i think you are thinknig of TL....the transmission line has to be (preferably) 1/2 the wave length of the desired tuning frequincy, but 1/4 can work, just not as effectively, to save space.
here is what little i know about horns, not stating any of this as fact, just what i heave read and can rememver off of the top of my head...
Horns idealy are 1/4 as tall as the deisred tuning wave length (i beleive, just been tiddling on the net about them) and have to be 1/2 as long as the deisred wave length tuning and have to continuously be getting larger untill you reach the deisred size, their roll off slope is generally less than that of other enclosures so low low tuning isnt as vital as say, a ported enclsore. but they are generally HUGE.
to give you and idea of a TRUE bass horn, go to a rock concert, thoes big 10x10x10 black boxes by the stage.... thoes are TRUE bass horns...tuned pretty low (if you cant see the woofer, if you can see the woofer they are either ported or TL).
Bass horns are extremely efficent...as much as 24dB more efficent than an IB alignment. FYI that is 8 times MORE efficent...
here are a few links to true bass horns.
30Hz bass cabinet
http://vincent.brient.free.fr/bass_horn.htm
Even this is pretty bit (about the size of a big ported enclsore) and it has a -3@300Hz....
http://diy.cowanaudio.com/unity.html