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Bipole and dipole surround speaker designs are fraternal twins: both have identical speaker drivers mounted to opposite sides of the speaker cabinet (their only distinguishing characteristic is polarity).
In a Dipole arrangement, the front and rear facing speakers are wired out of phase, that is, while one set pushes out the other pushes in. In a Bipole surround speaker, however, the two sets of drivers are wired in phase and both sets move in and out together.
Some higher end surround sound speakers allow you to switch between dipole and bipole operation--this enables you to tailor the speakers to your home theater. These speakers sound best mounted above or below ear level but a few feet down from the ceiling. Dipoles are excellent when they are placed near the listening areas because you can aim the "null" at the listeners thus, you can achieve high levels without blasting you in the ear.
Bipoles and dipole surround speakers can sound good either on the side walls or the back wall of a home theater--the overall shape of the room will decide where they should be placed. Bipoles are a poor attempt at omnipoles (having dispersion/cancellation problems). Audition a couple familiar movies with different surround sound speaker placement before you decide on a permanent location.