PMC IB-1S loudspeaker Transmission Lines
The use of transmission lines, or TLs, for bass-driver loading has blown in and out of favor for decades, and, one must admit, the physics behind it is not particularly reassuring. In theory, the transmission line behind the bass driver is a long tunnel that is damped, tapered, and folded to absorb all the energy from the rear radiation without impeding the driver's movement. Significant parameters in TL design are the volume of the air-space immediately behind the driver, the cross-sectional area of the initial portion of the line, the tapering of the area along the line to the mouth where it opens to the outside world, the balance between damping and air-flow in the line, and the arrangement of the bends in the line (John Wright, Hi-Fi News & Record Review, August 1970).
In common practice, however, TLs do not model what a physicist or radio engineer would call a proper transmission line. Practical TLs do not absorb all the energy, and a significant amount of low bass is re-radiated from the mouth of the tunnel to reinforce the system's low-frequency response. In addition, the TL is not purely resistive; it loads the bass driver with a reactive mass of air, thereby reducing its resonant frequency by a factor of 1.414. Thus, the line resonances and their distribution are arranged to reinforce the low-frequency response.
There are many knowledgeable audio people—Stereophile's Martin Colloms and John Atkinson among them—who believe that the performance of a transmission line does not justify its increased cost and complexity over a ported box of similar size. Nonetheless, with a well-designed tunnel whose length is one fourth the wavelength of the desired low-frequency response, a TL can achieve the measured performance of the ported enclosure, along with subjective properties that some of us prize. My large, home-grown TLs had an effortless bass power that I have heard equaled only in speakers too large for my digs.—Kalman Rubinson