I found this on the web today, a paper by W Marshall Leach Jr. It is about drivers used on basshorns, upper/lower cutoffs, cavity behind rear chamber and reactance annuling, driver and throat area specifications, impedance etc - its a good read so I thought I would post a link here. http://users.ece.gatech.edu/~mleach/papers/HornPaper/HornPaper.pdf
Adrian