds are dispersed throughout the room, creating an even more ambient quality. Dipole speakers are often used in higher-end home theaters, so consult with a professional if you're thinking of using them.