Unsound, I am only referring to the positioning of the drivers. Done this way the drivers are acting acoustically as one driver through the entire pass band, an almost perfect point source. Whether or not you use a 1st, 2nd or 3rd order cross and where you put it depends on the physical size of the tweeter as you want to keep the mid/woofers as close together as possible, the upper frequency limit of the mid/woofers and the lower frequency limit and power handling of the tweeter. If you choose the right drivers you can get away with a 1st order cross and still maintain the point source characteristics of the array. It does limit your choice of drivers generally to more expensive ones but it makes life much easier and guarantees a phase coherent speaker. The dipole plate design eliminates enclosure resonance and improves integration with the room acoustically. Placed at shoulder level these little speakers sound really big. With your eyes closed you would swear you were listening to big floor standers.
If you have access to a CNC mill you could even make the plate out of aluminum.
