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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Making subwoofers, the secrets of the masters. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers Time to saw the pianos Full-fledged sound transmission today is unthinkable without the low-frequency component of the acoustic signal. Therefore, more and more drivers who are not indifferent to the sound quality in the car, complement the stereo and a pair of speakers (as an American installer would say, "deck and two") with speakers that expand the reproducible sound range both towards the upper and especially the lower limit.
Since a signal of 20 Hz - 150 Hz does not have a practical effect on the stereo effect, it is customary to separate it into one channel, which is decorated with one acoustic case, thereby simplifying the circuit design and saving the car interior space. And due to the fact that the sound in the bass range spreads around obstacles, you can place the subwoofer cabinet in any free part of the car. The acoustic box is made of different materials: plywood, chipboard, plastic. There are also craftsmen who use wood from musical instruments.
The acoustic design of a subwoofer, if it is not a set of ready-made factory-made parts, usually includes: designing and calculating an acoustic system for a specific order, manufacturing an acoustic enclosure and installing the system in a car. Because subwoofer acoustics require creativity, each master installer finds his own key to bringing it to life. Having agreed on fundamental issues with the customer, the master proceeds to detailed calculations of future acoustics. Dozens of subwoofer enclosure designs can be summarized in a few basic types, with the best known being closed, phase-inverted, and passive speaker systems. When designing a subwoofer speaker system, one usually strives to obtain a good transfer function, that is, a good ratio of the sound pressure generated by it and the complex input signal. The transfer functions of the three mentioned systems are in principle similar to the transfer function of the high-pass filter of the second and fourth order, respectively, with the steepness of the falloff of the amplitude-frequency characteristic towards low frequencies of 12 dB per octave and 24 dB per octave, and the final frequency response in the operating frequency range of the subwoofer depends on characteristics of the speaker used, the crossover filter used, the design and material of the case, as well as the capabilities of the power amplifier. Given all these features in the acoustic design of the subwoofer, the craftsmen seek to reduce the displacement amplitude of the moving speaker system and thereby increase the level of input electrical power, as well as the maximum sound pressure, which will help reduce its overload and signal distortion. For a detailed study of the design, today you can use special computer calculation programs, laying the subwoofer parameters "at the input" and getting the recommended characteristics of the speaker cabinet "at the output". Sometimes, in order to calculate certain volumetric characteristics of a cabinet, the master installer has to use "equivalents": if the subwoofer manufacturer gives recommendations on the types of acoustic cabinets and their volume, it is not easy to calculate the subwoofer's own volume, that is, the volume of air displaced by it, due to the complexity of its geometry. Some meticulous installers sometimes use some kind of homogeneous loose substance (not capable of damaging the surface of the speaker, of course), filling it in and measuring the equivalent volume of air displaced. The acoustic enclosure is the most important element of the entire subwoofer speaker system. It should not only have an optimal internal volume to accommodate the speaker and the necessary components, but also have sufficient strength - not only mechanical, but also acoustic. The fact is that a dynamic type subwoofer works like a pump, compressing the air in front of the diffuser and rarefying it from the back side in the axial direction. In this case, the pressures on both sides are equal, but inverted in phase, therefore, if there is no or insufficient insulation of the front and rear surfaces of the speaker, an "acoustic short circuit" will occur. To avoid it, it is necessary to provide a phase shift of the acoustic signal emitted from the rear side of the diffuser, for example, by half a wavelength. This is achieved by installing a "soundbar-partition", most often closed in the form of a case. The larger this baffle (or the larger the body volume), the theoretically lower the tonality of the signal, which will not be acoustically closed. If the volume of the body (i.e. the size of the air mass contained in it) affects the height of its resonant frequency, then the shape of the body affects primarily the formation of standing waves as a result of the body's own resonance, which occurs at a particular frequency. To combat the formation of standing waves, they resort to shifting the speaker from the center of the acoustic enclosure. In this case, the cubic shape of the acoustic case with the speaker located at an equal distance from all its walls is considered the most unfortunate.
