Any speaker can be used for the surround channels in a 5.1, 6.1 or 7.1home theater set-up. But some believe the theater experience can be enhanced with bipole or dipole speakers. The designs for these are similar and often a single speaker will be designed to switch between the two types.
Bipole speaker have two drivers - not aimed at the listener - that are placed on opposite sides of the speaker cabinet. They work in phase (send out sound at the same time) and tend to diffuse the audio, using room acoustics to fill the room with sound.
Dipole speakers start with the same design as a bipole, but the drivers operate out of phase. This creates sound that is even more spread out than that produced by a bipole. Localized sounds, however, such as a specific sound placed in the surround channel on a movie sound track, can lose their effectiveness through dipole speakers.