On the tail of the excellent work published by Zilch and others, I've been testing a cheapo waveguide that might be of interest.
MCM sell a $10 circular 6.5" waveguide that fits on 1" screw mount compression drivers. Part number 54-580. http://www.mcminone.com/product.asp?...%5Fid=54%2D580
It works very well with a nominal directivity of 90 degrees circular.
Cut-off frequency is about 1.5kHz and it starts to lose direction control below about 2.5kHz although still useable down to a least 2kHz. Could be a good option for something like a 2407 driver.
Note that some rough EQ has been added to the on axis response to make it easier to see the what's happening.
Cheers,
Ralph.