Transmission Line

PMC's Unique ATL (Advanced Transmission Line) Approach

http://www.pmc-speakers.com/company/technology.php

I think Bose does this with their wave radio. PMC's are the speakers that I am considering.

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).