The Subwoofer DIY Page v1.1
Transmission Line Systems
last updated: 14 February 2018

The transmission line system is a waveguide system in which the guide reverses the phase of the driver's rear output, thereby reinforcing the frequencies near the driver's Fs. Transmission lines tend to be larger than the other systems, due to the size and length of the line required by the design. The payoff is an extended low end response and a characteristic sound that's appealing to many.

There is a lot of information about transmission line speakers available on the Internet.  Unfortunately a lot of that information is either based on "classical" transmission line design (which is basically obsolete), and/or is just flat out inaccurate. Fortunately these days there are quite a few software tools (including the freeware Hornresp tool) that can be used to simulate transmission lines, which makes the design process a lot easier.

Note also that the design process for a "full range" transmission line speaker is likely going to be quite different to the design process for a transmission line subwoofer.  That's primarily due to the fact that you will need to suppress resonances in the former by stuffing and/or lining the box because you don't want those resonances coloring the response of the speaker, while in the case of a transmission line subwoofer  you would likely try to avoid stuffing it as much as possible because any stuffing in the box will reduce the output at low frequencies. As this is a site for designing subwoofers, it will concentrate on the latter approach. 

Usually, only drivers which have low Qts (0.25 - 0.4) , Qes (0.3 - 0.4) and Fs values are suitable for transmission line systems. However higher-Q drivers can be used. Just ensure that you simulate the results in a proper modeling program first before committing to a build! Transmission line enclosures are usually a bit more complex to build than your common vented box, and you should spend a lot of time making sure that your simulation is correct before committing it to wood to make sure that you're not disappointed with the results.

There is a variant of the transmission line called the mass-loaded transmission line.  This is basically a transmission line that is terminated by a vent. There are several advantages to using this approach, including addressing the pipe resonances along the line and reducing the box size without giving up any noticeable response at low frequencies.  

Transmission Line resources and projects on the internet: