Just wave!

My understanding is that the strongest reinforcement from a transmission line is when line is 1/2 wavelength long. At that frequency, the backwave emerges from the terminus in-phase with the front wave, which gives full reinforcement. Unfortunately 1/2 wavelength lines are impractically large (and no, you can’t shrink the cross-sectional area significantly and still get good performance).

The lowest frequency at which the line is effectively reinforcing the front wave is when the line is about 1/4 wavelength long. At that frequency the backwave emerges from the terminus 90 degrees apart from the front wave, in "phase quadrature", which gives partial reinforcement.

Unfortunately at the frequency where the line length is equal to 1 wavelength, the backwave emerges 180 degrees out-of-phase with the woofer and we have a cancellation notch. Fortunately this notch largely disappears in the farfield response, but ime it is often still noticeable.

Various techniques exist to mitigate the 1 wavelength cancellation notch, including: Offsetting the woofer from the closed end of the line; incorporating a Helmholtz absorber into the enclosure; using a LOT of stuffing to absorb as much of that 1-wavelength energy as possible (and maybe using a high-Qts woofer so that the low bass is still strong); and using two woofers at significantly different locations along the line so that they are not both notching at the same frequency.

Incidentally no stuffing material, not even the best New Zealand lamb’s wool, slows down the speed of sound in the line to any significant extent.

I am NOT a transmission line expert; just happened to learn a few of the pitfalls the hard way.

Duke

Ime TL’s are very good in the midrange, as there is essentially zero reflection back into the cone. That may well be their biggest advantage.

As for where the terminus (line opening or vent) is located relative to the woofer I’m sure that makes a difference but haven’t really analyzed it. My instinct would be to spread them apart as far as is reasonably possible in as many planes as you can, in pursuit of modal smoothing. Like if the woofer is up high on the front, and the terminus is down low on the back, if you toe the speakers in, now the woofer and terminus are displaced relative to one another in all three dimensions.

But I cannot reliably say that’s the best strategy for choosing where the terminus winds up - other considerations that I’m not taking into account may dominate. For instance the internal geometry may matter more.

I remember delivering a pair of my rear-ported speakers to the home of a customer who was replacing transmission lines that had the terminus on the front at the bottom. One of the first things he commented on was that the bass was smoother with my fairly low-tuned rear-ported box. Whether that was because of the terminus location versus my port location, or my speaker’s freedom from the half-wavelength bump and/or one-wavelength notch, I do not know. I was surprised that my speaker was competitive with the transmission line in the bass region.

Duke