So Ivica,
doesn't that basically bring down the argument to that of slot size (rectangular or circular "slot") for being the source of the better dispersion?
The above arrangement of size reduction is perhaps also relevant to Espen's concern: it happens all the time! Look at how the original HR9040 throat reduces the 33mm entry to just 28mm as a matter of a few millimetres
(Source:
http://www.diy-audio.narod.ru/litr/K..._Exp_Horns.pdf)
......
The above Keele reference lists a P.W.Klipsch 1945 (filed) patent that was commented on
here to the effect of (i parphrase) ...
first, expansion is only in at single plane for a part of the horn length to bring radiation angle up to desired target in this plane; 2nd, expansion takes place at right angles to said 1st plane while continuing to expand in the angle reached in the 1st expansion. Expansion stops where targeted mouth area is reached.
So, all horn designers do what they please and place "obstacles" at will in order to get their designed pattern
Ralph