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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Subwoofer for car. Part 4. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers Beauty sits in a dungeon ... ... and on the street, in this case, there is only a phase inverter tunnel. Such is the fate of the most beautifully finished speaker when placed in a bandpass loudspeaker. In the history of our bass publications, this type of subwoofer acoustic design is the last one. And the point here is not that he is the last in his qualities. It's not like that at all. Just in order to cope with the bandpass, both the self-builder and the professional need to "enrich their memory with the knowledge of all those riches ..." and further down the text, who passed and remembers, of course. The bandpass loudspeaker is the most capricious design that can be imagined in the trunk of a car, and this applies to the manufacturing phase, and, most importantly, to the operation of the final product. As we will show a little further, the scope for errors that generate monstrous sounds that are unattainable in simpler configurations is huge. It is natural to ask - who the hell will carry us to these galleys? The answer is simple - greed and love. Greed - up to sound pressure. With any competent manufacture, the bandpass, in terms of the volume of bass radiation it creates, defeats rivals not even by knockout, but "for a clear advantage". Love is for beauty. The bandpass gives out all its mighty decibels through a pipe, that is, a phase inverter tunnel. The speaker is hidden inside and there is no need to look for a place for it from where it would radiate into the cabin. Finding a place for a tunnel, rarely more than 100 - 120 mm in diameter, is much easier and the most ingenious designs are possible. Another advantage that some manuals like to emphasize, I am silent, because it is imaginary. We are talking about the fact that, since, they say, the loudspeaker is a bandpass speaker, then it does not even need a crossover filter. What a savings! None, because it is required, but more on that later. Now - in order. The 4th order strip loudspeaker consists of two chambers, one of which is closed, and the other communicates with the surrounding space through a tunnel, like a conventional bass reflex loudspeaker.
The speaker is installed in the wall separating these chambers. It turns out as if they took a phase inverter and put a closed box "from the face" on it, or vice versa, they took a speaker in a closed box and provided it with a veil in the form of a phase inverter. Any of these models contains the great secret meaning of the bandpass, which comes down to the concept of a symmetrical load. What is the trouble with a conventional direct-radiation loudspeaker? The fact that the acoustic resistance of the medium, which the speaker must excite and vibrate, is very small. Small in comparison with the forces of elasticity and friction raging in the moving system of the speaker itself. The speaker almost extinguishes the energy supplied to it in its womb, turning it into heat in the end. From 0,2% to 0,5% passes into acoustic energy. Wow efficiency? In a way, it's like trying to row with a toy paddle tied to an excavator bucket. There is a lot of noise and panting, diesel fuel flows like a river, but all this perishes in oily mechanisms, because the environment almost does not resist. You can, of course, make the paddle bigger, which is what you have to do in bass speakers, where the cone acts as an paddle. In a conventional speaker - whether closed or with a phase inverter, behind the diffuser - a completely or partially enclosed space, where acoustic energy either perishes, but before that partly controls the oscillations of the diffuser in a closed box; or it is concentrated and splashes out through the phase inverter tunnel, significantly increasing the efficiency. dynamics in the frequency band where it occurs. But, if in these loudspeakers behind the "back" of the diffuser there is some kind of supervision, then its "face" is left to the mercy of fate and tries in vain to cause energetic tremors in a sluggish, almost weightless air mass. The creators of the bandpass, by enclosing the speaker between closed volumes, just ensured that both sides of the diffuser are loaded with rather energetically resisting air masses, which, resisting, involuntarily concentrate the radiated acoustic energy and give it out "on the mountain" already in amplified form. In real life it looks like this. The graph shows the frequency response of loudspeakers in open space, built on the basis of the same head with optimal parameters for this design. The Infinity Kappa 100w.br was chosen as the speaker for all the considered examples.
This speaker in its parameters represents an almost perfect compromise, allowing you to implement on its basis both a closed loudspeaker, and a phase inverter and a strip. This does not happen very often, less often, in any case, than manufacturers promise, attributing unconditional biscuity to their creations. Already on this graph it is clear that below 90 - 100 Hz the bandpass loudspeaker (red curve) gives an enviable head start to the phase inverter (blue curve), not to mention the closed box (green). It’s frightening, however, a kind of frank hump on the characteristics of the bandpass, because of which one can easily suspect that the bandpass with excellent recoil will desperately mumble on one note. Moreover, many really created designs do just that, which scared away many supporters of expressive bass (but attracted lovers of thunderous bass). All wrong. Yes, it happens, they mumble, but not because of this. Take a look at the second graph - the frequency response of the same trinity, but taking into account the internal acoustics of a medium-sized car, for which the characteristics were optimized:
In these, in fact, real conditions, the music lover's favorite - a closed box, gives an ideal, up to half a decibel, horizontal frequency response. The phase inverter is a noticeable hump in the area of \u40b\u45b"tasty" bass, which many people prefer to a sterile horizontal frequency response - it's not for nothing that all bass correctors on amplifiers, if any, are tuned to a frequency of 6 - 4 Hz. And the bandpass version, with the same speaker and with the same input power, produces XNUMX dB more sound pressure. This, by the way, is equivalent to increasing the power of the amplifier by XNUMX (in words - four) times. Where did the hump go? And it disappeared, compensated by an increase in recoil at frequencies below the critical one for the cabin size selected in this example. Well, how are your hands itching? Wait a little, wash something to relieve itching and read again what was written just above: with optimal parameters. And what parameters are optimal and what will happen if you do not get into them? Here, the scope for maneuver and for an accident is wider than that of a closed, for example, box, exactly three times, because just as many times more characteristics are to be chosen. Two volumes (according to the number of chambers) and setting up a tunnel in one of them. Also tell Bose (thank you that the second tunnel is not allowed to be done ... Let's go in order: The volume of the rear (closed) chamber Vr( determines, in essence, the "working range" of the finished bandpass. It can be seen from the graph that the larger the volume of the closed chamber, the lower both boundaries of the working band become, and the lower the return (efficiency ) loudspeaker. Of course, as well as in a closed box, with increasing volume, the maximum allowable input power decreases, since control over the movement of the diffuser is lost. If the volume of the rear chamber is reduced, the entire bell-shaped frequency response shifts up in frequency, efficiency increases, but at the cost of narrowing the bandwidth.
