How do transmissions lines differ from ported and vented speakers?
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 6dB per octave roll off after the resonant frequency, while a vented box has a 24bB 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, yet can suffer from an ‘oil can” effect due to high pressures behind the bass cone, interrupting its movement. Both sealed and vented designs can suffer from rear radiating sound bouncing off the cabinet walls, and passing through the bass driver causing what can be described as a boxy sound. Transmission line by contrast sound very natural because there is no build up of pressure behind the bass cone, with the rear
