I want to get rid of all the above. It is difficult to find a music lover who at least once did not scold bass reflex acoustics and did not look for an alternative. With the latter, everything is not so simple. Among the possible options, labyrinth acoustics is relatively widespread. The problem of labyrinths is that they are not technological and require a high production culture, which naturally affects the cost. A relatively budget version of the maze is the transmission line, it allows you to achieve a smooth frequency response, while maintaining high sound pressure in the low frequency range, but is less demanding on calculations, production costs and structurally simpler than the classic labyrinth acoustics. Under the cut, we are talking about its history, features and modern application.
General information
The transmission line is a hollow waveguide of variable or constant cross-section. One end of the waveguide is closed, the second is open. The dynamic emitter is located on the closed end side. The pipe, as a rule, is twisted and qualitatively damped. The bottom line is to reduce the amplitude of oscillations of the dynamic emitter diffuser in the region of the lowest frequencies near the resonant frequency of the tube and at the same time compensate for the decrease in the return from the speaker by the natural vibrations of the transmission line in the main, lowest-frequency mode.
In the overwhelming majority of cases this can be achieved when the length of the translation line coincides with a quarter of the length of the oscillations at the frequency of the natural resonance of the speaker. Gaponenko in his book “Speakers with their own hands” describes this with the following formula:
Where L - the so-called “Acoustic” length, which exceeds the actual geometric line length by an amount:
where S is the cross-sectional area of ​​the translation line.
In other words, it is necessary to tune the housing to the resonant frequency at which the air at the outlet of the waveguide will move in phase with the vibrations of the diffuser. A correctly designed transmission line is characterized by high accuracy in the low frequency range while maintaining sufficiently powerful, accentuated bass.
The bottom line is that it is easier to design a TL than other types of labyrinth design, and there will be no typical phase-inverter problems. Typical gundosy and turbulent sounds are not typical for such acoustics. The main advantage of such speakers is fidelity in the low frequency range, while maintaining a fairly small size.
The “reverse side” of the translation line, like structurally related labyrinths, is criticality to the correct calculation. Significant errors in the calculations will significantly affect the sound, unnecessary rattling resonances will appear, or an impressive frequency response unevenness. The good news is that it’s easier to calculate than the more complex types of mazes.
Voith's Well-Forgotten Trumpet
The earliest mention of the use of the transmission line that I was able to find was the experience of Paul Voight. The world
Paul voigt
The fact is that at that time Voight was developing speakers for cinemas, which were traditionally made into shoutboxes for that time. Then he switched to radios and home acoustics, where the broadband dual-diffuser speaker with a mechanical crossover used by him did not differ in a powerful bottom. This necessitated the search for a new acoustic design more suitable for such speakers.
Having started development, he experimented and at some point decided to install the speaker in a not very traditional place, i.e. not at the beginning of a cone-shaped horn, but on one of its sides. In this design, the port is used to adjust the back flow. The adjustment itself is carried out by increasing or decreasing the amount of damping material, depending on the type of driver used. The resonant frequency depends on the length of the waveguide, as well as the position of the speaker.
The modern version of TQWT
The transmission line, later called the Vojta pipe - in the context is very similar to a classic horn equipped with additional walls. Voight himself called the speaker TQWT (Tapered Quarter Wave Tube) - a conical quarter-wave tube. The case received this name for the reason that, as in all other classical types of TL, for the first mode, a quarter of the wavelength fits in the pipe, for the second three quarters, for the third five, etc.
A relative disadvantage of this design is the inability to choose a low cutoff frequency, since in this case it is possible to obtain an expression of distortion at low frequencies. The rest of the design allows you to create a relatively compact floor acoustics with “flat” bass, similar in characteristics to more complex labyrinths.
TQWT - is practically not used in mass acoustics, but it is very often used by radio amateurs when creating their own speakers. The problem is that there is still no complete, developed theory that describes the acoustic processes of TQWT systems, which cannot be said about well-described phase inverters.
ATL - half-rack transmission line
When transmission lines are mentioned, it is usually about floor systems. It is believed that the form factor and the volume of shelving units require the most compact solutions, which is a phase inverter. However, there is a company that has found a comparative effective design solution for the transmission line in the shelves. The founders and developers of PMC are principled opponents of FI acoustics and are convinced that the future lies in their innovation. PMC is one of the few modern companies that specialize in transmission line speakers.
Over the decades, the company has developed dozens of models for studio and home speakers with a transmission line, some of which still exist. Until the 2000s, they mainly produced floor systems, since classical TL often assumed just such a form factor.
Later, engineers somewhat complicated the design and created the so-called “Latest generation transmission line” or ATL (Advanced Transmission Line). The peculiarity of this design is in additional elements, allowing to obtain the advantages of TL in the shelves.
The relative minus of this design is that the ATL is close to other labyrinth acoustics in terms of complexity and manufacturability, which is guaranteed to increase the cost. The only good news is that one of the leaders of PMC Peter Thomas believes that:
“We really believe that quality should increase with increasing prices ... our customers are far from fools.” (From an interview with Sasha Metson in 2010). "
Summary and a few words in defense of FI
Despite the tangible minuses of phase-inverted acoustics, the physics of its work is well described, and most of the acoustic effects are predictable. This certainly allows you to get a predicted result, which is very important for mass production. A number of companies have mastered the transmission lines, but it remains less technological and more expensive.
It is possible that at a certain moment the transmission lines will become quite accessible and massive, but this will not happen before the moment when the main processes occurring in the transmission line are theoretically described. If we talk about massive and inexpensive (up to $ 500) speakers, it will be extremely difficult to find something other than FI and speakers with a passive emitter.
Those who are tired of the problems of phase-inverter speakers, while the aesthetics or dimensions of the rooms do not allow the use of a closed box, perhaps you should think about purchasing or creating your own transmission line. I will be grateful for any opinions regarding the broadcast line, people who have occasion to independently create such speakers are especially interesting.
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