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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Subwoofer for car. Part 3. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers Sound at the end of the tunnel "Volodya, you'll be at the warehouse - grab the ports for faziks..." (overheard in one of the Moscow installation studios) (When AvtoZvuk was still small and sat under the wing of Salon AV, the first two parts of the trilogy about subwoofers were published - about what to expect from different types of acoustic design and how to choose closed box speaker. A significant part of those who, thinking about life, decided to treat the bass armament of their car with understanding, this, in principle, could already do. But not all. Since there is at least one more, extremely popular type of acoustic design, which is not inferior in prevalence to a closed box. (A phase inverter in domestic literature, bass reflex, ported box, vented box - in English - all this, in fact, is a sound engineering implementation of the idea of a Helmholtz resonator. The idea is simple - a closed volume is connected to the surrounding space using a hole containing a certain mass of air. That's it the existence of this mass - that very column of air, which, according to Ostap Bender, puts pressure on any worker, and works wonders when the Helmholtz resonator is hired to work as part of a subwoofer.Here, a tricky thing named after a German physicist acquires the prosaic name of a tunnel (in bourgeois port or vent) . How does a phase inverter work? Why does the presence of a neatly made hole of a certain size in the loudspeaker case dramatically affect the work of the entire ensemble? As already mentioned, in passing, in the previous parts of this epic canvas, the phase inverter tunnel serves to delay the sound wave that occurs inside the loudspeaker box for a strictly defined time and release it outward in the same phase as the speaker created by the "front" side . Here, in the wild, they will combine their decibels and hit the ears (with the right calculation) so that it will not seem enough. That's why, in fact, they love the phase inverter - for the increased efficiency compared to a closed box. But not only. Brute force is not an argument if it is not backed up by signal fidelity. Here we mean another, much less trivial feature of the phase inverter - its ability to produce the required sound pressure at a significantly lower amplitude of the diffuser oscillations. This sounds somewhat paradoxical. Everyone knows that it is the presence behind the diffuser of a closed volume that restrains the vibrations of the diffuser, so why are they suddenly smaller in a "leaky" case? And because of the mass, as it was said. That's why the hole in the phase inverter housing is made as a rather long tunnel - a pipe, in other words, to keep a certain mass of air inside. At relatively high frequencies, above 200 Hz, the inertia of the air mass in the tunnel makes it acoustically completely opaque. It's like it's completely blocked.
Lower in frequency, the air pocket in the tunnel begins to come to life and move, as the pressure pulsating inside the box pushes it from behind. The inertia of the air mass leads to the fact that it does not move in time with the wave acting on it, but with some shift. This shift reaches 180 degrees in phase, that is, it begins to be out of phase with the sound wave emanating from the back of the diffuser at a certain frequency, which is called the bass reflex tuning frequency.
Here, almost all the efforts of the speaker go to rocking the intractable air mass inside the tunnel, so that there is almost nothing left for natural oscillations and the amplitude of the diffuser oscillation is minimal. (And the sound is coming, and what a sound it is! It's just that at this frequency, almost all of it comes from the tunnel). And since it is precisely the large amplitudes of the cone's oscillations that give rise to distortions that are noticeable to the ear, the situation in terms of sound is the most favorable. Even lower in frequency, things, however, begin to change for the worse. For very slow low-frequency oscillations, the mass of air in the tunnel is no longer inertia, and the back side of the diffuser pumps it back and forth like a pump.
In this case, a situation arises as if the speaker is not installed in the case at all, that is, the waves from the back of the diffuser and from the front meet in antiphase and largely eat each other, as in a normal acoustic short circuit. Therefore, below the tuning frequency, the output of the phase inverter drops twice as fast as that of a closed box. Worse, however, is something else - the diffuser no longer slows down anything and the amplitude of its oscillations at very low frequencies begins to grow simply catastrophically. The subsonic filters found on some, usually thoroughbred, crossovers and amplifiers are made almost exclusively to counteract this bass reflex bad habit. So, what exactly will we get by choosing a phase inverter as an acoustic design for our project? ((I want to warn you right away - the calculation of a phase inverter without computer programs intended for this is possible and there are calculation formulas and nomograms for it. However, on the threshold of the third millennium, I cannot qualify such methods other than masochism. Here is a picture that explains (almost) everything. A 10-inch speaker is taken, which is suitable for installation in a phase inverter in its parameters, and the characteristics that will be obtained when it is installed in an optimal phase inverter for it (20 l, tuned to 42 Hz) and a closed box of the same volume are simulated.
