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In a Dipolar speaker, the two sets of speakers are out-of-phase with each other, while the drivers are one side are pushing, the opposite side is pulling. The result is that there is a "null" or a dead zone of sound in the area along the 90 degree axis of the speaker (see illustration below). Why is that good? When properly set up, a pair of dipole speakers used as surround speakers will provide a very open, enveloping rear effects soundstage without allowing you to pinpoint the location of the speakers themselves. That's a good thing. But for all this to work properly, the speakers need to be positioned "in-line" with the listening position as shown on the illustration below. If you are sitting out of the null area, the effect is ruined. What if you can't or don't want to place your surround speakers and listening position as required? That's where bipoles come in handy.
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In a Bipolar speaker, the two sets of drivers are in-phase with one another - both sides push air at the same time. The result is greater sound output where the dipolar speaker's null would be. Theoretically, a bipolar speaker approaches a 360° soundfield - it squirts sound all around the room. That's a good thing if you need to position your surround speakers behind your listening position or anywhere outside of the null area. Some people prefer the greater localization of bipolar speakers when used in digital discrete (Dolby Digital 5.1, DTS) systems.