Subwoofer for car. Part 2. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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Charging…

"Then I began to count the volume of the violin box, and this work was long and exciting. ... The volume cannot be reduced - the violin will sibilate, it will start to mutter dully. If you increase it, it will squeal piercingly, the basses will become dull and weak...", A.A. G.A. Weiner, "Visit to the Minotaur".

Context

In the previous part of our conversation, it became clear what are the good and bad aspects of various types of acoustic design. It would seem that now "the goals are clear, get to work, comrades .." No such luck. Firstly, the acoustic design, in which the speaker itself is not installed, is just a box assembled with varying degrees of care. And often it is impossible to assemble it until it is determined which speaker will be installed in it. Secondly, and this is the main fun in the design and manufacture of car subwoofers - the characteristics of a subwoofer stand little out of the context of the characteristics, even the most basic ones, of the car where it will work. There is also a third. A mobile speaker system equally adapted to any kind of music is an ideal that is rarely achieved. A good installer can usually be recognized by the fact that, when "taking a reading" from a client ordering an audio installation, he asks to bring samples of what the client will listen to on the system he ordered after it is completed.

As you can see, there are a lot of factors influencing the decision and there is no way to reduce everything to simple and unambiguous recipes, which turns the creation of mobile audio installations into an occupation strongly related to art. But some general guidelines can still be outlined.

Tsifir

I hasten to warn the timid, lazy and humanitarianly educated - there will be practically no formulas. As long as possible, we will try to do without even a calculator - a forgotten method of mental calculation.

Subwoofers are the only link in car acoustics where measuring harmony with algebra is not a hopeless business. To put it more bluntly, it is simply unthinkable to design a subwoofer without calculation. The parameters of the speaker act as the initial data for this calculation. Which? Yes, not the ones that hypnotize you in the store, be sure! To calculate, even the most approximate, the characteristics of a low-frequency loudspeaker, you need to know its electromechanical parameters, which are innumerable. This is the resonant frequency, and the mass of the moving system, and the induction in the gap of the magnetic system, and at least another two dozen indicators, understandable and not very clear. Upset? Not surprising. Just about twenty years ago, two Australians turned out to be upset - Richard Small and Nevil Thiel. They suggested using a universal and rather compact set of characteristics instead of mountains of tsifiri, which perpetuated, quite deservedly, their names. Now, when you see a table in the description of the speaker called Thiel/Small parameters (or simply T/S) - you know what I'm talking about. And if you do not find such a table - go to the next option - this one is hopeless.

The minimum set of characteristics that you need to find out is:

• Speaker natural resonant frequency Fs
• Total quality factor Qts
• The equivalent volume of Vas.

natural frequency is the resonance frequency of the speaker without any acoustic design. It is measured in this way - the speaker is suspended in the air at the greatest possible distance from surrounding objects, so now its resonance will depend only on its own characteristics - the mass of the moving system and the stiffness of the suspension. There is an opinion that the lower the resonant frequency, the better the subwoofer will come out. This is only partly true; for some designs, an unnecessarily low resonance frequency is a hindrance. For reference: low is 20 - 25 Hz. Below 20 Hz is rare. Above 40 Hz is considered high for a subwoofer.

Full quality factor

The quality factor in this case is not the quality of the product, but the ratio of elastic and viscous forces that exist in the moving system of the speaker near the resonance frequency. A moving speaker system is a lot like the suspension of a car, where there is a spring and a shock absorber. The spring creates elastic forces, that is, it accumulates and releases energy in the process of oscillation, and the shock absorber is a source of viscous resistance, it does not accumulate anything, but absorbs and dissipates in the form of heat. The same thing happens when the diffuser and everything attached to it vibrates. A high figure of merit means that elastic forces predominate. It's like a car without shock absorbers. It is enough to run into a pebble and the wheel will begin to jump, unrestrained by anything. Jump at the very resonant frequency that is inherent in this oscillatory system.

