A transmission line loudspeaker is a loudspeaker enclosure design that uses the topology of an acoustic transmission line within the cabinet, compared to the simpler enclosures used by sealed (closed) or ported (bass reflex) designs. Instead of reverberating in a fairly simple damped enclosure, sound from the back of the bass speaker is directed into a long (generally folded) damped pathway within the speaker enclosure, which allows far greater control and use of speaker energy and the resulting sound.
Inside a transmission line (TL) loudspeaker is a (usually folded) pathway into which the sound is directed. The pathway is often covered with varying types and depths of absorbent material, and it may vary in size or taper, and may be open or closed at its far end. The aim of such a design is to try and ensure that undesired resonances and energies are selectively absorbed or reduced ("damped") due to the effects of the duct, or alternatively only emerge from the open end in phase with the sound radiated from the front of the driver, enhancing the output level ("sensitivity") at low frequencies. The transmission line acts as an acoustic waveguide, and the internal filling material both reduces reflection and resonance, and also results in a reduction in the speed of sound within the duct, which serves to lower the tuning frequency.
Transmission line loudspeaker designs are more complex to implement, making mass production difficult, but their advantages have led to commercial success for a number of manufacturers such as IMF Electronics, TDL Electronics, and PMC. As a rule, transmission line speakers tend to have exceptionally high-fidelity low-frequency response far below that of a typical speaker or subwoofer, reaching into the infrasonic range (British company TDL's studio monitor range from the 1990s quoted their frequency responses as starting from as low as 17 Hz depending upon model with a sensitivity of 87 dB for 1 W @ 1 meter), without the need for a separate enclosure or driver. Acoustically, TL speakers roll off more slowly (less steeply) at low frequencies, and they are thought to provide better driver control than standard vented-box cabinet designs, are less sensitive to positioning, and tend to create a very spacious soundstage. Modern TL speakers were described in a 2000 review as "match[ing] reflex cabinet designs in every respect, but with an extra octave of bass, lower LF distortion and a frequency balance which is more independent of listening level".
Although more complex to design and tune, and not as easy to analyze and calculate as other designs, the transmission line design is valued by several smaller manufacturers, as it avoids many of the major disadvantages of other loudspeaker designs. In particular, the basic parameters and equations describing sealed and reflex designs are fairly well understood; the range of options involved in a transmission line design means that the general design can be somewhat calculated but final transmission line tuning requires considerable attention and is less easy to automate.
Purpose and design overview
A transmission line is used in loudspeaker design to reduce time, phase, and resonance-related distortions, and in many designs to gain exceptional bass extension to the lower end of human hearing, and in some cases the near-infrasonic (below 20 Hz). TDL's 1980s reference speaker range (now discontinued) contained models with frequency ranges of 20 Hz upwards, down to 17 Hz upwards, without needing a separate subwoofer. Irving M. Fried, an advocate of TL design, stated that: "I believe that speakers should preserve the integrity of the signal waveform and the Audio Perfectionist Journal has presented a great deal of information about the importance of time domain performance in loudspeakers. I’m not the only one who appreciates time- and phase-accurate speakers but I have been virtually the only advocate to speak out in print in recent years. There’s a reason for that."
"It is difficult and costly to design and manufacture a time- and phase-accurate speaker system. Few of today’s high-end loudspeakers are time- and phase-accurate designs. The audio magazines need to appeal to a broad spectrum of advertisers including many who make speaker systems which are time incoherent. The magazines, and the reviewers who write for them, have ignored or downplayed the issue of time- and phase-accuracy in order to maximize advertising revenue. I am not alone in recognizing this situation."