The upper of the two black curves, of course, is ours. Compared to a closed box, the response is substantially higher throughout the entire frequency band below about 150 Hz. What does "essentially" mean? Take a look: at a frequency of, say, 60 Hz, the difference is about 4 dB. And this is equivalent to increasing the power of the amplifier by 2,5 times. That is, with a modest 100-watt amplifier, such a sub will play as if 250 watts were connected to it. For the same money. But from the red curves depicting the dependence of the amplitude of the diffuser oscillations on the frequency, ours is the lower one. Just where most of the bass energy is concentrated - below 100 Hz - the amplitude starts to drop and remains much lower than in a closed box, although the sound pressure generated is twice as much! In a closed box, the oscillation amplitude grows steadily and, when the power specified as maximum is applied, it goes beyond the operating range (red dotted line) already to 70 Hz, and below it is generally a disaster. It is there that such familiar rales accompanying bass notes will be generated. At the phase inverter, grace with amplitudes continues up to about 30 Hz, and there the amplitude begins to grow indefatigably. However, there is already almost no sound, so it makes sense to “strangle” this part of the spectrum with a subtone filter (if any) and enjoy impact efficiency with a minimum of distortion in the truly audio range. "Great!" - an impatient and decibel-hungry reader will exclaim, close these pages and immediately go to fix holes in his own subwoofer. Comrade, stop! See what can happen next. Let, leaving everything unchanged, we turn out the old speaker from our 20-liter box and install another - designed to work in a closed case.
His characteristic in a closed, native box for him - the bottom one on the graph - was very nice. And after being converted into a phase inverter, it will become like the top one, that is, it will give a pronounced “slap” between 50 and 100 Hz. It was as a result of the creation of such combinations that phase inverters received at one time the insulting nickname boom-box (“booze”), later used, this time quite rightly, for some kind of portable radio. What was the difference between the two speakers? In two parameters that should be in a certain harmony for a given acoustic design, otherwise - leave hope for everyone who sounds here, so to speak. These parameters are the resonant frequency Fs and the total quality factor Qts. For the "closed speaker" they were Fs = 25 Hz, Qts = 0,4. And for the "phase inverter" - 30 Hz and 0,3. It seems that the difference is not so great, but the results are significantly different. The energy bandwidth parameter invented at one time Fs / Qts immediately shows who is who: its value for the first speaker is 62,5, and for the second - 100. The rule is simple - if Fs / Qts is noticeably less than 100 - forget the word "phase inverter". If close or more - remember again, and forget about the closed box.Around 90 - 100 - "twilight zone", where, with certain concessions, you can use one and the other. But what will happen if you insist on your own and push the speaker into an unusual design? Let's try, as long as the drama unfolds on paper and on a computer screen, that is, "with little blood, on foreign territory." To begin with, we put the "phase inverter speaker" in a closed box and try to vary with the only parameter that we have - the volume of this box.
There are three curves on the graph. The flattest - the result of installation in a box with a volume of 50 liters, the steepest falling below 100 Hz - with a box volume of 10 liters. And in the middle - our original characteristic in a 20-liter volume. We see: the volume changes from indecently small to impractically large, but there is no good characteristic - it either starts to subside too early or subsides too quickly.
The speaker, born for a closed box, as can be seen from the following graph, has the ability to either hit the optimum (middle curve) or "cut" on the volume, while receiving a rather noticeably "humming" characteristic (upper curve obtained in a volume of 10 liters ). And vice versa? Is it possible, when installing a "closed" speaker in a phase inverter, to set it up in such a way as to obtain a flat frequency response? Theoretically - yes, the benefit of a phase inverter is that with a constant volume it is possible to tune the frequency by changing the diameter and length of the tunnel (in practice - always the length, of course). We start the experiment with the upper, absolutely terrible curve (volume 20 l, tuning frequency 50 Hz) and, gradually, rebuilding the phase inverter, suddenly, at a tuning frequency of 20 Hz, we notice that we have come to a very nice curve (lower on the graph).
Opanki, let's now calculate which tunnel is needed for this - and go! After half a second of computer time, we find that in order to tune a 20-liter volume to a frequency of 20 Hz, you need a tunnel with a diameter of 75 mm and a length of 1 m 65 cm. That is, it is as tall as a miniature lady, and not as small as a detail of a compact subwoofer. But on the other hand, a "phase inverter" speaker will allow with minimal hassle (push the pipe - push the pipe) to rebuild the frequency no worse than an equalizer. The graph shows the results of such activities in the tunnel tuning frequency range from 35 to 52 Hz, which required a tunnel length of 190 to 400 mm - not God knows what even at the highest value.
In the next part of the subwoofer saga (of course, not the last one - the topic is boundless, and God is merciful and may prolong the years of the author), we will deal directly with the question of the practical implementation of our plans - for those who want to do it themselves or for those who wants to be able to distinguish the work of a competent installer from the attempts of an ignorant hack. Agree - even when riding in a taxi it is useful to know that the path from Sokolniki to Izmailovo passes somehow away from Chertanovo ... Author: Andrey Elyutin, AvtoZvuk; Publication: avtozvuk.com
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