Details about the subwoofer in the home theater. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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At first it was exotic: in a home theater it was still all right, but thank you for occupying the trunk floor under the speaker in a hefty box. But everything flows, everything changes, and the inclusion of a subwoofer in a car audio circuit has become quite commonplace over time. The dilemma to put or not to put has given way to a more multifaceted problem which one is better to put? And really what?

The subwoofer is not only a source of low frequencies, but also makes the sound of the entire speaker system louder, reduces the amount of distortion due to the fact that additional load is removed from the remaining speakers. It is believed that for adequate bass transmission in serious audio complexes, a woofer is simply necessary. Given the importance of the sub-bass section of a system, speaker manufacturers offer the widest range of products, so there is plenty to choose from, from individual heads to active subwoofers and everything else in between. And what and how to install is the installer's job. You can make boxing yourself, use a "semi-finished product", a ready-made box or a ready-made subwoofer right away.

The sound of a subwoofer depends on many factors in the acoustic conditions of the cabin, the characteristics of the speaker head, the type of acoustic design, the shape and material from which it is made. In addition, the sub, being part of the car audio system, cannot live a life separate from it. In this connection, the work of the sub-bass link should be considered comprehensively. What are we going to do.

Configuration options

There are many of them, but there are six main options for including a subwoofer in the system. The first two use passive crossovers between the amplifier output and the speaker, the rest with active crossovers, which are located in the signal path between the signal source and the amplifier input. Naturally, each has its own advantages and disadvantages, summarized in Table N1.

The option using one amplifier is certainly the most economical, because a pair of speakers and a subwoofer "cling" to one device at once, and the signal is divided into frequency bands by an inexpensive passive crossover. On the other hand, most standard crossovers include a high-pass filter with a 6dB/octave slope and a low-pass filter with a 12dB/octave slope. The disadvantages of this configuration is that speakers driven by the same amplifier will not be sufficiently protected at peak powers. In addition, passive crossovers tend to have fixed cutoff frequencies, which limits the scope for system tuning. In addition, at all the power of one amplifier may simply not be enough.

The typical low pass filter slope (12 dB/octave) for passive crossovers is not quite adequate for bass reproduction. As you know, our auditory organs are not able to localize sound with a frequency below 100 Hz. That is, if there is a woofer in the car audio complex that reproduces frequencies below this mark, the listener, in theory, should not know where the bass is emitted from. At the same time, if the characteristics are not sufficiently steep, mid-bass frequencies, and sometimes even mid-range frequencies, become audible, and as a result, you can clearly determine where the subwoofer is located in the cabin. Thus, the work on creating a spatial picture is nullified.

The situation can be corrected by using filters of a higher order of the 3rd (18 dB / octave) or 4th (24 dB / octave). You can also model and create crossovers with the parameters required for a specific speaker. In principle, with a slight stretch, 2nd order filters are suitable for working with bandpass subwoofers, since they are the least susceptible to high-frequency overtones and they have the narrowest bandwidth. In general, standard 2nd order filters are not recommended for use as part of a sub-bass section.

An ideal, but rather expensive option is a configuration consisting of an active crossover, a bass amplifier and a subwoofer. Why, in general, is clear: the components, if there are sufficient financial resources, can be selected with characteristics that are adequate to each other and to the client's requests. Thus, great opportunities open up when setting up the system, which ultimately leads to positive results in the sound of the entire audio complex. The amplifier in such a circuit, in principle, can already be equipped with a built-in crossover, although this is not always required.

If the parameters of the built-in filters meet their requirements, then you can save money and do without an active crossover. In this case, first of all, you should pay attention to the order of the low-pass filter, which, as already mentioned, should not be lower than the third. It is also important to have smooth cutoff frequency adjustments or the possibility of setting them using replaceable resistor modules of the function, which allow you to integrate a subwoofer without audible "seams" at the junction of the bass and mid-bass ranges. However, in many budget amplifier models, the opposite is true. The standard configuration assumes the presence of filters with insufficient slope characteristics (6 and 12 dB per octave, respectively, for high-pass and low-pass filters), as well as two fixed cutoff frequencies, which, as already mentioned above, is not quite acceptable for bass. Then it remains to turn off the built-in crossover and offer the client to fork out for a more suitable device.

