Subwoofer for car. Part 1. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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You need bass in a car. This statement, trivial from the point of view of many, including the author, I preface everything that follows, especially in favor of those who do not share this point of view, so that they can, with a clear conscience, delve into other materials published in this issue of the Salon AV".

In the meantime, we will try, within the allotted volume of magazine pages, to decide what needs to be done in order to get as much bass for our money as we need (or as much as we want) and as we want (or as we need).

The well-known confusion in understanding the principles of forming the bass section of car acoustics is largely due to the information policy of advertising, and often reference publications. There, a potential buyer is first of all told the size of the speaker, then its power, then another mythical "frequency range" and complete it with a victorious price chord.

All? It wasn't there! This is where everything just begins. In English, the speaker itself is called a driver - a drive, and this is very correct. Just as an engine becomes a car only by enriching itself with everything that humanity has developed for this, so a speaker will become a loudspeaker only in its acoustic design.

With high-frequency and mid-frequency heads, the situation is relatively simple: the high-frequency heads carry their acoustic design on themselves, and the mid-range ones require a minimum size.

Bass players are another matter. Here, almost everything is determined by the choice of acoustic design, and depending on this choice, all the parameters reported to you will be subject to revision: both power, and frequency range, and, in a certain sense, the price. For with a skillful choice of parameters, you can achieve the nauseating sound of the most expensive and thoroughbred bass speaker.

Caravan, caravan….

The magazine briefly touched on the main types of acoustic design, now it's time to "announce the entire list". It's not that long:

	Acoustic screen
	Closed box
	Phase inverter
	Loudspeaker with passive radiator
	Strip loudspeaker (4th order)
	Strip loudspeaker (6th order)
	Quasi band loudspeaker
	Three-chamber bandpass loudspeaker (4th order)
	Three-chamber bandpass loudspeaker (6th order)
	acoustic maze
	Rolled up mouthpiece
	Aperiodic load (acoustic impedance)

The task of any low-frequency acoustic design is solved according to the ancient principle of "divide and conquer". "Separate" means that the vibrations emitted by one side of the diffuser must be somehow separated from the vibrations created by the other side of it, simultaneously and in antiphase with the first. "Conquer" means that with the "superfluous" sound waves cut off in this way, one can act in different ways.

Historically, the first acoustic design was an acoustic screen. He keeps the defense, not letting oscillations from one side of the diffuser to the other and not allowing them to cancel each other up to frequencies at which the shortest distance between the front and back sides of the diffuser becomes comparable to the half-wavelength of the emitted frequency. And below this frequency, the acoustic screen "signs in complete inability" and allows anti-phase waves to extinguish each other as they please. To suppress an acoustic short circuit at a frequency of, say, 50 Hz, the shield must have a size of 3 meters by 3. Therefore, this type of acoustic design has long lost its practical value, although it is still used as a reference when measuring speaker parameters.

Structurally, the simplest acoustic design of those used in practice is a closed box (sealed or closed in foreign terminology). Here, unnecessary vibrations are dealt with decisively and coolly: locked in a closed space behind the diffuser, they will sooner or later fade away and turn into heat. The amount of this heat is negligible, but in the world of acoustics everything is in the nature of small perturbations, so how this thermodynamic exchange occurs is not indifferent to the characteristics of the acoustic system. If sound waves are allowed to dangle unattended inside the loudspeaker case, a significant part of the energy will be dissipated by the volume of air contained inside the case, it will heat up, albeit slightly, and the elasticity of the air volume will change, moreover, in the direction of increasing rigidity. To prevent this from happening, fill the internal volume with sound-absorbing material. While absorbing sound, this material (usually cotton wool, natural, synthetic, glass or mineral) also absorbs heat. Due to the heat capacity of the sound-absorbing fibers, which is much greater than that of air, the temperature rise becomes much smaller and the dynamics "seem" that there is a significantly larger volume behind it than it really is. In practice, in this way it is possible to achieve an increase in the "acoustic" volume in comparison with the geometric one by 15 - 20%. This, and not at all the absorption of standing waves, as many believe, is the main point of introducing sound-absorbing material into enclosed loudspeakers.

