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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Loudspeakers in the car. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers To create a good modern car audio system, installing ready-made "speakers" and attaching them to the radio is clearly not enough. Therefore, after talking about radio tape recorders, it is advisable to consider modern speaker designs used by motorists. In the second part of this article, a table of the main parameters of domestically produced dynamic heads will be given. In the next issues of the magazine, we will continue to describe the stages of selection, placement and installation of audio system components in a car. When choosing electrodynamic heads or loudspeakers, colloquially referred to as "speakers", for a car audio system, it must be remembered that there is no ideal in nature. Each brand will have its adherents, so finding out which of them are "worthy of all" is at least pointless. Preference should be given to those that perform their functions better. Do not forget that developers, improving some indicator or parameter, often compromise at the expense of others. Therefore, there are no and cannot be universal solutions that are equally applicable in all cases. Please also note that there is no single methodology for testing automotive speaker systems (ACs). In addition to a number of standardized methods, many manufacturers use their own, exaggerating their dignity and even resorting to outright lies when evaluating their own products. What is worth, for example, the fantastic power of hundreds of watts indicated on some modest-looking heads of dubious origin. Of all the known types of acoustic transducers in car audio systems, dynamic direct-radiation heads and piezoceramic midrange and tweeters have found mass application. The dynamic loudspeaker was invented and patented by the Americans Rice and Kellogg in 1925, and the most noticeable changes in its design are associated with the emergence of new materials for the manufacture of diffusers and magnetic systems. Despite its inherent shortcomings, it is quite universal, and all other types of emitters (tape, electrostatic, etc.) have a limited scope. Using them in a car is fraught with problems, but it can be of some interest when creating unique audio systems. In order to make it easier to navigate when choosing acoustic emitters, we recall their main parameters and accepted English designations used by most foreign manufacturers. Impedance, Ohm - the total electrical resistance of the loudspeaker head, most often normalized in absolute value at a frequency of 1 kHz and equal to 4 ohms, less often - 8 ohms. There are also heads with an impedance of 10 or 6 ohms (the latter figure is typical for the products of Japanese companies). At one time, car speakers with an impedance of 2 ohms were quite widespread (this made it possible to obtain significant power at a low supply voltage), but now they have become a rarity. Less common piezo emitters in the operating frequency band (above 5 kHz) have a fairly high capacitive impedance - tens to hundreds of ohms. This must be remembered when choosing an amplifier - some of them are unstable on a capacitive load. Characteristic Sensitivity Level (SPL) is the average sound pressure that the loudspeaker develops. It is measured at a distance of 1 m with an input power of 1 W (usually at a fixed frequency of 1 kHz, unless otherwise specified in the documentation for the head). The actual sensitivity of car drivers is about 90 dB/W 1/2 m, although some woofers and piezo horns have sensitivity higher than 100 dB/W 1/2 m. However, be aware that some manufacturers use a measurement with a fixed voltage of 2,8 B, which gives more impressive numbers for low-resistance heads. Since piezoelectric emitters have a fairly high impedance, a power of 1 W develops on them at very high voltages, often exceeding the maximum allowable, which is why their sensitivity is measured at a higher voltage level (usually from 5 to 12 V). The distance at which the sound pressure is measured can be even 0,5 m for some emitters. Therefore, advice: in order not to make a mistake in choosing, pay attention to the footnote, which indicates the conditions for measuring this parameter. Frequency response range, Hz, kHz, indicates the frequency boundaries in which the sound pressure deviations do not exceed certain limits. Sometimes a clear frequency response unevenness is indicated, in other cases it can be estimated from the schedule attached to the product. Often there is no additional information at all. Rated electric power (Nominal power handling), W - long-term input power. Denotes the amount of power that a loudspeaker can handle for an extended period of time without damage to the cone surround, overheating of the voice coil, or other annoyances. Peak electrical power (Peak power handling), W is the maximum input power that the loudspeaker can withstand for a short time without risk of damage. Harmonic distortion factor (Total Distortion), %, is indicated very rarely. Since this parameter is frequency dependent, values are given for several fixed frequencies or as a graph. There are several more parameters for midrange and bass heads that fully describe their electrical and mechanical characteristics when operating in piston mode (more on this below). These parameters were first introduced by A. Thiele and later by R. Small. In honor of the authors, they are called the Thiel-Small parameters. Their full list is quite large, but the minimum required set includes the following. Natural resonance frequency (Fs), Hz, loudspeaker heads in open space. At this point, its impedance is maximum. Equivalent volume (Vas), m3 . This is a closed volume of air excited by the head, having the flexibility equal to the flexibility of the moving system of the head. Total quality factor (Qts - dimensionless quantity) loudspeaker head at resonant frequency takes into account all losses. The following parameters are components of the full quality factor and are relatively rare in the documentation. Mechanical quality factor (Qms - dimensionless quantity) loudspeaker head at resonant frequency takes into account mechanical losses. Electrical quality factor (Qes - dimensionless quantity) loudspeaker head at resonant frequency takes into account electrical losses. The total quality factor of the head is less than 0,3 ... 