A good overview about horns you can find here:
http://www.peavey.com/Media/pdfs/qwp1.pdf

Although it's not JBL specific (Pavey) the differences between the following - and other - horns are explained
- conical horns
- exponential horns
- radial horns (Smith-horn)
- constant directivity horns (EV)
- Mantaray horns (Altec)
- Bi-Radial-Horns (JBL).

In the following thread you can found links to even more profound literatur:
http://audioheritage.org/vbulletin/s...roat+waveguide

@german readers:
There exists a german translation of the book "Winston E. Kock: Refraction Sound Waves",
easy to understand, lots of pictures
____________
Regards
Peter