A variation of this (and not the previous, as is often believed) type of acoustic design is the so-called "infinite screen". In English sources, this type of design is called infinite baffle or free-air. All the given names are equally disorienting. We are all adults here and we understand that in practice there can be no infinite screen. In fact, an infinite screen is considered to be a closed box with a volume so large that the elasticity of the air enclosed inside it is much less than the elasticity of the diffuser suspension, so the speaker simply does not notice this elasticity and the characteristics of the speaker system are determined only by the parameters of the head. Where the border passes, starting from which the volume of the box becomes, as it were, infinite, depends on the parameters of the speaker. However, when solving practical problems of such volume alwaysit turns out the internal volume of the trunk, which, even in a small car, will give an "infinitely large" volume reaction even for a large speaker. Another thing is that not every speaker will work well in such a design, but we will discuss this separately when we talk about choosing a speaker for acoustic design (or vice versa).

With all the (by the way, seeming) simplicity of a closed box as an acoustic design for the low-frequency section of car acoustics, this solution has many advantages that are not found in other, more sophisticated designs.

Firstly, the simplicity (or almost simplicity) of calculating the characteristics. A closed box has only one parameter - the internal volume. You can choose the right one if you try! The margin for error here is reduced to a minimum.

Secondly, in the entire frequency range, up to zero, the diffuser oscillations are restrained by the elastic reaction of the air volume inside the box. This significantly reduces the likelihood of speaker overload and mechanical damage. I don't know how comforting this sounds, but for avid bass lovers, speakers in closed boxes sometimes burn, but almost never "spit out".

Third, only the closed box is an acoustic filter second order, that is, it has a frequency response decay below the resonance frequency of the head-box system with a slope of 12 dB / oct. Namely, such a slope, only in the opposite sign, has the frequency response of the internal volume of the car, below a certain frequency. If you guess, calculate or measure (as anyone happens to) - it becomes possible to get a perfectly horizontal frequency response at low frequencies.

Fourthly, with a competent choice of head parameters and volume for it, a closed box has no equal in the field of impulse responses, which largely determine the subjective perception of bass notes.

The natural question now is - so what's the catch? If everything is so good, why do we need all the other types of acoustic design?

There is only one trick. efficiency In a closed box, it is the smallest compared to any other type of acoustic design. At the same time, the smaller we manage to make the volume of the box, while maintaining the same working frequency range, the less its efficiency will be. There is no more insatiable creature in terms of power input than a closed box of small volume, which is why the speakers in them, as was said, although they do not spit out, they often burn ...

The next most common type of acoustic design is a phase inverter (ported, vented, bass-reflex), which is more humane in relation to the radiation from the rear side of the diffuser. In a phase inverter, part of the energy that is "put against the wall" in a closed box is used for peaceful purposes. To do this, the internal volume of the box communicates with the surrounding space by a tunnel containing a certain mass of air. The value of this mass is chosen in such a way that, combined with the elasticity of the air inside the box, create a second oscillatory system that receives energy from the rear side of the diffuser and radiates it where necessary and in phase with the radiation of the diffuser. This effect is achieved in a not very wide frequency range, from one to two octaves, but within it the efficiency increases significantly, according to the principle "no waste - there are unused resources".

In addition to higher efficiency the phase inverter has another major advantage - near the tuning frequency, the amplitude of the cone's oscillations is significantly reduced. This may at first glance seem like a paradox - how having a hefty hole in a loudspeaker cabinet can hold back a cone's movement, but it's a fact of life nonetheless. In its operating range, the phase inverter creates completely greenhouse conditions for the speaker, and exactly at the tuning frequency, the oscillation amplitude is minimal, and most of the sound is emitted by the tunnel. The allowable input power is maximum here, and the distortion introduced by the speaker, on the contrary, is minimal. Above the tuning frequency, the tunnel becomes less and less "transparent" to sound vibrations, due to the inertia of the air mass enclosed inside it, and the loudspeaker works as closed. Below the tuning frequency, the opposite happens: the inertia of the inertia gradually disappears and at the lowest frequencies the speaker works almost without load, that is, as if it was taken out of the case. The amplitude of oscillation increases rapidly, and with it the risk of spitting out the cone or damage to the voice coil from hitting the magnetic system. In general, if not protected, going for a new speaker becomes a real prospect.

A means of protection against such troubles, in addition to prudence in choosing the volume level, is the use of filters of infra-low frequencies. By cutting off a part of the spectrum where there is still no useful signal (below 25 - 30 Hz), such filters do not allow the diffuser to run wild at the risk of your own life and your wallet.

The phase inverter is much more capricious in the choice of parameters and tuning, since three parameters are already subject to selection for a specific speaker: the volume of the box, the cross section and the length of the tunnel. The tunnel is very often made so that the length of the tunnel can be adjusted for an already finished subwoofer by changing the tuning frequency.

