Loudspeaker dispersion-

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.

That's not the wide band dispersion, that would be above a certain frequency.

As an aside this talk confirms my belief in the good cabinet design (snail) of the B&W flagship Nautilus Loudspeaker. It certainly minimizes speaker cabinet interaction, doesn't it?

I'm not sure that the baffle acting as an extension of the driver diameter could explain this. If the cabinet is one foot wide (like many are) then horizontal dispersion would be impacted at app 1khz, but the 90 degree horizontal spec cited by the OP would seem to indicate that this is not the case. Even if this speaker is really narrow, I doubt that many foot wide columns see the horizontal dispersion of (effectively) 12" tweeter.

Although the idea makes sense intuitively, I doubt that the baffle width explains the spec. My guess (just a guess): It's probably destructive interference as cited by Bob in his first post and expanded upon by Johnny.