The D'Appolito, MTM Configuration

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.

@erik_squires .."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..".

I would agree with this assessment, and while I have tried other configurations where sound and stage went to the ceiling, 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.

Seeing more MTM configurations surfacing (or resurfacing) in the past year than I recalled seeing in prior years. Was just thinking about this recently, wondering why some design engineers just rediscovered this. Some of my original attempts were to essentially mimic the original Proac Response 3 speakers from a few decades ago. A good friend still enjoys my first set. For my 3rd built attempt I decided to pull the drivers even closer together. Some rough test mule cabinets helped get there, until making the final cabinets out of Walnut ply. I like the result more this way for this added cohesion i’m hearing. Don’t think I could ever go back to 2-ways or TMM/TWW.

I always liked this design option. If the.crossover point is low enough and the driver centers are close enough, comb filtering shouldn't be an issue.  Always many variables in play, and pros and cons to each design choice.

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