Actualy a horn is very effective using any standard division of a wave (1/4, 1/2, and 1) One full wavelength is of course the best, since It does not have to phase with the rear of the speaker, but, can you imagin having to fit that horn somewhere? I have seen it done however. The best example (It could take time to load...) is the Royal Devices lsitening room.
http://www.royaldevice.com/custom3.htm
I would love a chance to listen to about a 47 Hz tone on THAT.
