On the subject of Voigt pipes, Ron has built a couple and been quite impressed. They have interesting properties.
Most speakers try to play down the box vibrations, and prevent them coming back at the cone rear at higher frequencies. At very low frequencies the reflex design exploits these reflected waves to enhance bass response, but this is a resonant behaviour and causes the 'one-note bass' we hear so much about.
The Voigt pipe does this too, and gets around the tail-off in bass the transmission line usually suffers.
By using a pipe of particular length closed at one end, and positioning the driver at a point exactly one third down, then waves coming off the back of the speaker travel to the closed end, 1/3 the length, bounce back and reinforce the vibrations coming off the front of the cone. This way there is proper phase alignment at some specified point, related to the resonance of the driver (a bit lower, actually). The front wave is then taken down the remaining 2/3 of the pipe, loading it correctly at that frequency, and the result is a strong wavefront from both the speaker, and the vent - both in phase. In practice, this gives a very transmission line bass 'sound', placing rather than playing bass notes, but without the 4dB/octave rolloff from the resonant peak. However, no free lunch - the downside is the boxy sound which is very difficult to eliminate.
Cheers,
Hugh
