A pure transmission line speaker will completely absorb the back-wave from the driver, making it behave like an infinite baffle. The advantage an infinite baffle gives is that there's no cancellation from the output from the front of the driver and the output from the back of the driver. The output from the back is thrown away and only the output from the front is used.
In reality, this is almost impossible to do fully, and that's especially true where a speaker box is concerned. You'll never be able to absorb 100% of the back-wave (with current tech), so a compromise is reached in the "transmission line" speaker by using some of the back-wave energy for the overall output. This is done by making the pipe length, damping and taper so that it channels the back-wave out and it is now in phase with the front-wave. This extra energy adds to the front-wave, boosting output.
The pipe is tuned so that by the time the back-wave travels through it, it will have shifted in phase to match the output from the front. And while it seems like it would be relatively easy to do that, designing and constructing a box to hold that pipe is more complex than nearly ever other type of speaker. And in my opinion it's questionable whether the results are better than a much less complex (and typically much smaller) ported box.
But even when he know this, that doesn't stop the DIY speaker builder from taking a crack at it. And I have to admit that while thinking about the subject for this video, I find myself drawn to the idea again.
The one shown in the video thumbnail was very quickly drawn for this video. It shows the typical implementation of the folded, tapered duct with the output opening on the front. Along with this the duct would have to be stuffed with damping material to tune the response and reduce the ripples the long pipe adds to the response. These ripples extend up throughout the effective frequency range of the driver and can be difficult to get rid of. And I'm not unconvinced that the ripples are not what people like about these speakers - that extra "stuff" that's not there in other designs.


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