There are three "common" types of surround speakers available today plus some unique types created by different speaker manufacturers. "Monopole" speakers consist of a speaker or group of speakers all firing on the same plane in the same direction. This includes the vast majority of all speakers made. What people think of as "normal speakers" are termed Monopole. With regard to current surround sound formats, monopole speakers are the least desirable because they are the least effective in creating an "enveloping sound field" (ambience). They are good at localization, but that alone is not enough to produce the desired surround effect.
If you take a monopole speaker and add another speaker placed 180 degrees opposite of it (i.e. back to back) firing in the same phase, you have a "bipole" speaker. Firing in phase means all drivers on both sides are at the same excursion point at the same time. This creates the exact same sounds coming from both sides of the speaker at the same time. By design, Bipole speakers send no sound directly toward the listener. A bipole speaker will produce good "ambience" as all the sound is reflected off the walls of the room, but is not effective in producing "localized" sounds.
If you take the basic design of a bipole speaker with the rear facing drivers firing exactly opposite of the front, you have a "dipole" speaker. Dipole speakers produce a very diffuse sound, which is good for ambience, but, like bipoles, are not very effective at localization. Dipole design further reduces direct sound to the listening position.
Both bipole and dipole speakers should be mounted on the sides of the listening position and use reflected sound off of the walls to produce their effects. So if monopoles can offer localization but not enveloping ambience, and bi-pole/dipole speakers deliver ambience without localization, what can provide both important characteristics at the same time?
Klipsch produces a unique surround speaker that utilizes a technology called Wide Dispersion Surround Technology (WDST™). Each WDST™ enabled speaker contains two Tractrix® Horn drivers and a woofer. Each horn covers a 90-degree arc and the combination of the two covers a full 180 degrees. This coverage gives excellent ambiance without having to use the walls to reflect sound. The controlled pattern of each horn (what we call "controlled directivity") leads to excellent localization of sounds because there is sound directed at the listening position, regardless of where in the room you are seated. And because the WDST™ surround speaker does not rely on wall reflections, it can be mounted in many different places in a room, leading to greater flexibility with placement. It is rare to have perfect side-wall positions available due to the placement of doors, drapes, furniture and such. WDST™ design delivers enveloping ambience WITH localization for the ideal surround sound result AND gives you the flexibility of placement to solve room design problems.