As applied to a loudspeaker, this means an overshoot of the frequency response at the resonance frequency, the greater, the higher the total quality factor of the system. The highest quality factor, measured in thousands, belongs to the bell, which, as a result, does not want to sound at any frequency other than the resonant one, fortunately, no one requires it from it.

A popular method for diagnosing the suspension of a car by wiggle is nothing more than measuring the quality factor of the suspension in an artisanal way. If we now put the suspension in order, that is, attach a shock absorber parallel to the spring, the energy accumulated during compression of the spring will not all return back, but will be partially ruined by the shock absorber. This is a decrease in the quality factor of the system. Now back to dynamics. Nothing that we go back and forth? This, they say, is useful ... Everything seems to be clear with the spring at the speaker. This is a diffuser suspension. And the shock absorber? Shock absorbers - as many as two, working in parallel.

The full quality factor of the speaker consists of two: mechanical and electrical. The mechanical quality factor is determined mainly by the choice of suspension material, and mainly by the centering washer, and not by the external corrugation, as is sometimes believed. There are usually no large losses here, and the contribution of the mechanical quality factor to the total does not exceed 10 - 15%. The main contribution belongs to the electrical figure of merit. The hardest shock absorber operating in the oscillatory system of a speaker is an ensemble of a voice coil and a magnet. Being an electric motor by its nature, it, as it should be for a motor, can work as a generator, and this is exactly what it does near the resonance frequency, when the speed and amplitude of the voice coil movement are maximum. Moving in a magnetic field, the coil generates a current, and the load for such a generator is the output impedance of the amplifier, that is, practically zero. It turns out the same electric brake that all electric trains are equipped with. There, too, when braking, the traction motors are forced to work in the mode of generators, and their load is the batteries of brake resistances on the roof.

The magnitude of the generated current will naturally be the greater, the stronger the magnetic field in which the voice coil moves. It turns out that what more powerful speaker magnet, tem below, other things being equal, its quality factor. But, of course, since both the length of the winding wire and the width of the gap in the magnetic system are involved in the formation of this value, it would be premature to draw a final conclusion only on the basis of the size of the magnet. And the preliminary - why not? ...

Basic concepts - the total quality factor of the speaker is considered low if it is less than 0,3 - 0,35; high - more than 0,5 - 0,6.

Equivalent volume

Most modern loudspeaker heads are based on the "acoustic suspension" principle. We sometimes call them "compression", which is wrong. Compression heads are a completely different story, associated with the use of horns as acoustic design.

The concept of acoustic suspension is to install the speaker in such a volume of air, the elasticity of which is comparable to the elasticity of the suspension of the speaker. In this case, it turns out that in parallel to the spring already in the suspension, another one was installed. In this case, the equivalent volume will be such that the newly appeared spring is equal in elasticity to the existing one. The value of the equivalent volume is determined by the rigidity of the suspension and the diameter of the speaker. The softer the suspension, the greater will be the size of the air cushion, the presence of which will begin to disturb the speaker. The same happens with a change in the diameter of the diffuser. A large diffuser at the same displacement will compress the air inside the box more strongly, thereby experiencing a greater reciprocal force of elasticity of the air volume.

It is this circumstance that often determines the choice of speaker size, based on the available volume to accommodate its acoustic design. Large cones create the prerequisites for a high output subwoofer, but also require large volumes. The argument from the repertoire of the room at the end of the school corridor "I have more" must be applied with caution here.

Equivalent volume has interesting relationships to resonant frequency that are easy to miss without realizing. The resonant frequency is determined by the stiffness of the suspension and the mass of the moving system, and the equivalent volume is determined by the diffuser diameter and the same stiffness.

As a result, such a situation is possible. Suppose there are two speakers of the same size and with the same resonant frequency. But only one of them obtained this frequency value due to a heavy diffuser and a rigid suspension, and the other, on the contrary, a light diffuser on a soft suspension. The equivalent volume of such a pair, with all the external similarity, can differ very significantly, and when installed in the same box, the results will be dramatically different.

