I am planning on a 90 degree waveguide to match my 12" P.Audio Winner 12S with an intended (active) crossover of 1.2-1.3kHz (a mini Summa without all the sci-fi box-building capabilities of the good doctor Geddes). I am thinking of making the throat seperately from the rest of the horn to allow me to play around a little if need be.

How big to I make the terminus (assuming a 2" radius round over)? Do I make the waveguide mouth before the round-over the same size of the effective diameter of the 12 (around 10-11") or does the waveguide have to be a little larger? I recon a near match is a big improvement over the RCF H100 horn atm but I might just as well hit the nail on the head first time, though I cannot go much bigger due to limits of the lathe this thing will be tuned on.

Any takers?