Isnīt a center speaker supposed to have two midrange drivers?
We have several models using a single tweeter and midrange/bass driver instead of the more common approach with a tweeter placed between two midrange drivers. This is a compromise where both concepts have their advantages/disadvantages. Many people think that a center speaker is a special speaker with special featurs, but this is not true. Any decent regular speaker can be used as a center. The only thing that generally separates a dedicated center speaker from any other speaker is the magnetic shielding, to eliminate the risk of the speaker affecting a picture tube tv set. But as CRT sets are quickly becoming obsolete, and the new plasma / LCD displays are not sensitive to magnetic fields, there is nothing stopping you from using a regular speaker as a center.
A center speaker is usually placed lying down to better fit underneath a plasma screen for example, but from a sonic point of view a standing speaker is better. A very good solution is placing the tweeter and midrange vertically, and the woofer horisontally, but the are required becomes much larger. When the speaker is lying down with the drivers placed horisontally phase errors occur as soon as the listener is off-axis.
Using a single midrange/tweeter is the best compromise to obtain a good radiation pattern and minimum phase errors. Besides, the speaker becomes smaller. Using two bass/midrange drivers on each side of a tweeter will inevitable lead to severe phase errors for any listener not sitting straight ahead of the speaker, or off axis. Also, the cabinet volume has to be twice the size to obtain the same lower end crossover point.
This is a well known problem, and the main reason for most speakers to have the tweeter and midrange mounted vertically.
The common setup for monitor speakers is using a single midrange/tweeter. In order to tonally match this, a center speaker with two midrange drivers twice the cabinet volume will be needed. Some manufacturers compromise and make the cabinet smaller, but this results in a center speaker that doensīt cover the lowest frequencies - and they are not fully matched.