So, having established what the vital parameters mean, we will finally begin to choose a betrothed. The model will be like this - we believe that you have decided, on the basis of, say, the materials of the previous article in this series, with the type of acoustic design and now you need to choose a speaker for it from hundreds of alternatives. Having mastered this process, the reverse, that is, the choice of a suitable design for the selected speaker, will be given to you without difficulty. I mean, almost effortlessly.

Closed box

As mentioned in the above article, a closed box is the simplest acoustic design, but far from primitive, on the contrary, having, especially in a car, a number of important advantages over others. Its popularity in mobile applications does not fade at all, so let's start with it.

What happens to the characteristics of the speaker when installed in a closed box? It depends on a single quantity - the volume of the box. If the volume is so large that the speaker hardly notices it, we come to the option of an infinite screen. In practice, this situation is achieved when the volume of the box (or other closed volume located behind the diffuser, or, more simply, what is there to hide - the trunk of a car) exceeds the equivalent volume of the speaker three times or more. If this relationship is met, the resonant frequency and total quality factor of the system will remain almost the same as they were for the speaker. This means that they must be chosen accordingly.

It is known that the speaker system will have the smoothest frequency response at a total quality factor equal to 0,7. Smaller values ​​improve the impulse response, but the rolloff starts quite high in frequency. At large, the frequency response acquires a rise near resonance, and the transient characteristics deteriorate somewhat. If you focus on classical music, jazz or acoustic genres, a slightly overdamped system with a quality factor of 0,5 - 0,7 would be the best choice. For more energetic genres, the emphasis on the bottoms, which is achieved with a quality factor of 0,8 - 0,9, does not hurt. And finally, rap lovers will be drawn to the full program if the system has a quality factor equal to one or even higher. The value of 1,2 should, perhaps, be recognized as the limit for any genre that claims to be musical.

It must also be borne in mind that when installing a subwoofer in the passenger compartment, low frequencies rise, starting from a certain frequency, due to the size of the passenger compartment. Typical values ​​for starting up the frequency response are 40 Hz for a large car like a jeep or minivan; 50 - 60 for the middle, like a figure eight or "loin"; 70 - 75 for a small one, with Tavria.

Now it's clear - to install in the infinite screen mode (or Freeair, if it doesn't bother you that the last name is patented by Stillwater Designs), you need a speaker with a total quality factor of at least 0,5, or even higher, and a resonant frequency of at least 40 hertz commercials - 60, depending on what you bet. Such parameters usually mean a rather rigid suspension, only this saves the speaker from overload in the absence of "acoustic support" from the closed volume. Here is an example - Infinity produces versions of the same heads with indexes in the Reference and Kappa series br (bass reflex) and ib (infinite baffle).

The Thiel-Small parameters, for example, for a ten-inch Reference differ as follows:

It can be seen that the option ib in terms of resonant frequency and quality factor - ready for operation "as is", and judging by both the resonance frequency and the equivalent volume - this modification is much tougher than the other, optimized for operation in a phase inverter, and, therefore, is more likely to survive in the difficult Freeair conditions.

And what happens if, without paying attention to small letters, you drive into these conditions a speaker with an index that looks like two drops of water br? And here's what: due to the low quality factor, the frequency response will begin to collapse already at frequencies of about 70 - 80 Hz, and the unrestrained "soft" head will feel very uncomfortable at the lower end of the range, and overloading it there is easy.

So we agreed:

For use in the "endless screen" mode, it is necessary to choose a speaker with a high total quality factor (not less than 0,5) and a resonant frequency (not lower than 45 Hz), specifying these requirements depending on the type of predominant musical material and the size of the cabin.

