Theory
Proper transmission line loudspeakers employ a tube-like resonant cavity whose length is set between 1/6 and 1/2 the wavelength of the fundamental resonant frequency of the loudspeaker driver being used. The cross-sectional area of the tube is typically comparable to the cross-sectional area of the driver's radiating surface area. This cross section is typically tapered down to approximately 1/4 of the starting area at the terminus or open end of the line. While not all lines use a taper, the standard classical transmission line employs a taper from 1/3 to 1/4 area (ratio of terminus area to starting area directly behind driver). This taper serves to dampen the buildup of standing waves within the line, which can create sharp nulls in response at the terminus output at even multiples of the driver's Fs.
Essentially, the goal of the transmission line is to minimize acoustical or mechanical impedance at frequencies corresponding to the driver's fundamental free air resonance. This simultaneously reduces stored energy in the driver's motion, reduces distortion, and critically damps the driver by maximizing acoustic output (maximal acoustical loading or coupling)at the terminus. This also minimizes the negative effects of acoustic energy that would otherwise (as with a sealed enclosure) be reflected back to the driver in a sealed cavity.
Older acoustical models discuss transmission lines in terms of "impedance mismatch" or pressure waves "reflected" off the terminus opening back into the cavity. In fact, there is no "reflection". The driver mounted in a resonant cavity exhibits behavior akin to "cavitation" in which a series of gas pressurizations and rarefactions oscillate back and forth in a captive state. As the driver propagates this alternating train of weak adjacent pressure and vacuum pulses down the transmission line - waves that fit neatly within the cavity (anti node at terminus) remain largely captive (low acoustic output) while waves that do not (node or peak pressure at the terminus) exhibit high levels of energy transfer. Those that meet neither condition exactly produce output that is neither maximum nor minimum. There is no physical phenomenon that can cause "reflection". The electrical circuit analogy upon which the concept of "reflection" is based has no physical embodiment in an acoustical transmission line. As discussed below, the degree of acoustical coupling achieved and hence, loading, is determined by the difference between the distance from the driver to the terminus and the length of the quarter-wave peak of the fundamental wavefront (Fs) and its odd-ordered harmonics. The greater the difference, the lower the acoustical coupling. The smaller the difference, the greater the acoustical coupling and hence the lower the acoustical impedance.
Read more about this topic: Acoustic Transmission Line
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