what's so special about center speaker ?

Even the MTM design period isn't all that great. Biro Technology has a good article and it appears the designer has found that even a vertical MTM suffers 9-12db dips in frequency response, with what I believe is more ambient energy, assuming your ear level with the tweet. I don't know why there are so many mtm. Granted there's no lobing errors or polar axis tilt, better bass extension and spl, plus higer power handling, those problems can be dealt with in other designs. Needless to say I'm not a big fan any symetric array, whether or not its horizontal or vertical.

MTM's do have several advantages if properly constructed. One would have to do some very serious studying of the drivers, their dispersion patterns, optimum crossover frequencies, the spacing between drivers, placement on the baffle, etc... before building a marketable model. then again, this is true of ANY speaker design.

The reason that the horizontal MTM design is so widely used in the HT market is for the very reason that Ezmeralda and Danner "bag" on them. It has limited horizontal dispersion characteristics. Since you have a speaker on each side of it ( the mains in an ht system ), they are trying to limit side to side dispersion and keep the voices centered. Not only does this improve imaging and localization in this case, it also minimizes lobing and interference with the two other front speakers.

Some of the other benefits are that the designers can use multiple drivers for increased surface area, greater power handling and low frequency extension, lower distortion due to reduced excursion, smaller drivers for improved transient response, etc...

Obviously, another consideration would be cosmetics, as this design is typically less intrusive than a vertical array would be. This type of speaker will work best if mounted on top of the TV with a slight downward angle ( to minimize ceiling bounce ).

I think that Vance tends to look at ( and comment on ) these designs as if they were running them as "stereo" speakers rather than "special application" speakers. I agree that the potential for problems are much greater with a design of this type, especially when mounted horizontally. When properly executed and the layout of the room is taken into consideration, they are hard to beat for this type of situation. After all, most people try to watch a tv / movie while viewing the screen as centered as possible. Unfortunately, most of these designs and "home theaters" are not properly executed, so consumers end up with a lot lower performance than their systems are capable of. Like anything else, you have to buy what will work best for your specific situation. Sean
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To quote Vance Dickason on page 189 of The Loudspeaker design cookbook, 6th ed., "The center front channel speaker is commonly positioned horizontally , with the drivers in opposite acoustic polarity to the left/right front speakers. This is done strictly for aesthetic reasons." Note that last sentence. And on the top of page 189 he has graphs showing the effects of the various driver arrangements and their effects on off-axis frequency response: figure 10.11 correllates to figure 10.15 on the facing page of a horizontal MTM. From the first paragraph in the second column on page 189 "The consequence for the horizontally polarized speaker at 30 degree off-axis is a dip in the repsonse at the crossover frequency of nearly 15db." The other three driver arrangements perform far more admirably given the task. And this is also why THX certification is not given to driver arrangemnts of a horizontal mtm (should the manufacturer even bother to apply).

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.