Some proponents of TL loudspeakers consider that using a TL is the theoretical ideal manner in which to load a moving-coil drive unit. However, it is also one of the more complex types of constructions. The most common and practical implementation is to fit a drive unit to the end of a long duct that is usually open at the far end. In practice, the duct is folded inside a conventional-shaped cabinet, so that the open end of the duct appears as a vent on the speaker cabinet. There are many ways in which the duct can be folded, and the line is often tapered in cross section to avoid parallel internal surfaces that encourage standing waves. Some speaker designs also use a spiral or elliptical spiral-shaped duct, usually with one speaker element in the front or two speaker elements arranged one on each side of the cabinet. Depending upon the drive unit, and quantity and various physical properties of absorbent material, the amount of taper will be adjusted during the design process to tune the duct to remove irregularities in its response. The internal partitioning provides substantial bracing for the entire structure, reducing cabinet flexing and colouration. The inside faces of the duct or line are treated with an absorbent material to provide the correct termination with frequency to load the drive unit as a TL. The enclosure behaves like an infinite baffle, potentially absorbing most or all of the speaker unit's rear energies. A theoretically perfect TL would absorb all frequencies entering the line from the rear of the drive unit but remains theoretical, as it would have to be infinitely long. The physical constraints of the real world, demand that the length of the line must often be less than 4 meters before the cabinet becomes too large for any practical applications, so not all the rear energy can be absorbed by the line. In a realized TL, only the upper bass is TL loaded in the true sense of the term (i.e., fully absorbed); the low bass is allowed to freely radiate from the vent in the cabinet. The line therefore effectively works as a low-pass filter, another crossover point in fact, achieved acoustically by the line and its absorbent filling. Below this “crossover point” the low bass is loaded by the column of air formed by the length of the line. The length of the line is specified so as to reverse the phase of the rear output of the drive unit as it exits the vent. This acoustic energy combines with the output of the bass unit, extending its response and effectively creating a second driver.
Essentially, the goal of the transmission line is to minimize acoustical or mechanical impedance at frequencies corresponding to the fundamental free-air resonance of the bass driver. 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.
Transmission line loudspeakers employ this tube-like resonant cavity, with the length 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. 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 the 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 resonance frequency Fs.
In a transmission line speaker, the transmission line itself can be open ("vented") or closed at the far end. Closed designs typically have negligible acoustic output from the enclosure except from the driver, while open-ended designs exploit the low-pass filter effect of the line, and the resultant low bass energy emerges to reinforce the output from the driver at low frequencies. Transmission line enclosures may have smoother impedance curves than other loudspeakers, possibly from a lack of frequency-specific resonances, but can also have relatively low efficiency.
One key advantage of transmission lines is their ability to conduct the back wave behind the transducer more effectively away from it – reducing the chance for reflected energy permeating back through the diaphragm out of phase with the primary signal. Not all transmission line designs do this effectively. Most offset transmission line speakers place a reflective wall fairly close behind the transducer within the enclosure – posing a problem for internal reflections emanating back through the transducer diaphragm. Older descriptions explained the design in terms of "impedance mismatch", or pressure waves "reflected" back into the enclosure; these descriptions are now considered outdated and inaccurate as technically the transmission line works through selective production of standing waves and constructive and destructive interference (see below).
A second benefit is that the resulting music is time coherent (i.e., in phase). Fried quoted in 2002, a listening test performed and reported in December 2000's Hi-Fi News (as he believed) in which a high-quality recording was obtained using reputable but non-time-coherent loudspeakers and this recording was then time phase corrected; an expert listening panel "voted unanimously for the superior realism and accuracy of the time corrected output" for high-quality sound reproduction.
One of the significant and common problems with a transmission line loudspeaker system is the unwanted phase-cancellation effects of higher line harmonics bleeding from the transmission line and adversely affecting the overall sound field. For example, in the PMC PMC6 mid-sized transmission line monitoring loudspeaker, there is a dip around 300 Hz that is caused by the fifth harmonic of the transmission line’s resonant frequency. This type of problem is quite common, and it was readily apparent in other transmission line loudspeakers. For example, the large IMF TLS80 MkII from 1977 also had an anomaly, but this time at the lower frequency of about 140 Hz, consisting of an almost one-octave-wide deleterious 3-dB or so dip in the on-axis response. Another problem is that the sound radiation from the exit of the line is spread over a quite broad frequency range caused by the hump of the quarter-wave transmission line resonance, whereas the high-Q port resonance of a vented-box loudspeaker rolls off much more quickly and extends over a much narrower frequency band. These sorts of issues with transmission line loudspeakers can lead to tonal accuracy problems that cannot be resolved.