From the CEDIA Seminar Part 1 original thread
Toquemon-
The following configurations of center channels work "better". That is, they exhibit less or none of the undesirable cancellation/lobing effect that takes place with a D'Apollito in which the tweeter is exactly in the center of the cabinet and the mid-woofers are thus separated completely from each other.
The cancellation effect is usually more noticeable on mid-woofers of 5.25" and up (if both are driven all the way up to the crossover point). It is less noticeable on midwoofers of 4" and under. So, the tighter together you can get each of the mid-woofers, by squeezing the tweeter out of the center, for instance, as in example #2, the more closely the two drivers approach being a point source. Explained another way, the smaller-diameter-driver cancellations take place more closely to the speaker so the cancellations are more "resolved" by the time you get back to the listening position.
The original "benefit" of the D'Apollito configuration was that, set up vertically, the tweeter frequencies dispersed widely from left to right (in the horizontal plane) while the tweeter's frequencies were limited vertically; they were more focussed. This is why you see so many THX approved left and right speakers using the D'appolito configuration. Stood up vertically as Joe D'Appolito originally intended, the left and right mid-woofers tend to cancel each other at many frequencies as those frequencies head toward the ceiling or floor. So you don't get a strong floor or ceiling axial reflection at specific frequencies. Unfortunately, when you lay the D'Apollito on it's side as is done for the center channel, all the design's attributes become liabilities.
Here are four work-arounds that many designs currently on the market feature.
1. Three-way designs with a midrange under a tweeter and flanked by woofers work great as long as that midrange is brought down low enough into the vocal range. However, most of the faux 2" and 3" size "midranges" you see are crossed over above 1KHz so they don't necessarily answer the problem their vertical alignment (tweeter above midrange) would seem to imply.
2. Small mid-woofer drivers driven up to their 2.5KHz or so crossover can work well if they're pushed as closely together as possible, like many of the Atlantic Technology designs with the tweeter nestled in between the "V" formed by the two. AT also has the switchable proximity (TV) notch filter on many of their centers which makes for clear vocals without the false and unnatural "chestiness" added to voices.
3. A D'Apollito-appearing horizontal design which uses one well designed passive radiator and one true mid-woofer works very, very well. Boston Acoustics has used this trick for quite a while and I can tell you that in listening double blind, the Boston would always trump the more traditional double-driven mid-woofer designs. It was that much better.
4. Same can be said for 2 1/2 way designs in which one of the two driven woofers is rolled off fairly quickly , usually at around 500 Hz, while the second woofer goes up to the ~2.5KHz crossover point. Works very well.