The location of the speaker relative to the walls of the case The design of the enclosure largely determines the acoustic characteristics of a subwoofer system, although the materials used in its construction are equally important. Today they can be: wood, plastics, organic glass, ceramics and even concrete. Most craftsmen consider medium particle boards (MDF) to be the most practical: they have good soundproofing properties, are affordable, have a uniform density (unlike plywood), high specific gravity, and also lend themselves well to carpentry. It should also be noted that craftsmen who make competitive car audio systems often use exotic or transparent materials in the manufacture of subwoofer enclosures, not because of their unusual properties, but in pursuit of external originality or a desire to demonstrate the internal structure of the system. Closed systems
Optimal subwoofer cabinet size Systems of this type have good acoustic characteristics with structural simplicity. The volume of the enclosure determines the limits of the frequency range in which the installed subwoofer will perform optimally: if the volume is insufficient for a given speaker, then the pressure inside the enclosure will be higher than outside, and the lowest tones will be attenuated. With a further decrease in its volume, the loss of the low-frequency component will increase, and higher tones, on the contrary, will be emphasized, enhancing the “kick effect”, instead of dense and clear bass. Therefore, when space is limited, it is better to use a smaller subwoofer, such as an 8" subwoofer, rather than limiting the enclosure of a 10" or 15" speaker as much as possible. Increasing the volume of the cabinet above the manufacturer's recommended subwoofer can increase the output at the lowest frequencies, but the installer will face even more problems when installing it in a car. The graph above characterizes the recommended optimal cabinet sizes for subwoofers of various diameters. Phase inverted systems In search of more efficient acoustic design schemes for low-frequency speakers, craftsmen began using cases with phase inverters and various types of acoustic ducts decades ago. Interest in them has especially increased in recent years, with the development of hi-fi equipment due to the fact that they allow you to expand the range of reproducible frequencies. Some craftsmen find it worthwhile to spend the extra effort to go beyond a closed design and get a gain in low frequency response. In phase-inverted cases, cylindrical or rectangular tubes are used, usually tuned to a certain frequency. The acoustic design of a subwoofer using such enclosures requires great theoretical knowledge and experience from the installer, since in order to obtain a good transfer function, it is necessary to coordinate, for example, such factors as the mutual influence of the radiation resistances of the diffuser and the phase inverter pipe through a mutually oscillating mass of air. Errors, for example, when calculating the quality factor, as well as designing and setting up a phase inverter, are the reason that the speaker system "mumbles" or the bass is "smeared". And even if the phase inverter pipe is tuned to the required frequency, it can become a source of non-linear distortion if, for example, the volumetric air velocity in it exceeds the permissible one (it should not exceed 5% of the speed of sound): in this case, the air flow becomes turbulent. The sensitivity of the transfer function (sound pressure) of a phase inverted system to the frequency detuning of the phase inverter is very high, and after final assembly, it may be necessary to fine-tune. When evaluating the efficiency and sound quality of a particular subwoofer system, practitioners often use the analysis of the sound pressure level, for example, in one-third octave bands of its operating frequency range. Real-time analysis tools can be used to measure this level and other important characteristics of a finished subwoofer system (see "12 Volt Wizard" No. 3, 1997). The legitimacy of the subjective assessments of audio experts is also recognized. Heavy boxes for light bass
This is not to say that it is extremely difficult to make an enclosure or speaker for a car subwoofer. If you follow the instructions and recommendations of experts, then even with a limited set of hand tools, a capable craftsman is able to make a quality speaker system. As noted above, craftsmen most often use MDF-type particle board, which has the necessary mechanical and acoustic properties. Less commonly used multilayer plywood. Depending on the qualifications of the master in carpentry, sawing and fitting the surfaces of the box in the corners may be difficult for him. It is especially difficult to achieve a high-quality connection using only hand tools. "Popular" in the manufacture of such structures, the articulation of surfaces according to the "dovetail" principle is not suitable, since most particle boards, like multilayer plywood, do not have the necessary mechanical strength of the structure of the adhesive composition and particle filler. The figure shows some types of corner joints used in the manufacture of boxes for automotive subwoofer systems. Master's look Victor Polyakov, "Russian Game" company We have seen in practice that customers who value the sound quality of the car audio system prefer bass-reflex designs despite the additional costs. Moreover, subwoofer manufacturers make our task easier with recommendations for calculating cabinets, pipes, and crossover cutoff frequencies. We also made sure that the use of high-quality materials for acoustic enclosures is justified. We use Jamo fine particle board. For subwoofers, an XNUMX-inch slab is ideal, which is not only strong, well-machined, but also has good acoustic properties without the need for additional surface treatment. For interior decoration, special foam rubber is used, and for the overall design completeness, the cases are covered with carpet and special acoustically transparent materials. Our company now has a complete range of colors for such materials, and it is important that the customer is better aware of the available options and options for the acoustics in his car. Dmitry Gurinovich, "Remers-Center" company Many customers tend to save a hundred or two on materials for car acoustics, so we found that it was possible to successfully use 20 mm plywood and 15 mm chipboard to make subwoofer cabinets, especially since today you can use hot plastic, special Acoustic glue or Flex glue, which holds and seals very well. Vinyl coating gives a good appearance and tightness, which is important for the conditions of the car. We believe that such optimal sufficiency is the right approach. When we receive a very expensive order, solid wood comes into play. In general, the choice of materials directly depends on the financial situation of the client. Sergey Dudyrev, SV Art company When calculating the boxes, first of all, you need to keep in mind which speaker you want to enclose in the case. It is no secret that the manufacturer completes its products with a description of the technical characteristics, in which the parameters of, say, the same speakers should be indicated. It is clear that this greatly simplifies the work of the installer. Some firms that manufacture acoustics offer different configuration options for boxes in order to get different sound colors from the same speaker. This is a big plus when working with a client who has certain musical tastes. If, during the installation process, you have to deal with a speaker whose parameters are not known in advance or require more accurate rechecking, then these necessary data can be obtained by your own measurements - using a sound generator. It is believed that this method is more accurate, but it is more laborious and, accordingly, requires large material and time costs. In principle, for any speaker, it is possible to manufacture a box of any configuration. There are no hard limits - it all depends on the goals pursued by the installer. Let's take an ordinary closed box as an example. With its small volume, the damping of the speaker is very high, and it is accordingly able to withstand a large amount of power. However, when the volume is increased, a boom may appear. By increasing the volume of the box, we reduce the boominess, but at the same time, the limit of the input power also decreases. We usually design boxes where the speaker puts out decent power without humming. We manufacture boxes from high quality plywood. Moreover, in order to achieve the best quality, we make the walls of the box from two layers of 10 mm plywood connected with a special glue. Thus, the wall thickness is at least 20 mm; for a 10" speaker, the allowable minimum is 15mm. From the inside, the plywood walls must be pasted over with sound-absorbing material. The trapezoidal shape of the box was chosen by us not by chance. This configuration immediately eliminates a pair of standing wave directions. It is desirable that the walls of the box are not parallel, so the trapezoid suits us perfectly. In addition, this form of the box is more justified due to the configuration of the luggage compartment of the car.
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