Loudspeaker dispersion-

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.

Johnnyb53, I've read elsewhere the same comment about the relationship between the diameter of a driver's diaphram and the wavelength at which the driver will start to beam.

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

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.