Hello mcraghead. Transmission lines taper from larger to smaller while horns taper from smaller to wider, the opposite. There are mathematics dictating the rate of taper. The transmission line loads the rear of the speaker, and tends to damp resonances. Horns allow the speaker's cone to move freely, very effectively coupled to the room. Several companies produce transmission lines without talking much about them. The foremost advocate of transmission lines, Bud Fried, produced speakers which have been copied by others who have been successful at selling them. I encourage you to build a transmission line enclosure. Be very careful to get the dimensions right and be sure to choose the right driver. The narrow front, deep cabinet using multiple small diameter woofers is ideal for a transmission line. The design is more difficult to build than an bass reflex cabinet, but the effort will be worth it. A sealed enclosure of the right dimensions, paired with the right driver, can be very easy to make and quite effective. Enjoy the music.
Differences Between Folded Horn Speakers and Transmission Line Speakers
I've been looking at various DIY speaker builds and came across a folder horn speaker and I've also seen transmission line speakers. I've tried to google the differences in the two, as they look similar, but I suspect that there are differences. The only visual difference that I've noticed (I've only seen one folded horn, so the comparison pool is very small) is that the folded horn seem to have a larger opening than the transmission line speakers. Maybe it's just the one speaker that I saw, so I don't know that would always be the case. I'm hoping someone on the forum is much more knowledgeable about these things than I am.