Complicated charm - part 2
The history of T+A began with power lines, which fascinated designers many years ago. Later they were marginalized, so we see enclosures of this type every few years, and this, in turn, allows us to recall the principle of their operation.
Not all T+A (loudspeaker) designs were and still are performance-based. transmission lineHowever, the name of the Criterion series is forever associated with this solution, perfected by the company since 1982. In each generation, these were entire series with powerful flagship models, much larger than today, but how the largest dinosaurs died out. So we saw designs with two woofers 30 speakers, four-way and even five-way (TMP220) circuits, cabinets with unusual acoustic circuits, also with low frequencies placed inside (between a chamber with a hole or a closed chamber and a long labyrinth - for example TV160).
This topic - a labyrinth of different versions of power lines - T + A designers have gone as far as no other manufacturer. However, in the late 90s, development towards further complications slowed down, minimalism came into fashion, systemically simple designs won the trust of audiophiles, and the “average” buyer stopped admiring the size of the speakers, more and more often they are looking for something slender and elegant. Therefore, there has been a certain regression in loudspeaker design, partly common sense, partly derived from new market requirements. Reduced and the size, and "patency", and the internal layout of the hulls. However, T+A has not given up on the concept of power line improvement, a commitment that comes from the tradition of the Criterion series.
However, the overall concept of a loudspeaker enclosure acting as a transmission line is not a T+A development. It remains, of course, much older.
The idealized transmission line concept promises an acoustic heaven on earth, but in practice creates serious unwanted side effects that are difficult to deal with. They don't solve cases popular simulation programs – difficult trial and error still needs to be used. Such a problem has rather discouraged most manufacturers looking for profitable solutions, although it still attracts many hobbyists.
T+A Calls Its Latest Approach to Transmission Line KTL (). The manufacturer also publishes the case section, which is easy to explain and understand. Apart from a small midrange chamber, which, of course, has nothing to do with the transmission line, half of the entire volume of the cabinet is occupied by a chamber formed immediately behind both woofers. It is "connected" to the tunnel leading to the outlet and also forms a shorter dead end. And everything is clear, although this combination appears for the first time. This is not a classic transmission line, but rather a phase inverter - with a chamber with a certain compliance (always depending on the surface that is “suspended” on it, i.e. in relation to the surface of the opening leading to the tunnel) and a tunnel with a certain mass of air.
These two elements create a resonant circuit with a fixed (by mass and susceptibility) resonant frequency - just like in a phase inverter. However, characteristically, the tunnel is exceptionally long and with a large cross-sectional area for a phase inverter - which has both advantages and disadvantages, so this solution is not used in typical phase inverters. The large surface area is an advantage as it reduces airflow velocity and eliminates turbulence. However, since it sharply reduces compliance, it requires an increase in the mass of the tunnel due to its lengthening in order to establish a sufficiently low resonant frequency. And a long tunnel is a drawback in a phase inverter, as it provokes the appearance of parasitic resonances. At the same time, the tunnel in CTL 2100 is not so long as to cause the desired phase shift of the lowest frequencies, as in a classical transmission line. The manufacturer himself raises this issue, stating that:
“The transmission line offers serious advantages over a bass reflex system, but requires an extremely advanced design (…), the sound path behind the woofers (in the transmission line) must be very long - like an organ - otherwise the low frequencies will not be generated.”
It is really interesting that when drawing up such a declaration, the manufacturer not only does not comply with it, but also publishes material (case section) confirming this discrepancy. Fortunately, low frequencies will be generated only by the action of not a transmission line, but simply a delayed bass reflex system, which “in its own way” introduces beneficial phase shifts without requiring a tunnel with a length correlated with the expected cutoff frequency - this depends on other system parameters, mainly from the Helmholtz resonant frequency dictated by compliance and mass. We know these fences (also rendered as power lines, which makes them more glamorous), but the fact is that T + A added something else to it - the same short dead channel that has not been here since the parade.
