A transmission line speaker acts like a sealed system in many ways which is one reason it is so desirable.
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A transmission line features a complicated exit path filled with absorbent material that will ideally absorb all the back radiation that enters it as opposed to having the air rush out as it does in an ordinary ported design. The purpose is the same in both cases, to simulate the effect of a much larger enclosure by reducing the back pressure created by the woofer. They can work very well if properly designed but like everything else has its virtues and defects. I have had them but long ago.
As I understand it, higher level frequencies are absorbed by the damping in the transmission line, so the transmission line port reinforces the bass response - and the roll off is more gradual than with ported enclosures - so PMC's seem to have more useful bass than other designs of the same size. Only downside seems to be that they require an astute designer, no problem at PMC.
The t-line is a resonating pipe and the danger is that harmonics of the pipes' fundamental resonance might escape (the line) and color the sound of the speaker system. In practice, t-lines usually behave identically to reflex ports, but involve greater cost and complexity (passed on to the buyer of course).