[BuffaloBill]

Sealed boxes have a similar roll off to transmission line, 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 usually 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 sounds 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...transmission line has enormous benefits compared to sealed and vented including improved resolution and reduced distortion, even frequency response and deeper, faster and better defined bass.

(A properly designed compression driver and horn will have even less excursion and provide a more even frequency response compared to other designs.)