Ramarcus,
We have built some pretty elaborate custom transmission lines over the years. The do become somewhat large if you are to get some true low end extension, which is probably why they have never gained any real commercial traction. I don't know of any dedicated commercially available subwoofers using the TL principle.
The length of the line determines the extension of the line, 1/4 wave length of the desired frequency, ie if you want the line to extend all the way down to 20 Hz you'd be looking at a line thats about 17.4 feet long , 25Hz would call for a 11.3 foot long line and so on. The area of the line should taper from about 120 ptc. of SD (Driver Radiating Area) at the beginning of the line to about 20 ptc at the exit of the line.
The latest one we made was for the custom pair of PA speakers used at the Paul Stookey concert at last January's THE Show that we sponsored together with Cary Audio, it was a dual 12" line with the two starting chamber spilling into a completely smooth with a consistent taper from start to end, shoot me an email if you like some photos.
Good Listening
Peter
