The D'Appolito, MTM Configuration

Thanks for the extra info, @audiokinesis - I tried refreshing my memory via Wikipedia but it seemed woefully limited on details. Even the AES synopsis of his article seems to indicate he had much more to say about the interaction of driver arrangements, lobing and crossover slopes than what I put in my original post.

For sure, without a horn / waveguide tweeter in the middle it seems hard to get the same tight vertical dispersion as the outer drivers, so using a simple dome tweeter seems to short-change the design. 

For those, besides Duke:  Duke is pointing out that a simple MTM arrangement, using a dome T falls short of D'Appolito's design.  In particular, the mids would have tight vertical dispersion, but the tweeter would have very wide vertical dispersion.  Using a horn/waveguid would give similar vertical dispersion, and therefore reduce high frequency reflections (i.e. increase detail) and also sound smoother off-axis.

The use of a horn also would seem to be a way to enable 4th order filters, thanks to the physical set-back of the driver. I must say that this is definitely one area where using an active / digital crossover can really make your life easy.

The MTM configuration can work well, but I think in most cases, steep crossover filters, ideally 4th-order, are a requisite, in order to prevent comb filtering. I’ve heard a few other MTM designs that didn’t have as good of coherence as one would expect. So as usual, it really comes down to the talent of the designer and implementation. Lots of ways to skin the proverbial cat.

As I recall, while D'Appolito's early designs did not use 4th order filters he eventually came to the same conclusion. 

Also, MTMs are typically not capable of producing vertical dispersion as evenly as a coaxial design.

I thought this was a feature, not a limitation.  AFAIK, the idea was to narrow the vertical dispersion to minimize floor and ceiling reflections as much as possible, approaching some of the benefits of tall planar speakers or line arrays.

@gdaddy1 - I meant to discuss the web site where they have several pages of news from the Florida show. :)

In this particular case it was the Acora Acoustics VRC-1’s that made me think of all the speakers I’ve heard with this kind of arrangement, but they don’t need my help promoting them. :)  I do like their choice in midrange though, since I use it in my center. :)  I need to raise the prices!