(MJK) Some answers.
What are the attempted advantages of a TL speaker cabinet with equal sizes channels all the way to the port, as opposed to each channel being wider than the last?
(MJK) A constant area TL with need to be L = c / (4 x f) long to produce a fundamental 1/4 wave mode that is tuned near the driver's fs value. There will be higher harmonics generated at 3 x fs, 5 x fs, 7 x fs, and so on. You can mitigate the peaks and dips in the response by offsetting the driver 1/5 to 1/3 of the TL length and by adding stuffing to the first 2/3rds of the length.
The worst ripple and most ragged response will be achieved with a TL that expands from the closed end to the open end. The open end will never be large enough to be a horn loading so the standing waves will be very strong. To achieve a fundamental tuning frequency near the driver's fs the length will be much greater than L = c / (4 x fs). The harmonic's (3/4, 5/4, 7/4, ...) standing waves will be closely spaced just above the tuning frequency. This is a very difficult design to tame and get a decent SPL response. I would not recommend this style of TL.
Also, what is being achieved by a decreasing size horn channel compared to an increasing size channel?
(MJK) My recommendation is to use a severely tapered TL with a closed end to open end area ratio of at least 10:1 if not higher. This will produce a shorter and smaller TL compared to a TL with a constant or expanding area along the length. You can taper the line in steps of decreasing area or a constant slope. A simple single fold TL will work well so all you need is a slanted dividing wall in the enclosure.
A decreasing area TL will reduce the required length for a given tuning frequency. The length will be much less than L = c / (4 x fs). It will also push the higher harmonic's (3/4, 5/4, 7/4, ...) standing waves much higher in frequency where the stuffing is better at damping the peaks and reducing the resulting ripple. Mount the driver at 1/5 of the length and stuff the first 2/3rds of the TL with fiber.
Are there any simple answers to these questions? I probably won’t comprehend a high level answer.
(MJK) No simple answers. TLs are difficult to design and get an optimum result. An accurate computer model to simulate the TL and iterate the design is a requirement to get it right. Rules of thumb and other commonly accepted myths for TL design will produce a hit of or miss result, you would need to be very lucky.
