you can calculate the dispersion with some trigonometric skills, but dispersion belongs to horn's the shape. I mean conical one have worst dispersion (but easyest to calculate), exponential/hyperbolic better (but harder to calculate).
Bitzo, what you mean is that conical horns have better dispersion characteristics, but worse LF loading characteristics as opposed to exp (the best loading) and tractrix (planar to round wave forming).
\'bits of ply round a driver\'
Just to clarify, the tweeters in front of the mids and plugs never made any difference to the sound. The size of the wave lengths meant that everything defracted around the tweeter housing without breaking up. The ten degree angles were added to the 40 degree tweeters so that the dispersion would more or less match the horn. Still a compromise, but the best one I could think of at the time.