My guess is that not many of you have heard of a folding transmission line in a loudspeaker, much less ever heard one working. The late Bud Fried, from Fried Audio (pronounced "freed" as opposed to what one might do with eggs), was a big proponent of the design style to get bass out of a cabinet without dealing with a sealed box.
Transmission lines can be very effective to extend bass response, control the woofer's movement and lower distortion in a speaker over a broad range giving generally better overall results than those of a simple port.
If you're going to punch a hole in a woofer box to get lower bass, this might be the best way to do it. Unfortunately it is tough to implement because the idea behind it is to make a long tube or tunnel for the rear of the woofer's energy to travel - long enough that by the time it gets outside it is twice out of phase - meaning it is 360 degrees out of phase as opposed to 180 degrees.
At the long wavelengths found in low bass the sound being out of time by one cycle, relative to out of phase by half a cycle, is fairly meaningless to the ear and thus you really can't hear what's happening and you ignore this difference.
Creating a tunnel long enough to delay the output of the woofer long enough to be in phase takes some cabinetry work and what's usually done to make this work is called a folded path, where you make a sort of maze inside the speaker that goes back and forth until you get the correct length for the sound. Picture it like a line at Disneyland, where you have a folded line, people moving back and forth in a small space.
Transmission line loudspeakers typically have a broad mouth or opening just below the main woofer where the back pressure of the main woofer exits and supports the front of the woofer. It's a very clever design but for cost reasons it isn't done very often.
Tomorrow the sealed box Monopole.