Finally, one more circuit as part of an active crossover and an active cabinet subwoofer (that is, what is sold with an amplifier). The method is perhaps the easiest. The manufacturer himself takes care of coordinating the parameters of the amplifier and the woofer, he himself calculates the working volume necessary for it and completes the bass unit with a crossover. True, there are many opponents to the use of active subwoofers in serious audio complexes. The main arguments: low power amplifiers and low-end crossover. In most cases, one can agree with these statements, although the above advantage (ease of installation) cannot be taken away from active subs. Perhaps the only way to improve their performance is to use a more "decent" active crossover.

What should be the crossover

There is something to think about here and it is desirable to do this at the initial stage of building the system. It is possible to have a device that has exclusively subwoofer outputs or use a crossover with low-pass and high-pass filters (in the event that one amplifier drives the subwoofer speaker along with other drivers). If the option of switching on amplifiers according to the biamping scheme is considered, then a three-way crossover with subwoofer, low-mid-frequency and high-frequency outputs is useful here.

The optimal crossover parameters are shown in Table. 2

Table 2

As we have said, the slope of the signal attenuation characteristic is the most important parameter when choosing a crossover. An insufficiently high filter order (below 3rd, that is, below 18 dB per octave) will lead to the fact that, along with the bass we need, the subwoofer will reproduce unwanted mid-bass or even mid frequencies. As a result, the effect of bass directionality is obtained, which is exactly what is not required of it.

How much power does a sub need

This is usually based on the total power of the system and the cutoff frequency of the subwoofer. The percentage power distribution in a typical car audio system, depending on the frequency range, looks like according to Table. 3 as follows.

Table 3

It is noteworthy that less than half of the power is at frequencies above 300 Hz. Therefore, at least 50% moves to a range below this border. This leads to a rule that applies to many configurations: the power of the bass amplifier should be one and a half times the power of the front speakers. So, if a 4-channel device 4 x 25 W is used at the front, then the power supplied to the subwoofer is calculated by the formula: 1,5 x (25 W + 25 W). We get that about 75 watts should be taken to the bass.

More may be required. For example, when the cutoff frequency of the low-pass filter is higher than 150 Hz, if the sensitivity of the subwoofer speaker is low, or if the subwoofer is in the luggage compartment and it is difficult for bass to enter the cabin. Well, when the client just wants "a lot of bass". It is clear that when choosing an amplifier (or amplifiers), first of all, they take into account the number of woofers in the system, the value of their impedance (woofers are usually made 4-ohm, with ordinary or dual voice coils) and the diameter of the cone.

The most effective and frequently used way for installers to increase the power of a bass amplifier is to bridge its channels (of course, if such an option is available). The inclusion of a 2-channel bridged circuit usually gives more watts than the total power of its channels, when operating in stereo mode. For example, a 2 x 50 W unit is supposed to deliver 150 W in the "bridge", and a 4-channel (4 x 50 W) device in stereo configuration will develop up to 150 W per channel.

Another option is the parallel connection of speakers (Fig. 1). In this case, the positive terminals are interconnected (the negative ones, of course, too) in order to ensure the in-phase movement of the heads. The increase in subwoofer output is due to a twofold decrease in the total impedance of parallel-connected voice coils. Since the standard impedance of car speakers is 4 ohms, the total impedance of two drivers connected in parallel will be only 2 ohms, which allows the bass amplifier to reach its full potential.

Fig. 1

It is important to remember that it is rare for an amplifier to withstand a load with a resistance below 4 ohms in a "bridge". Which in principle excludes the use of two 4-ohm speakers connected in parallel in this mode, since the amplifier included in the "bridge" "feels" only half the load that the speakers provide to it.

Types of subwoofers

What should be the acoustic design of the woofer depends on many factors. Each type of subwoofer box, whether fully sealed (sealed), bass-reflex (ported) or band-pass (band-pass), has its own advantages and disadvantages. In addition, a lot depends on the specific platform for building an audio complex, that is, the machine itself quite often dictates its conditions to specialists. And yet: is there an unquestionably better type of bass cabinet? The answer is negative. Agree: if there was, then why do we need all the others? On the contrary, closed boxes are installed in the trunks of vehicles with approximately the same regularity as phase inverters.

