Transmission line, ported and vented designs are three different concepts on how to load the bass driver in a speaker enclosure. Transmission lines and sealed boxes have a 12dB per octave roll off after the resonant frequency, while a vented box has a 24dB per octave roll off. Ported speaker are the most common as they are cheap to build and easy to design, though the quality of the bass reproduction is questionable in many designs and such a steep roll off can have knock on problems further up the frequency range.
Sealed boxes have a similar roll off to transmission lines, however the efficiency of such systems is lower due to the inward radiating energy not being utilised by the cabinet. Distortion also occurs as the air in the box is compressed and expanded adiabatically as the cone moves in and out, the force required to move the cone changes non linearly with respect to the cone excursion. Both sealed and vented designs require careful absorption to avoid rear radiating sound bouncing off the cabinet walls, and passing through the bass driver causing boxy resonances.
Transmission line by contrast sound very natural because there is no build up of pressure behind the bass cone, with the rear radiating air being forced through an internal labyrinth to reinforce the bottom end of the frequency band. This also means no rear sound is re-radiated through the bass driver. The other advantage is that the air in the transmission line loads the bass driver and lowers its resonant frequency. This allows for the extended low end response and keeps the bass driver well damped, requiring less excursion than sealed or ported speakers to produce the same output. PMC has painstakingly developed their own advanced transmission Line (ATL) and has taken loudspeaker design to new levels, by using a cabinet construction and highly specified drive unit and crossover components. The PMC ATL design has enormous benefits including Improved resolution & reduced distortion, Even frequency response and Deeper, faster and better defined bass.