Due to the presence of two interconnected oscillatory systems, the phase inverter is a fourth-order acoustic filter, that is, its frequency response theoretically has a drop of 24 dB / oct below the tuning frequency. (Really - from 18 to 24). It is almost impossible to get a horizontal frequency response when installed in the cabin. Depending on the ratio of the cabin size (and, therefore, the characteristic frequency from which the frequency response of the internal acoustics begins to rise) and the bass reflex tuning frequency, the total characteristic may deviate from a delicate hump to crazy Amur waves. The hump, that is, a smooth rise in the frequency response at lower frequencies, is often just what is needed for optimal subjective perception of bass in a noisy space, but sharp changes in amplitude with an unsuccessful choice of parameters earned the phase inverter, completely undeservedly, the nickname boom-box (“booze”) . To be fair, we note that a thumping effect can also be achieved from a closed box - I will explain how next time; and a properly sized bass reflex is capable of delivering very clear and musical bass with reasonable power input.

A variation of the bass-reflex design is a loudspeaker with a passive radiator (or radiator). Foreign language terms: passive radiator, drone cone. Here, the second oscillatory system, which makes it possible to utilize the energy taken from the rear side of the diffuser, is implemented not in the form of an air mass in the tunnel, but in the form of a second diffuser, not attached to anything, but weighted to the required mass. At the tuning frequency, this diffuser oscillates with the largest amplitude, and the main one with the smallest. As they move up in frequency, they gradually change roles.

Until recently, this type of acoustic design has not been used in mobile installations, although it is used quite often in home environments. The reason for the dislike was the unjustified efforts to get a second cone (usually the same speaker, but without a magnetic system and a voice coil) and the difficulty in placing two large cones where a cone and a small tunnel should be placed in a conventional phase inverter. However, most recently, car subwoofers with a passive radiator have appeared - the need has forced them. The fact is that recently new generation speakers have begun to appear with a very large diffuser stroke, designed to work in small volumes. The volume of air "blown out" by them during operation is very large, and now it would have to be made significant in diameter (otherwise the air speed in the tunnel will increase so much that it will hiss like a steam locomotive). And the combination of small volume and large diameter of the tunnel makes it necessary to choose a longer length for the tunnel. So it turned out that phase inverters of a conventional design for such heads would be decorated with meter-long pipes. To avoid such unnecessary incidents, they preferred to concentrate the required oscillating mass in a passive radiator with a diffuser stroke, the same as that of an active speaker.

The third type of subwoofer, quite often used in auto installations (although less often than the previous two) is a bandpass loudspeaker. Sometimes the name "loudspeaker with symmetrical loading" (symmetric loading) occurs. If the closed box and the phase inverter are acoustic high-pass filters, then the bandpass, as the name implies, combines high- and low-pass filters.

The simplest bandpass loudspeaker is a single 4th order (single reflex). It consists of a closed volume, the so-called. the rear chamber and the second, equipped with a tunnel, like a conventional phase inverter (front chamber). The speaker is installed in the partition between the chambers so that both sides of the cone work on fully or partially closed volumes - hence the term "symmetrical load".

Of the traditional designs, the bandpass loudspeaker is by any means the champion in efficiency. In this case, the efficiency is directly related to the bandwidth. The frequency response of a bandpass loudspeaker is bell-shaped. By choosing the appropriate volumes and tuning frequency of the front chamber, it is possible to build a subwoofer with a wide bandwidth, but a limited return, that is, the bell will be low and wide, or with a narrow band and very high efficiency. in this lane. The bell will then stretch out in height.

Bandpass is a capricious thing in the calculation and the most time-consuming to manufacture. Since the speaker is buried inside the case, you have to go to tricks to assemble the box so that the presence of a removable panel does not violate the rigidity and tightness of the structure. Matching the frequency characteristics of the subwoofer, interior and front acoustics is also associated with a well-known headache. Impulse characteristics are also not the best, especially with a wide bandwidth. How is this compensated?

First of all, as mentioned - the highest efficiency.

Secondly, the fact that all sound is emitted through the tunnel, and the speaker is completely closed. When arranging such a subwoofer, considerable opportunities open up for the installer (or amateur) with imagination. It is enough to find a small place at the junction of the trunk and the passenger compartment, where the mouth of the tunnel can be accommodated - and the path to the most powerful basses is open. Especially for such installations, JLAudio, for example, produces flexible plastic sleeves-tunnels, with which it proposes (and many agree) to connect the subwoofer output to the cabin. Like a vacuum cleaner hose, only thicker and stiffer.

