what's so special about center speaker ?

ALL of those variables ( including the "off axis dip" ) would change with different drivers, altering the spacing of those drivers on the baffle, changing the size of the baffle, altering crossover frequencies, etc... Vance even states that the results that he shows for the horizontal mtm array are NOT directly comparable to the vertical mtm array as they are completely different speakers. He's using different drivers in a different box for the two sets of graphs.

While i think that his book is EXTREMELY useful, it is not the "end all" or have all of the answers. It is nothing more than an ongoing collection of research notes and data that he has collected and continues to update. In case you haven't noticed, many, many, many of his graphs do not use the same spacing or plotting characteristics. They are therefore NOT directly comparable in many instances. One would have to extrapolate data from the given charts, some of which are less than clearly marked, for any type of meaningful comparisons under many of the circumstances that he has documented.

Besides all of that, why would anyone that had any knowledge about speakers purposely wire an entire speaker out of phase / reverse acoustic polarity from other speakers in a multiple channel array ??? I can't say that i've inspected dozens upon dozens of center channel mtm's, but i have seen more than a few. None of them were wired with the opposite acoustic polarity to what one would expect. Doing something such as that would cause unstable and very diffuse imaging resulting in speech / vocals that were NOT "anchored" between the L - R mains. This would completely defeat the purpose that a center speaker was designed to achieve. Sean
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Nowhere did it say the speaker was wired out of phase, wiring was not even mentioned. It stated "the drivers in opposite acoustic polarity to the left/right front speakers." This would also refer to horizontal tm designs, along with the horizontal mtm's in relation to the left right being vertical driver arrangement. But what is amusing is that many speakers will actually have the tweeter wired out-of phase with the lower frequency driver to maintain a smooth frequency response throughout the crossover region depending on crossover chosen. So dependig on whether or not the center speaker purchased was of the same make as the front l/r, one could actually have thier high-frequencies in the center channel out of phase with the left and right.

I agree with all of the points made. It is not that uncommon in multiple driver designs for one or more of the speakers to be wired out of phase with the others.

I would also like to clarify that i "mistook" the "opposite acoustic polarity" as being wired out of phase. If two different cabinets were wired out of phase in relation to each other, one driver would be pushing outward while the other would be pulling inward. This would result in "opposite acoustic polarity", hence the confusion. It did not strike me at the time of reading this thread that Vance was referring to the polarization of drivers ( vertical vs horizontal ) and dispersion characteristics. Sean
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According to the Biro's designer, one does not need a center channel if the mains are less than 2.5 meters apart (personal communication). With my "improperly designed" MTM JMlab's I keep the centers no farther than 7 1/2 feet.

Kslim, you can do fine without a center channel in a small room. If all your audience fits in a couch and in a few pillows on a rug, you're fine. If you get into bigger projection units and TVs you will need the center channel to anchor the dialog.

Now, who says you can't have TWO small two-way speakers as center channels? Why keep one in a closet? One can place one above and one below the TV, or any way that suits you best.

Ever heard of The Penguin Principle?