There are many different types of designs that fall under the category of "transmission line". Some offer a consistent size "tunnel" from start to finish, some are tapered, some are flared, front or rear loaded, etc... Each has their own benefits / drawbacks. I think that "Audio" magazine had a pretty decent article about the variations that one could encounter on these designs, but that was probably at least 25 years ago or so.
While i don't know if they are still around, Kinetic Audio used to use a TATL ( Tapered Acoustic Transmission Line ) design. Some brands, like Von Schweikert and DCM, who claim to have TL designs, are not anywhere near what one would consider to be a "true" TL.
Audio Xpress ran an article on a "DIY" speaker project that utilized TL back-loading of the woofer. Several graphs and charts demonstrating how hard it was to properly fine-tune and the amount of reflections / standing waves / non-linear output that one encounters using this approach. One of the hardest to build & design correctly, hence the lack of information and commecial offerings.
Old Colony Sound Labs just released a computer program that allows for various TL designs to be simulated, so those that are truly interested in DIY'ing a set of TL's might want to check into this. Sean
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