The D'Appolito, MTM Configuration

When auditioning the Kef LS50s, I also demoed the R700 which sounded like the LS50 but more extended.  It sounded very good then but was also demoing many other speakers searching for a speaker that would resonated with me.  

We slightly modified D Appolito design in our Sofia loudspeaker line . using very small tweeter between two flat sides of woofers . quasi coaxial design

what’s between my two MTM speakers now, and just to the outside of them on either side, the result produces amazing midrange and depth of stage. There is a cohesiveness in how the mids and high frequency blend together in a uniformed manner. Its just more engaging to me.

@decooney, you nailed it, as I experience the same with the ProAcs. It is amazing how deep the sound stage goes behind the wall that is behind the speakers.

Joseph D'Appolito's original design had a few characteristics which have not made it into everything "MTM". 

He used an odd-order crossover (3rd order), which would have a lobe at 45 degrees above the tweeter axis in a normal tweeter-on-top two-way configuration.  With the MTM topology, we get one lobe above the tweeter axis and another lobe below the tweeter axis, resulting in significantly more uniform vertical coverage in the crossover region than we'd get with an even-order crossover.

Also, the original tweeter was a horn-loaded Dynaudio dome, and the physical set-back of the tweeter's voice coil due to the horn depth contributed to time-alignment in the crossover region.

Finally, the narrower radiation pattern of the horn-loaded tweeter resulted in better pattern-matching through the crossover region than we'd get with a conventional dome tweeter. 

The advantage of the more-commonly-used 4th order crossover for an MTM configuration is that it allows a lower crossover frequency, but along the way some of the desirable attributes of Joseph D'Appolito's exceptionally well-thought-out approach have been traded off.

I've used an MHM format ("H" for horn tweeter) several times over the years, mostly in custom studio monitors.

Duke

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.