Such channels are also found in cases with transmission lines, but more classic ones, without a communication camera. They cause the wave reflected from the blind channel to run back in phase, compensating for the unfavorable resonances of the main channel, which can also make sense in the case of a phase inverter system, since parasitic resonances are also formed in it. This idea is confirmed by the observation that the blind channel is half as long as the main one, and this is the condition for such an interaction.
Summing up, this is not a transmission line, at most a phase inverter with a certain solution, known from some transmission lines (and we are not talking about a longer channel, but about a shorter one). This version of the phase inverter is both original and has its advantages, especially when the system requires a long tunnel (not necessarily such a large section).
A definite disadvantage of this solution, in proportions suggested by T+A (with such a large cross-section tunnel), is that the tunnel system occupies about half of the total volume of the casing, while designers are often under pressure to limit the size of the structure to a value below the optimum for achieve the best results (using fixed speakers).
So we can conclude that T + A is also fed up with the transmission line and comes up with cases that actually play the role of phase inverters, but still can claim noble lines. The tunnel went through the bottom wall, so high enough (5 cm) spikes were needed to prepare a free distribution of pressure. But this is also a solution known ... phase inverters.
Transmission line at a glance
Behind the woofers there is a large chamber, and only from there tunnels go - one is shorter, closed at the end, the other is longer, with an exit in the bottom panel.
The starting point for the transmission line enclosure was to create ideal acoustic conditions for damping the wave from the back of the diaphragm. This type of enclosure had to be a non-resonant system, but only to isolate the energy from the back side of the diaphragm (which could not be "simply" allowed to radiate freely because it was in phase with the front side of the diaphragm). ).
Someone will say that the reverse side of the diaphragm freely radiates into open partitions ... Yes, but phase correction (at least partially and depending on frequency) is provided there by a wide partition that differentiates the distance from both sides of the diaphragm to the listener. As a result of the continuing large phase shift between the emission from both sides of the membranes, especially in the lowest frequency range, the disadvantage of an open baffle is low efficiency. In phase inverters, the back side of the diaphragm stimulates the resonant circuit of the body, the energy of which is radiated outward, but this system (the so-called Helmholtz resonator) also shifts the phase, so that the resonant frequency of the body is higher over the entire range, the radiation phase of the front side of the speaker diaphragm and the hole is more - less compatible.
Finally, a closed cabinet is the easiest way to close and suppress the energy from the back of the diaphragm, without using it, without compromising the impulse response (resulting from the resonant circuit of the bass reflex cabinet). However, even such a theoretically simple task requires diligence - the waves emitted inside the case hit its walls, make them vibrate, reflect and create standing waves, return to the diaphragm, and introduce distortions.
Theoretically, it would be better if the loudspeaker could freely “transmit” the energy from the back of the diaphragm to the speaker system, which would dampen it completely and without problems - without “feedback” to the loudspeaker and without vibration of the cabinet wall. Theoretically, such a system will create either an infinitely large body or an infinitely long tunnel, but ... this is a practical solution.
It seemed that a sufficiently long (but already finished), profiled (slightly tapering towards the end) and damped tunnel would meet these requirements at least to a satisfactory degree, working better than the classic closed casing. But it also proved difficult to obtain. The lowest frequencies are so long that even a few meters long transmission line almost never drowns them out. Unless, of course, we “repackage” it with damping material, which will degrade performance in other ways.
Therefore, the question arose: should the transmission line end at the end or leave it open and release the energy reaching it?
Almost all power line options - both classic and special - have an open labyrinth. However, there is at least one very important exception - the case of the original B&W Nautilus with a labyrinth closed at the end (in the form of a snail shell). However, this is in many ways a specific structure. Coupled with a woofer with a very low quality factor, the processing characteristics fall off smoothly, but very early, and in such a raw form it is not suitable at all - it has to be corrected, boosted and equalized to the expected frequency, which is done by the Nautilus active crossover.
