Bipole loudspeakers have sound coming out the front and the rear, just like a dipole. They are bi-directional at higher frequencies, unlike the sealed box I described earlier, which we would say is mono-directional at the same higher frequencies. At lower frequencies where subwoofers work, the sound would be omni-directional meaning the sound is all around the source of it.
Bipoles, however, are in phase and dipoles are out of phase between the front and the back. The easiest way to picture a bipolar loudspeaker is to take two identical mono-directional loudspeakers and place them back to back so one set of front radiating surfaces point both forward and the second set pointing backwards.
Perhaps the most famous of these speaker types came from Mirage loudspeakers, formerly a company owned by API of Canada and now owned by Klipsch of the US. Mirage speakers were all the rage for many years and most of their high end models were all bipolar designs with equal front and back radiators for sound. More recent bipolar loudspeakers might include Definitive Technology versions among others.
Bipole loudspeakers don't suffer the same wrap around frequency cancelation problems associated with dipoles; where the sound gets low enough in frequency (like in the bass and midbass areas) to meet each other and cancel because one is out of phase with the others. They do, however, have the same sorts of problems with comb filtering that dipoles have and depend every bit as much on the skill of the designer to make them work.
Bipoles and dipoles both enjoy amazing spatial depth qualities if designed correctly and implemented in the room properly - but those are a lot of ifs to contend with. In fact, some not-so-well designed bipoles and dipoles have unnaturally deep sound stages because of the their comb filtering problems - and this can be very tricky to ignore for us Audiophile types - so excited we get about depth and sound stage depth (me included).
Tomorrow we'll start to cover dipoles - the most difficult to integrate loudspeaker in a room.
