Without any fuss David J McBean has updated his program for calculating horn loudspeakers (Version 11.00):
http://www.users.bigpond.com/dmcbean/
Horn systems can have an oblate spheroidal waveguide, a single spherical, tractrix or Salmon's family hyperbolic-exponential flare segment, or up to four conical and/or exponential flare segments in series, coupled to a throat chamber, loudspeaker driver and acoustically lined or vented rear chamber. Horns with multiple drivers and arrays of multiple loudspeakers can be modelled.

You may use it for free - it's great IMHO.
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Peter