Loudspeaker dispersion-

What are the speakers? Most panel speakers (e.g., electrostatics, planar magnetics) have very limited vertical dispersion, which is why they make them tall. Some speakers have wave guides that determine the dispersion pattern. A tightly spaced vertical array of speaker drivers (such as in a dynamic line source speaker) has very limited vertical dispersion.

D'Appolito (and most other) MTM arrays have a limited vertical dispersion as well. There's something about the tweeter with midranges both above and below that limits the vertical dispersion. In many cases limited vertical dispersion is intentional to minimize the sound bouncing from the floor and ceiling and smearing the image.

04-04-14: Bob_reynolds
Here's my guess...

Put one driver on a baffle and you should have the same dispersion horizontally and vertically. Place a second driver vertically adjacent to the first driver and vertical dispersion will suffer.

Yes, and doubly so when the tweeter has mid/woofers above and below the tweeter as in a D'Appolito or MTM array. tHe larger waves of the midrange drivers keep the shorter tweeter waves from having much of a vertical dispersion at all. MTMs have a famously narrow vertical dispersion. An example is the Atlantic Technology AT-1, near fullrange MTM tower. Its measurements showed a vertical dispersion of 5 deg. above the tweeter axis and 10 deg. below. The commentary also mentions that crossover design can influence dispersion, especially suckouts.

A pistonic driver's dispersion depends on its employed frequency range relative to the piston's diameter. For example, suppose we have a 2-way monitor with a 6-1/2" woofer and 1" dome tweeter, a pretty common configuration. A 6.5" dia. diaphragm starts beaming at about 2100 Hz, whose wavelength is 6.457. Let's suppose the speaker designer chooses a crossover point of 2.5Khz to improve power handling. That pretty much guarantees that the dispersion will narrow significantly at and near the crossover point.

Just thinking out loud here .... Most tweeters have 1 inch domes or diaphram. At what frequency would that type of tweeter start to beam?

As the frequency approaches the diaphram diameter, the dispersion narrows. In the case of the 1" tweeter--barring some kind of waveguide--it's in full beam mode at about 13,500 Hz. See the wavelength caluculator to find the wavelength of any frequency.