Before we turn to subwoofer enclosures, let's talk about why they are needed in principle. The whole problem is that the rear of the speaker generates as much sound as its front, but with the opposite directionality. Purely hypothetically, if you are put in a seat in a car where these two components affect the auditory organs with equal intensity, then they will be neutralized, and you will not hear anything at all. In practice, speakers devoid of proper acoustic design will radiate high and medium frequencies, but not the bass we need. This is because the sound wave emanating from the rear of the head has to make a longer path to the listener's ear. As a result, high and medium frequencies are amplified, while low ones, on the contrary, are weakened.

In a word, "corpusification" is indispensable. What are the boxes made from?

The preferences of experienced installers are very different, but most agree on one thing: without wood, in one form or another, good bass cannot be achieved. The two most common materials today are medium particle board (MDF) and plywood. These materials are distinguished by optimal acoustic characteristics, are sufficiently durable, easy to process and affordable. Chipboard is even more affordable, inferior in its characteristics (sound insulation, density, strength) to the two previous materials. Various types of plastic or plexiglass are more often used when there is a lack of space or for demonstration purposes, but this is already a certain concession from the side of music.

Boxless subwoofers (free-air)

What makes them great is their ease of execution.

The installer does not need to "sculpt" the box, and the owner does not need to clutter up the trunk. The speakers are mounted in a pre-reinforced and machined rear shelf, and the cargo compartment acts as a kind of enclosure that prevents sound from the back of the head from penetrating into the cabin. The problem is that it is not always possible to make the trunk completely "deaf", and the above phenomenon of low frequency cancellation results in the absence of what is called "deep bass" and in a decrease in speaker efficiency.

Fully enclosed boxes (sealed boxes)

A type of subwoofer enclosure that is reasonably effective at blocking the propagation of sound waves emanating from the rear of the loudspeaker. The "stuffy" air inside the box in this case acts like a spring, controlling the vibrations of the diffuser. Fully enclosed boxes are easier to design than, say, bass reflex and bandpass subwoofers. They are not too picky about the displacement required for optimal driver performance, and are more forgiving of erroneous data provided by the manufacturer in the product's accompanying documents.

The airbag protects the speaker from low-frequency impulse noise, which sometimes occurs when the system is turned on, infrasonic energy is supplied to it, or simply when driving on rough roads. This is the preferred option if you need to take up a minimum of space in the trunk or cabin, since it is believed that the volume of a fully closed case can be insignificant to reproduce "deep bass", although in this case you need to know when to stop. When the volume decreases below acceptable limits, significant losses of the low-frequency component occur. With an increase in volume, the return in the region of infrasonic frequencies will increase, but then the effect of saving space is leveled. In any case, in terms of output, such a subwoofer will be inferior to larger-sized counterparts of the bass-reflex or bandpass variety. At the same time, compared to them, the frequency response of closed cases in the decay region is smoother.

Bass reflex subwoofers (ported)

Bass reflex cabinets, unlike fully enclosed ones, use the sound coming from the rear of the speaker for the benefit of powerful bass, since the resulting sound pressure is the sum of the emissions of the cone and port.

Accordingly, the use of this acoustic design allows you to increase the efficiency of the bass head. At the same time, phase inverters are more complex to manufacture and more picky in terms of calculating the working volume, setting the port to the optimal frequency, matching the radiation resistances of the diffuser and the phase inverter pipe, etc. Therefore, it is better to double-check the parameters indicated by the manufacturer.

But well-done work, as a rule, is rewarded according to merit. It is believed that a bass-reflex subwoofer is capable of reproducing bass a whole octave below the case.

True, there is one "but": in this case, the box itself should be five times larger. So the use of a case with a phase inverter is often due to the customer's willingness to part with a certain volume of the cargo compartment.

Bandpass subwoofers

This type of acoustic design can be called a hybrid of the two previous ones, since in its design it has the characteristic features of both.

