I placed a little extra right behind the driver, and then made the rest of the line uniform. Some argue that the density should be greater just behind the driver, and gradually taper off, while others argue that it should be uniform through out the line. Acoustastuff and long hair wool are the prefered materials (the new Miraflex by Owens-Corning may be OK), these both have convoluted surfaces along the length of the fibers, which aid in the function of the material. Fiberglass or polyfill can be used, but these are not the best. The material chosen affects both the apparent speed of sound in the line AND the apparent increase in box volume. The line should be stuffed with an average of about 0.5 lbs to 0.7 lbs of acoustical stuffing material per cubic foot of line volume.This is the hard part, as the line stuffing affects both the total apparent line length AND the total apparent box volume simultaneously. The line length should be 1/4 wavelength tuned to the resonant frequency of the chosen speaker IN THE BOX VOLUME CREATED BY THE TOTAL T-LINE CROSS SECTIONAL AREA TIMES LENGTH, and as if the box were a closed box.
This means a very tight sizing for the woofer mount and the begining of the line. The area can be gradually tapered down to the actual area to save space, but it can remain the same as when it started, as long as it is not more than than about 20-25% more area. Normally, it is difficult to make the line area exactly equal to the cone area, as the frame and mounting immediately force an area larger than the cone area.
Editor's Note: This methodolgy is obselete and here for historical purposes.