Folding a Transmission Line
For aesthetic and practical reasons the TL is almost always folded on itself to assume the aspect of a normal loudspeaker box, whether bookshelf or floor-standing. In Figure 1a we see how the unitary area of a TL with a square cross-section varies with the variation of the length, measured at the centre of the same, in the case of a bend angle of 180°; in Figure 1b we see the same length of line, but without the bend. A simulation that does not take the fold into account refers to this second model.
If there is a change of cross-section at the bend, then the real length of the transmission line will also be different from the simulated length because l1+l2+l3 is always greater than l. In Figure 2a we see an example of a TL of unit width and a cross-section that halves at the bend; in Figure 2b the change of cross-section interpreted by a simulation that does not take the bend into account.
Tests with measurements have shown that these variations, especially those present in the TL end sections, considerably affect the final response of the system, and do so to an increasing degree as the size of the box increases.
SpicyTL discretises the section variation with 6 and 8 RLC elements for the 90 or 180 degree bend respectively.