You can probably google the 'd'appolito configuration' to see what the design
goals are, a hit like so comes up.
http://www.diyaudio.com/forums/showthread/t-14774.html
You can also make a speaker that looks like an MTM but not follow the rules.
I haven't seen any concrete evidence that TM, MTM, TMM is better over one another, there seems
to be a personal opinion on what people like and too many variables that affect
performance of each design. ie, you can make a great TM and lousy MTM and visa versa,
the generic driver arrangement doesn't guarantee results.
If you are building a 3 way loudspeaker, TMW, you can get great performance
out of this simple old school design and more tolerant to errors.
Since the woofer is doing a higher frequency job, keep it close to the midrange
for best synergy. Keep the tweeter next to the midrange, and finally align the
height of the loudspeaker design so the tweeter is at your ear level height
at the listening position {standing or sitting down, you choose, etc}.
If you want to add an extra midrange, MTMW, you can do it. Do a generic MTM
or follow the 'd'appolito configuration', but no guarantees of satisfaction on either.
If you use a long ribbon tweeter, then you have issues as the center to center
spacing on the midranges are too high and you can get worse sound. Ideally you
want the two midranges close to each other as much as possible and sneak the
tweeter in the middle, often center offset dome tweeter, maybe with cut flanges
to make it smaller.
Or.. TMMW is fine and probably much better overal as you can have more flexibility when
tweaking the sound using the active crossover as midrange center to center spacing is the
lowest vs. MTM.
Two midranges have the potential to offer 1/2 the distortion of the single
driver at the same SPL level. That's why line array rules. Make a line array of
16 small 4" midranges and you have 1/16 less distortion of the single driver
at the same SPL. This allows you to push the line array to higher SPL levels
and have clean sound still.
