What is a "transmission line" speaker?

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.

Go to pmcloudspeaker.com/ , go to LB1 page and click on "transmission line" highlighted in blue. Advantages of this loading can include superior control over the woofer, attenuation of higher frequencies from the back of the woofer and absence of the impedance peak found in bass reflex loading.

go to the meadowlark.com site. there are explanations there, along with great speakers.