To elaborate on what Paul and Monte have said, the differences between these speaker designs is related to their sound radiation patterns. Their usefulness in any given application depends on the priorities related to that application.
Monopoles radiate sound in one direction only; forward, in the direction they are facing. The coverage area will be determined by the width of their dispersion pattern in the frontal plane. Monopoles are very "directional", IOW, it's easy for the ear to tell where the sound is coming from.
Bipoles radiate sound in two planes and the output of both planes is in-phase, IOW, the drivers move out and in at the same time. With surround speakers, bipoles are usually designed with the drivers mounted at ~45 degree angles to the wall. The in-phase propagation gives a localizable nature to the sound and bipoles usually have a very broad coverage area. However, they also have significant reflected sound from the front and rear walls, so they can add a sense of ambiance.
Dipoles are similar to bipoles in that they are usually designed with the drivers at 45 or greater angles to the wall. The difference is that dipoles fire the opposing drivers out-of-phase; IOW, when one driver is moving out, the opposing driver is moving in, and vice-versa. This causes an area of cancellation immediately in front of the speaker. This is referred to as a "null" in the forward plane. The only thing the listener hears is the indirect sound of the reflections off the front an back walls.
Soooo... which do you pick for your surround sound system??? The choice comes down to what type of listening you do the most.
If you listen to a lot of discrete multi-channel sources, (i.e., 5.1, 6.1 or 7.1 discrete movies on Blu-Ray, DVD-A, or SACD multi-channel audio), the the "ambiance" is mixed into the discrete signals and you want speakers that will reproduce the recorded ambiance without adding additional ambiance of their own: monoples.
If you listen to a lot of stereo music or older stereo movies and you add multi-channel processing to it, (i.e., Dolby PLIIx Movies/Music, Neo-6 Movies/Music, etc.), then you want a very diffuse sound field: dipoles.
If you want some of each, (directionality and diffusion), then you want a speaker that can be directional when needed and diffuse when needed: bipoles.
Personally, I have side surround speakers that can be configured as either bipoles or dipoles. I have them configured as bipoles and I never change them. I like the directionality when called for. Otherwise, I rarely notice the surrounds at all, (i.e., they are adding "ambiance" which should never be noticed as "discrete" sound.) My rear surrounds are monopoles, which provide all the directionality I need for the limited amount of 7.1 material that is out there.
Craig
