What is a "transmission line" speaker?

A "transmission line" speaker has a labyrinth at the rear of the driver(s), usually to augment bass response.

The labyrinth increases the distance that the backward firing woofer soundwaves must travel to exit the port of the speaker. So it's like a bass reflex speaker with a very long (in distance traveled) port.

A good example of an actual transmission line speaker was those designed by Irving "Bud" M. Fried. See if he has a url and some more info on this design method.

A true transmission-line speaker enclosure completely absorbs the back-wave before it would exit the port. Many common "transmission-line" cabinets today, behave more as a reflex, or tuned port cabinet, and let the back-wave exit the port with some significant energy, to reinforce bass response.

The length of the port is usually determined by what frequency the cabinet is tuned to. Port length is usually a multiple of the sine wave that represents the frequency, for example, 35 Hz. The wave length of that particular frequency is divided usually by 4 or 8 to give port length. That is, the port length is 1/4 or 1/8 as long as the sine wave of 35 Hz. Hope this helped, it might just be confusing.