What is a "transmission line" speaker?

The sound coming out of ANY vent can never be in phase with the front wave of a driver at all frequencies at all times. That is why a sealed design is always superior in this respect. The TL is not nearly as "bad" as most other designs, as the entire length of the labyrinth is typically lined ( and sometimes even stuffed ) with damping material. As such, the amount of out of phase leakage across a wide frequency range is quite minimal when compared to a port that allows sound to flow freely through an open hole and / or a passive radiator that "talks" at many various frequencies. Theoretically, the TL should also be better in this respect than a "stuffed port" or "Aperiodic" design due to the length of the line / absorption losses. Dynaudio calls their version of a "resistive" or "stuffed" port tuning a "vario-vent".

As far as efficiency levels go, everything that i've ever read / seen / heard tells me that TL's are not real efficient i.e. probably less than 90 dB's or so. One could build something with a higher sensitivity, but this would be done at the expense of extension. Getting high sensitivity with extreme bottom end capability becomes very expesive and requires a big cabinet. This is why many manufacturers fall prey to the laws of diminishing return and opt for some type of venting to augment what they can't get out of the driver naturally. Like anything else though, there are trade-offs involved when you take the "less than optimum / cost IS an object" approach. Sean
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Agree with Sean on virtually all of his points. I also never heard the Steve@HiFiFarm definition of Transmission Line.

In response to a few of the others' questions above, the original true Transmission Line was a cabinet design to "get rid" of the back-wave, and have a better loading of the woofer at the point that it would normally have an impedance spike. The original Transmission Lines did not have any output form the port, at all, and were designed specifically so they would not. Tapered transmission lines that taper down toward the port have better low end extension and linearity than straight pipes, or tapering wider toward the port. Sounds that do exit the port may be in phase, but are not "in-phase at all frequencies" with the direct radiations of the cone. Cancellation and addition, known as "comb-filtering" can occur. Since most modern "transmission line" speakers are quite a departure from the original idea, they should more aptly be called "modified transmission lines" or maybe some "catchy" name for marketing purposes.

I use a TQWT Voigt Pipe enclosure which is strange combination of bass-reflex, transmission-line, and back-horn. It makes use of some of the characteristics of all these designs in a "blend" that looks to be horrible on paper, but sounds great in the listening room.

Transmission line design is still a time consuming job that needs alot of work to get it right, and doesn't really lend itself to mass-market applications.

Thanks again to everyone. Gotta pretty good thread started here and drug in several of the 'Gon faithful. As usual, thisngs are not as simple as they appear.

Twl: Can you point me to a website that has info on the design of your cabinet ? Sounds pretty interesting. Sean
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You should check out the Buggtussel line of speakers. Some very good stuff. The company is owned by Kevin Blair, he is one of the leading people on transmission line design.Their website is www.buggtussel.com