A typical strip subwoofer consists of two chambers, one of which does not have access to the outside, and the second one has it in the form of a phase inverter tunnel. Through it, the bass in a concentrated form is fed into the salon. In a concentrated one, because the speaker is mounted in a wall separating two volumes filled with air, and the vibrations of the diffuser meet resistance both from the front and from the rear. As a result, the energy that is supplied to the loudspeaker is not dissipated, but is spent more rationally than in closed and bass-reflex boxes. Such a cunning design of the bandpass causes, on the one hand, its higher sound pressure (up to 6 dB compared to closed cases), and on the other hand, a tendency to work in a narrow low-frequency band. But strips are good because they give you the right to choose, because by changing the volume of the chambers you can either increase the amount of sound pressure created by the woofer or expand the boundaries of the frequency range. However, in order to successfully manipulate the volumes, as well as adjust the phase inverter tunnel in accordance with the acoustic conditions of the cabin, one must have extensive installation experience. Here, the "course of a young fighter" will clearly not be enough.

Speaker selection

It's no secret that the size and type of acoustic design is largely determined by the power, sensitivity and frequency response of the bass head. Working in the low-frequency range, the subwoofer must set in motion large masses of air, the area of ​​\uXNUMXb\uXNUMXbwhich is proportional to the area of ​​the cone. Accordingly, larger diameter drivers generate more sound pressure. The second factor contributing to the reproduction of thunderous bass is the length of the diffuser stroke, that is, the maximum allowable (from the point of view of safety for the speaker) limits of the amplitude of its oscillations.

When choosing a speaker, it is important to know what kind of power load it can safely withstand, how efficiently the voice coil dissipates thermal energy. This parameter is directly related to the diameter of the voice coil and the gauge of the wire wound around it. Iron rule: the power of the subwoofer must exceed the power supplied from the amplifier. Unfortunately, not all manufacturers of woofers honestly tell us about this characteristic, often indicating somewhat inflated data for advertising purposes (however, amplifier manufacturers also sin with this). Therefore, so that the speaker does not burn out, it is better to check everything yourself.

In turn, the sensitivity of the speaker determines how efficiently it manages the power given to it by the amplifier. In other words, the sound pressure that a driver creates at a distance of 1 meter when a 1 W signal is applied to it. However, sometimes the indicator of the sensitivity of the speaker, indicated in his passport, is not entirely adequate to the real state of affairs. The thing is that power (Pn) is created by voltage (U), which depends on the nominal resistance (Rh) of the driver. If it is equal to 4 ohms (which is typical for car acoustics), then a voltage of 2 V is supplied. But 8-ohm home speakers require 2,83 V. You can play on this difference, which many manufacturers do by indicating the absolute sensitivity of the woofer, that is, measured with an input signal of 2,83 V.

Acceptable sensitivity for a subwoofer is 90 dB, but as they say, the more the better, because loudspeakers with more output make less demands on the power of the amplifiers. This is clearly illustrated in Table. 4

Table 4

As you can see, even a 3 dB difference is very significant and allows you to save twice on the power of the amplifier. But do not rush to rub your hands. It also has its own pitfalls. Subwoofers with high sensitivity often suffer from higher cutoff frequencies, which means that saving on amplifier power can easily be lost at the deepest bass frequencies. In other words, the sensitivity of a speaker should not be considered in isolation from its frequency response (sound pressure versus frequency).

The frequency response of a subwoofer depends both on the head itself and on the type and size of its acoustic design and the acoustic features of the vehicle. It is generally accepted that the larger the volume of the case, the closer the lower limit of the frequency range will be to the maximum value allowed for a given speaker. How to calculate it without carrying out practical tests, that is, before the construction of the box begins? There are three "magical" stats called Thiele Small's parameters.

Thiel's Parameters - Small

When building a subwoofer, you can’t do without three basic parameters: the natural resonance frequency of the speaker head (fs), its full quality factor at the resonant frequency (Qts) and the volume of air with elasticity equivalent to the elasticity of the diffuser suspension (Vas). They are usually indicated by the manufacturer in the accompanying product documents. Using these parameters, it is possible, by simple calculations, to compare the approximate characteristics of several speakers (in particular, to obtain the lower limit of the frequency range of the head) already at the early stages of installation, when you have not yet decided which type of box will be used and which woofer needs volume for optimal performance. The formula looks like this: ffb= fs Vas. Let's say you're choosing between two 10" Ultimate AU1050 and Jensen JSW104 heads. Here are their parameters (Table 5).