Now about the "heavenly" volume. If you put a speaker in a volume comparable to its equivalent volume, the system will acquire characteristics that are significantly different from those with which the speaker came into this system. First of all, when installed in a closed volume, the resonant frequency will increase. Rigidity has increased, but the mass has remained the same. The goodness will also increase. Judge for yourself - by adding the stiffness of a small, that is, unyielding air volume, to help the stiffness of the suspension, we thus, as it were, put a second spring, and left the old shock absorber.

With a decrease in volume, the quality factor of the system and its resonant frequency grow in the same way. So, if we saw a speaker with a quality factor of, say, 0,25, and we want to have a system with a quality factor of, say, 0,75, then the resonant frequency will also triple. And what is the speaker like there? 35 hz? So, in the correct volume, from the point of view of the shape of the frequency response, it will turn out to be 105 Hz, and this, you know, is no longer a subwoofer. So it fits. See, you don't even need a calculator. Let's look at another. Resonant frequency 25 Hz, quality factor 0,4. It turns out a system with a quality factor of 0,75 and a resonance frequency somewhere around 47 Hz. Quite worthy. Let's try right there, without leaving the counter, to estimate how much the box will need. It is written that Vas = 160 liters (or 6 cu.ft, which is more likely).

(Here I would write a formula - it is simple, but it is impossible - I promised). Therefore, for calculations at the counter, I will give a cheat sheet: copy and put in your wallet if the purchase of a bass speaker is included in your shopping plans:

With us - about twice, so it turns out a box with a volume of 50 - 60 liters. It will be a bit much ... Let's go for the next one. And so on.

It turns out that in order for a conceivable acoustic design to come out, the speaker parameters not only have to be in a certain range of values, but also be linked to each other.

Experienced people reduced this linkage to the Fs / Qts indicator.

If the Fs/Qts value is 50 or less, the speaker is born for a closed box. In this case, the required volume of the box will be the smaller, the lower Fs or the smaller Vas.

According to the external data of "natural recluses" can be recognized by heavy diffusers and soft suspensions (which gives a low resonant frequency), not very large magnets (so that the quality factor is not too low), long voice coils (since the cone travel of a speaker operating in a closed box , can reach quite large values).

Phase inverter

Another type of popular acoustic design is a phase inverter, with all the ardent desire at the counter it is impossible to count, even approximately. But to estimate the suitability of the dynamics for him - you can. And we will talk about the calculation in general separately.

The resonant frequency of this type of system is determined not only by the resonant frequency of the speaker, but also by the setting of the phase inverter. The same applies to the Q-factor of the system, which can change significantly with a change in the length of the tunnel, even with a constant body volume. Since the phase inverter can be, unlike a closed box, tuned to a frequency close to or even lower than that of the speaker, the head's own resonant frequency is "allowed" to be higher than in the previous case. This means, with a good choice, a lighter cone and, as a result, improved impulse response, which is what the phase inverter needs, since its "innate" transient response is not the best, worse than that of a closed box, at least. But the quality factor is desirable to have as low as possible, not more than 0,35. Reducing this to the same Fs / Qts, the formula for choosing a speaker for a bass reflex is simple:

Speakers with an Fs / Qts value of 90 or more are suitable for operation in a phase inverter.

External signs of a phase-inverted rock: light diffusers and powerful magnets.

Bandpasses (quite briefly)

Strip loudspeakers, for all their loud merits (this is in the sense of the greatest efficiency, in comparison with other types) are the most difficult to calculate and manufacture, and matching their characteristics with the internal acoustics of a car with insufficient experience can turn into a pitch hell, so with this type When it comes to acoustic design, it is better to go over the rocks and use the recommendations of the speaker manufacturers, although this ties your hands. However, if the hands are still in an untied state and itching to try: for single bandpasses, almost the same speakers are suitable as for phase inverters, and for double or quasi-strip ones, they are the same or, more desirable, heads with an Fs / Qts value of 100 and higher.

How to calculate all this already moving away from the counter - next time.

Author: Andrey Elyutin, AvtoZvuk; Publication: avtozvuk.com

See other articles Section Speakers.

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