Even more efficient are 6th-order bandpass loudspeakers with two tunnels. The chambers of such a subwoofer are tuned with a spacing of about an octave. A double bandpass provides less distortion in the operating band, since the speaker is loaded with bass reflexes on both sides of the cone, with all the advantages of such a load, but has a steeper frequency response decline below the operating band compared to a single bandpass.

The intermediate position is occupied by the so-called quasi-strip loudspeaker, which is also with a serial setting, where the rear chamber is connected by a tunnel to the front, and the front by another tunnel - to the surrounding space.

Three-chamber strip-line loudspeakers are simply alternative design implementations of conventional strip-line loudspeakers, and are made up of two ordinary ones, after which the wall separating them is removed.

There are three more options for the acoustic design of low-frequency acoustics, which, although they exist, are practically not used. The first of the outsiders is an acoustic labyrinth, where the "energy removal" from the back of the cone occurs through a long pipe, usually folded for compactness, but still increasing the dimensions of the subwoofer to limits unacceptable in a mobile installation.

The second is an exponential horn, which, in order to obtain a sufficiently low cut-off frequency, must have cyclopean dimensions, which makes it rare to use it in the low-frequency link even in stationary systems, where there is more space than in a car.

The third type, which has single application precedents, is a loudspeaker with an aperiodic load in the form of a concentrated acoustic resistance (aperiodic membrane). We used to call it PAS - an acoustic resistance panel. The idea is that the load on the diffuser is a closely spaced semi-permeable obstruction, such as a dense fabric or a layer of black wool sandwiched between perforated panels. Theoretically, such a load is inelastic in nature and, like a shock absorber in a car suspension, dampens acoustic energy without affecting the resonant frequency of the speaker. But this is theoretical. But in practice, the presence of an air volume between the speaker and the PAS created such a hodgepodge of characteristics and reactions that the results became unpredictable.

So, from a quick look at the main types of acoustic design, it is clear that there is no perfection in the world. Any choice will be a compromise. And in order to make the essence of the compromise clearer, let's finish this correspondence meeting as it should be - by summing up the intermediate results. Let's compare the options considered in terms of the main factors that determine the success of their use in a mobile audio installation.

These factors should include:

K.P.D.

The value of efficiency inherent in a particular type of acoustic design ultimately determines how powerful the amplifier will be needed to achieve the required volume level, and at the same time how difficult the life of the speaker will be.

In the frequency range 40 - 80 Hz, which is most important from the point of view of reproducing information in the bass register, the places are distributed as follows: narrow-band bandpass loudspeakers are champions in this classification, especially two-tunnel 6th order ones. They are followed by a broadband two-tunnel and a conventional phase inverter. And finally, the most eager for power input are a closed box and a broadband single bandpass.

Insertion distortion

In the lower octave - one and a half musical range (30 - 80 Hz), all types of acoustic design behave decently at low power levels. The phase inverter and bandpass speaker are somewhat better than the others, but not by much. But with high power, the rivals stretch along the distance. The best results here are to be expected from a dual bandpass loudspeaker. Behind him is a single bandpass and phase inverter. And it closes the circuit - a closed box, which gives the greatest distortion at large signal amplitudes.

Impulse characteristics

Accurate reproduction of the fronts of bass instruments is perhaps the main quality for bass acoustics. There is little use in low bass thrusts if they are blurry and sluggish. In this regard, a closed box promises the best results (if calculated correctly). Single bandpass loudspeakers have good performance, but degrade as bandwidth increases. The worst response to an impulse signal is a dual bandpass loudspeaker, again, especially a wideband one.

Coordination in frontal acoustics

The work of the subwoofer should be, starting from a certain frequency, entrusted to the midbass of the front acoustics. For a closed box and a phase inverter, this is not a problem, and the system designer has a fair amount of freedom in choosing the crossover frequency, since both this frequency and the steepness of the decline are determined by external circuits. But narrow-band bandpasses often have their own frequency drop starting from 70-80 Hz, where not all midbasses can safely pick up a song. At the same time, the requirements for midbasses become more complicated, and working with a crossover does not become easier.

Let's put all of the above in a table, based on our usual five-point system:

Author: Andrey Elyutin, AvtoZvuk; Publication: cxem.net

See other articles Section Speakers.

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