Transmissionline housing

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Mini-TML (self-made)
Internal structure of the mini TML box "Pico Lino 2"

Transmissionline enclosures (short: TL or TML) are a special form of loudspeaker enclosures . This loudspeaker principle uses standing waves in a delay line to improve the reproduction of low frequencies. Despite the similarity to the bass reflex box , the operating principles of TML and bass reflex differ considerably from one another.

Mode of action

In a simple transmission line, a loudspeaker excites a tube that is open on one side at or near the closed end to vibrate .

The wave that runs towards the end of the pipe is largely reflected due to the different radiation impedances in the pipe and the outside air , and a standing wave forms in the pipe . The fundamental wave is formed at pipe length = lambda / 4, the harmonics at the odd multiples of lambda / 4. This is why one speaks of quarter-wave resonance housing. At these frequencies in particular, sound energy is then mainly emitted to the environment via the open pipe end, while the membrane of the loudspeaker is in the rest position. The length of the pipe is often (but not necessarily) chosen so that its lambda / 4 resonance roughly coincides with the resonance frequency of the loudspeaker:

Length = speed of sound / (4 × loudspeaker resonance frequency )

Due to the air masses oscillating at both ends, the pipe behaves as if it were roughly its diameter longer.

It must be prevented that, in addition to the lambda / 4 resonance, higher-frequency components of the music signal are emitted through the pipe opening, which would lead to a very unpleasant "pipe sound" due to an interference- related comb filter characteristic. A suitable construction is, for example, the arrangement of a correctly dimensioned volume of air as a 6 dB low-pass between the loudspeaker and the pipe (not very clear-cut). 

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An effective method to counteract the development of individual pipe resonances is to position the loudspeaker in the fast belly of a resonance. To eliminate the first harmonic, the loudspeaker chassis would be a third of the length (as seen from the closed end), and for the second harmonic, a fifth of the length.

In addition to acoustic measurements, the effect can also be determined from the frequency response of the impedance . 

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Numerous, partly incorrect or outdated rules of thumb and construction regulations are circulating for the practical implementation of TL housings. A realistic prognosis of the acoustic behavior of a loudspeaker chassis in a transmission line housing is now made possible by simulation software; the most common programs are AkAbak and AJHorn, as well as the Mathcad worksheets by Martin J. King.

To dampen higher resonances, mostly porous or fibrous damping material is introduced into the pipe. If there is too much damping material in the pipe, the pipe behaves like an acoustic sump and not like a transmission line.

The damping material is to be determined in its position by suitable measures. It also causes a reduction in the speed of sound, which means that the structural length is reduced. The slight lowering of the loudspeaker resonance frequency due to the resonating insulation material is of little importance for the dimensioning.

Here, too, the impedance frequency response can provide information as to whether the damping was sufficient and in the right places. An impedance maximum lies in the range of the loudspeaker resonance frequency .

A transmission line pipe is often difficult to accommodate in living spaces because of its length, for example around two meters for 40 Hertz pipe resonance. For this reason, and because material can also be saved, folded transmission lines are usually constructed.

Characteristic is an amplitude frequency response dip in the upper bass range (80 to 120 Hz) caused by destructive interference between the direct sound of the loudspeaker chassis and the sound radiated through the pipe opening . Depending on the housing design and damping, further interference and pipe resonance effects appear in the amplitude frequency response at higher frequencies.

Although bass reflex enclosures are usually more efficient and easier to construct, some (especially English) designers and music lovers prefer this construction. 

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For transmission lines, the loudspeaker should have a relatively low resonance frequency.

The influence of the damping material on the mechanical quality factor and consequently also the overall quality factor can prove to be advantageous when using loudspeaker chassis with a weak drive and large overall quality factors, whereby the strong mechanical damping always goes hand in hand with the risk of increased non-linear distortion . Inexpensive loudspeakers, which are less suitable for other types of enclosures due to their high quality factors, are therefore often used in transmission line enclosures. Contrary to outdated design guidelines, a high overall quality factor is not a prerequisite for a functioning transmission line.

literature

  • Arthur R. Bailey, "A Non-resonant Loudspeaker Enclosure Design," Wireless World, October 1965, pp. 483-486
  • Arthur R. Bailey, "The Transmission-line Loudspeaker Enclosure", Wireless World, May 1972, pages 215-217

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