Okay, seriously... There's nothing to stop someone using monopole speakers as rear surrounds,. In fact, with SACD it is recommended but for different reasons. Also, if you look at the recommended layouts for DD and DTS home cinema it appears to show monopole speakers for the surrounds, and many will take that at face value without investigating further. But as with so much in life, one size does not fit all. THX takes a different approach. Their site shows dipole surrounds for the sides.
Good monopole speakers such as the BX2 need space between the speaker and listener for the sound field to develop. Now, bear in mind that if you actually read Dolby's description of the role of the side (rear) surround speakers then it says "
The surround speakers create a lifelike sense of spaciousness, providing the ambient sounds for a movie or audience reactions in a concert video" So they're telling us that if you have a living room the size of a football pitch (I'm joking, but you get the point) then you can use monopoles because they'll be far enough away to be heard as ambient rather than pin point.
In the real world of a typical UK livingroom we just don't have that much space. In these situations dipole speakers are far more effective. They produce the ambient diffuse sound required at a much shorter listening distance.
So why so so many surround kits come with monoplole rears?..... Well, much of it is down to cost. It's cheaper to make a smaller range and use the same speaker all round. Monitor Audio is one of the few speaker manufacturers to make a dipole rear surround at a sensible price.
