Designing a Transmission Line Speaker enclosure
This more original enclosure design gets its name from the electrical transmission line used to convey audio or video signals. If the end of such a line is left unconnected signals travelling along it are reflected back to the source from the open end. If however, the line has a high degree of loss, or a matching circuit is connected to the end, the signal is totally absorbed and none is reflected back.
This practice also works in the more physical world of loudspeakers for absorbing the problematic rear wave. In this case a long path for the rear wave that is filled with acoustically absorbent material is provided. Whatever small amount of energy reaches the end, nay that may be reflected back undergoes further loss on the return journey so that little if any arrives back at the loudspeaker to modify its cone motion. Thus one of the major sources of distortion of the infinite baffle enclosure is eliminated.
The standard transmission line design
In practice the end of the line is left open so that what of the rear wave escapes into free air. Because of the time taken to travel along the line, this wave is therefore in phase with the front-emitted one, reinforcing it. The effect is thus similar to that of the bass reflex enclosure without the strong air and possible panel resonances.
This speaker has a high sensitivity and is more efficient than most, requiring only a modest amplifier power output. The provision of a long folded passage is expensive to manufacture commercially, which is why this sort of design is rarely produced in mass, with the exception of a few TDL designs (namely the RTL series, £300 - £900). Most come from smaller specialist manufacturers.
Such a design requires a lot of material and woodworking ingenuity, but this would not be impossible and the effort would be rewarded if the enclosure is solidly constructed.
An example of a transmission design