Table 5

As can be seen from the calculation results, despite the worse self-resonant frequency performance, the lower end of the Jensen JSW104 frequency range is about 10% lower than that of the Ultimate AU1050 (of course, if these speakers are installed in cabinets of the same volume).

If you have decided on the type of acoustic design and know what kind of speaker volume is needed for optimal performance, then the equation for determining the lower limit of the bass range of a particular subwoofer head model is somewhat modified:
f3 = 0,8ffb/ Vb (for closed case)
f3 = 1,0ffb/ Vb (for bass reflex housing)
(f3 indicates the lower limit of the range reproduced by the subwoofer in volume Vb).

So in a closed case with a volume of 0,75 cubic feet, the Ultimate AU1050 drops to 50,1 Hz, and the Jensen JSW104 to 44,8 Hz. In principle, these equations clearly demonstrate that with an increase in the volume of the case, the lower limit of the frequency range will tend to the maximum allowable for the woofer. This is due to the fact that when it (the head) is placed in a box of a certain volume, the subwoofer resonance frequency (f3) increases compared to the natural resonant frequency (fs) of the driver. Why is clear: a kind of air spring inside the box is added to the elasticity of the diffuser suspension. The larger its volume, the smaller this elasticity, and the lower the cutoff frequency of the subwoofer.

Free-air speakers have their own formulas: fob = fs/Qs and f3 = 0,9 fob. All the same Ultimate and Jensen showed the following results:

Note that in this application, Ultimate wins noticeably. In addition, the tendency to increase the lower limit of the frequency range immediately catches the eye when the speaker is operating in an open volume. Such is the price of the ease of making free-air subs. There is something to think about.

This begs the question: how low should the subwoofer play? It is known that modern cassette devices have a lower limit of the reproducible frequency range from 50 to 30 Hz. The same applies to FM radio stations. For CD receivers, this figure is usually around 20 Hz. However, even though a person, in principle, perceives frequencies below 20 Hz, it is not at all necessary to go so deep. It is rare that a soundtrack, including one recorded on a CD, has a lower limit of 30 hertz. This is rather an exception. And the rule is that the most bass musical instruments play in the band from 40 to 60 Hz. The rest are from 60 to 100 Hz. So the scope of the subwoofer usually starts (if you go from the bottom along the frequency response) from 40 hertz.

Finally, one last note. While Thiel Small's parameters allow many of the characteristics of a subwoofer to be calculated, it should not be forgotten that these calculations refer to ideal conditions (usually an anechoic chamber free of reverberations is considered "ideal"). Where will the sub play? That's right, in a car full of both reflective and absorbing surfaces. Therefore, the obtained characteristics are inevitably influenced by the acoustic conditions of the cabin, which are purely individual for each vehicle and change the frequency response in the low-frequency region of the spectrum quite noticeably. In essence, it is precisely for this reason that serious audio complexes are rarely equipped with ready-made boxes, since their manufacture takes into account, at best, the average parameters of cars.

Subwoofer software

As we have already found out, the calculation of acoustic design parameters for a bass head is not an easy task. Indirectly, this conclusion confirms the existence of specialized software (software), which can greatly facilitate the work of the installer. There are currently several such programs (Blaubox, WinSpeakerz, Term-Pro, JBL SpekerShop, etc.), but they are largely similar. You can choose a case for an existing speaker or, on the contrary, choose a woofer for an already built box.

Such programs allow you to compare the performance of one or another loudspeaker in different types of enclosures. Most likely, in the database you will find the loudspeaker you need with a listing of all the necessary characteristics. If not, then the base can be supplemented with the parameters of your driver that the manufacturer provided you with, and only then calculate all the necessary characteristics of the box to achieve the optimal frequency response and subwoofer power. True, these parameters, as we mentioned earlier, are still different from those that will be obtained when the box is loaded into the machine. This already requires experience, knowledge of the individual characteristics of the vehicle, which allow us to predict the behavior of the bass system in the car. Experience that comes with age. But everyone started somewhere. Worth a try.

Author: A. Smirnov; Publication: 12voltsmagazine.com

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