In open transmission lines, most of the energy emitted by the back of the diaphragm goes out. The work of the line partly serves to dampen it, which, however, turns out to be ineffective, and partly - and therefore still makes sense - to the phase shift, due to which the wave can be emitted, at least in certain frequency ranges, in a phase approximately corresponding to the phase radiation from the front of the diaphragm. However, there are ranges in which the waves from these sources come out almost in antiphase, so weaknesses appear in the resulting characteristic. Accounting for this phenomenon further complicated the design. It was necessary to correlate the length of the tunnel, the type and location of attenuation with the range of the loudspeaker. It also turned out that half-wave and quarter-wave resonances can occur in the tunnel. In addition, transmission lines located in cabinets with typical loudspeaker proportions, even if they are large and tall, must be "twisted". That is why they resemble labyrinths - and each section of the labyrinth can generate its own resonances.
The solution of some problems by further complicating the case gives rise to other problems. However, this does not mean that you cannot achieve better results.
In a simplified analysis considering only the ratio of maze length to wavelength, a longer maze means longer wavelength, thereby shifting the favorable phase shift towards lower frequencies and enhancing its performance. For example, the most efficient 50 Hz amplification requires a 3,4 m maze, as half of the 50 Hz wave will travel that distance, and eventually the tunnel output will radiate in phase with the front of the diaphragm. However, at twice the frequency (in this case, 100 Hz), the entire wave will form in the maze, so the output will radiate in a phase directly opposite to the front of the diaphragm.
The designer of such a simple transmission line tries to match the length and attenuation in such a way as to take advantage of the gain effect and reduce the effect of attenuation - but it is difficult to find a combination that significantly better attenuates twice the higher frequencies. Even worse, the fight against waves that induce “anti-resonances”, i.e., collapses on the resulting characteristic (in our example, in the region of 100 Hz), with even greater suppression, often ends in a Pyrrhic victory. This attenuation is reduced, although not eliminated, but at the lowest frequencies the performance is also significantly lost due to the suppression of other and in this respect useful resonant effects that occur in this complex circuit. Considering them in more advanced designs, the length of the labyrinth should be related to the resonant frequency of the loudspeaker itself (fs) to obtain a relief effect in this range.
It turns out that, contrary to the initial assumptions about the absence of the influence of the transmission line on the loudspeaker, this is an acoustic system that has feedback from the loudspeaker even to a greater extent than a closed case, and a similar phase inverter - unless, of course, the labyrinth is not jammed, but in practice such cabinets sound very thin.
Previously, designers used various "tricks" to suppress antiresonances without strong damping - that is, with effective low-frequency radiation. One way is to create an additional "blind" tunnel (with a length strictly related to the length of the main tunnel), in which a wave of a certain frequency will be reflected and run to the output in such a phase to compensate for the unfavorable phase shift of the wave leading to the output directly from the loudspeaker.
Another popular technique is to create a 'bonding' chamber behind the loudspeaker that will act as an acoustic filter, letting the lowest frequencies into the maze and keeping the higher ones out. However, in this way a resonant system with pronounced phase inverter features is created. Such a case can be interpreted as a phase inverter with a very long tunnel of a very large cross section. For cabinets that function as a bass reflex, speakers with a low factor (Qts) will theoretically be suitable, and for an ideal, classic transmission line that does not affect the speaker, high ones, even higher than in closed cabinets.
However, there are fences with an intermediate “structure”: in the first part, the labyrinth has a clearly larger cross section than in the next, so it can be considered a chamber, but not necessarily ... When the labyrinth is muffled, it will lose its phase inverter properties. You can use more speakers and place them at different distances from the outlet. You can make more than one socket.
The tunnel can also be widened or narrowed towards the exit…
There are no obvious rules, no easy recipes, no guarantee of success. There's more fun and exploration ahead - which is why the broadcast line is still a topic for enthusiasts.
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