When changing the volume of the anterior chamber (with a tunnel) Vf, the extreme limits of the working band remain in place, but the outlines of the "top" of the bell change. The tighter the anterior chamber, the lower its gain, but it appears in a wider frequency band. The volume of the anterior chamber shown on the graph, 8 liters, should be considered critical, because at this boundary the average often horizontal section of the frequency response already begins to "sag". With a further decrease in Vf, the characteristic will take on a frankly two-humped shape, and this cannot be corrected by anything.
What was ideal in the boundless space (tuning to 55 Hz) turned into a characteristic that was rapidly falling above 50 Hz. Docking it with the midbass is absolutely unrealistic, because the crossover frequency will have to be chosen at 60 hertz, and the midbass cannot be pulled so low. And if you do not tighten it, a hole will appear exactly where the most expressive elements of the bass register are concentrated - the lower midbass (80 - 100 Hz). On the other hand, what looked crooked in the previous picture blossomed into a very decent and fairly even response, reaching 90 - 95 Hz without any effort, where it can be married even with 5-inch midbass. (You have already seen this curve on the very first graph - I had it hidden.) I don’t even give the characteristic with the setting for 45 Hz from the previous example, so as not to frighten once again. But I will supplement the graph with what can happen with an error in the other direction - setting the tunnel to 75 Hz. See the narrow and nasty hump at 80 Hz? So he will mutter sickeningly, poisoning our lives, and the reputation of the bandpass. You might say, "So what, badly calculated artwork is bound to produce bad characteristics and corresponding sounds." Correct, but the price of a mistake is somewhat different. Here's an example for you: for the same head Infinity Kappa 100w.br, placed in a closed box, we will "mistake" in volume compared to the original 14 liters by one and a half times, and then, where ours did not disappear, by another one and a half. The frequencies for a box volume of 20 and 30 liters in free space do not differ at all, in the cabin they diverge below 50 Hz, but remain more than favorable.
And we have more than doubled the volume! A more nervous phase inverter reacts more sharply to falseness in the tunnel setting, but still retains decorum. For example, the results of tunnel restructuring from 30 to 45 Hz with a constant volume of 29 liters. The frequency response is writhing strongly, but you can still live.
Because of such "strictness in piloting" and ruthlessness to calculation errors, it seems that more unsuccessful bandpasses have been created than successful ones. Add to this the possibilities that choosing the wrong speaker promises - we operated with one of the best for such an occasion. In general, life could easily fail ... Another element of its specificity that scares away the bandpass is a much more complex and troublesome manufacture. Where the speaker "looks at the street", it can be installed from the street. Do everything, fasten everything, seal it, and finally tighten the head. Here it will have to be placed inside, and, therefore, at least one wall should be made detachable, losing rigidity and acquiring in return a headache with joint sealing. Work "not for a boy, but for a husband." But... Everything in life has its "but". Except maybe migraines... But the bandpass gives extraordinary layout possibilities. Let's say: the simplest constructive implementation, shown at the beginning of the article, looks like a box closed on all sides, which cannot be damaged, and, therefore, it can be placed within the trunk "to taste". If you follow the path of installing two speakers, the possibilities are generally unique. Look: two identical bandpasses can be fused into one three-chambered one, making them a common front chamber, and two tunnels can be replaced with one, with a double section. Then the whole structure can transmit bass to the cabin through a single window of God knows what size, attached, for example, in the center of the rear shelf or opposite the rear seat armrest opening. Fine?
Or vice versa: put the speakers "back to back" in a closed chamber, and organize the bass "exhaust" through tunnels located outside the volume of the box and connected to the holes originally intended for the rear speakers.
In a word, you can turn around here very sincerely, you just need to keep in mind all the time: the bandpass is a powerful, most, in fact, powerful weapon of a bass attack. And such weapons are not for recruits, therefore, if you, or even your quite professional installer, do not have sufficient experience in calculating, designing, manufacturing, tuning and measuring the characteristics of subwoofers, it is better to stop at something simpler. And if all of the above is there and you "hit the setting" - invite me to listen, huh? Author: Andrey Elyutin, AvtoZvuk; Publication: avtozvuk.com
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