0,35 is considered low, more than 0,5 ... 0,6 - high. Knowing the full quality factor and resonant frequency of the head, we can conclude that it needs acoustic design. If the Fs/Qts ratio is 50 or less, the head is designed to operate in a closed box. To work in a phase inverter, it is advisable to use heads in which this indicator is 90 or more. Car heads mounted in the doors or on the rear shelf work almost in a closed box. To work under these conditions, it is necessary to choose a head with a high total quality factor (not less than 0,5) and a resonant frequency of at least 45 Hz. One of the most important design characteristics of a dynamic head is the cone material, on which the sound quality depends to the greatest extent. The ideal head should have a completely rigid and massless diffuser mounted on a completely flexible suspension. All existing designs are far from this. As the frequency of the signal increases, starting from a frequency called the cutoff frequency of the piston action zone, the cone ceases to oscillate as a whole. The resulting interference of sound waves from different sections of the diffuser leads to the appearance of local peaks and dips in the frequency response, coloring the sound. Deformations of a real diffuser caused by insufficient rigidity lead to the appearance of natural oscillations in the diffuser material. They must be effectively suppressed, otherwise the appearance of intermodulation distortions (overtones) and "blurring" of the impulse signal attack is inevitable. The non-linearity of the suspension also causes intermodulation distortion. Thus, the diffuser material must combine a low specific mass with high rigidity and high attenuation. The search for a compromise with such conflicting requirements forces designers to use new materials that successfully coexist with the old ones. At the same time, the solution of one problem often leads to the emergence of new ones. Paradoxical as it may seem, but paper diffusers so far most successfully combine all the necessary characteristics. Paper diffusers have been used in heads since their "birth". Initially, they were glued, at present they are made mainly by casting and pressing methods with impregnation with synthetic compounds. Pressed conical diffusers are cheap and technologically advanced, but they have a number of disadvantages (mainly low rigidity) and are used only in inexpensive designs. Diffusers of higher quality are made by casting. Liquid paper pulp is applied to a matrix, usually from a metal mesh, and, upon hardening, forms a diffuser preform. With this technology, due to the use of a curvilinear generatrix and a variable diffuser thickness, decreasing from the center to the edges, it is possible to partially solve the problem of rigidity. Paper diffusers can be used in almost all types of heads. The advantages of such diffusers are excellent internal damping, the almost complete absence of local resonances, and a smooth transition from piston to zone operation. Smooth frequency response allows you not to worry about the behavior of the head outside the operating frequency band, which makes it possible to use the simplest crossover filters with low slope and minimal phase distortion. The subjective assessment of the sound quality is high. The main disadvantage of paper cones is their relatively low rigidity, which can affect the elaboration of fine details of the sound. The mechanical strength is low and this limits the maximum power input. The technological dispersion of the parameters of the heads of mass series is relatively large, which, with high requirements for sound quality, may require their preliminary selection. Parameters change over time and under the influence of the atmosphere, despite the impregnation of paper pulp and protective coatings. The latter circumstance limits the use of heads with paper cones in car audio systems without taking special measures. Unfortunately, this hinders the use of high-quality heads intended for "home" audio systems in the car. Polypropylene was first used as a cone material in the development of monitors for the BBC sound studios in 1975 and is now widely used in heads for a wide variety of applications. Due to the rather large internal damping, a properly designed polypropylene cone can provide a flat and smooth frequency response at high SPLs. To increase the rigidity, mineral additives are used - quartz, mica, magnesium silicate. The advantages of heads with polypropylene cones are very smooth frequency response, neutral sound, good impulse response, smooth transition to zone mode, weather resistance. The best samples of polypropylene diffusers are not inferior to paper ones in terms of sound transparency, but due to limited rigidity they lose in terms of "detail" of the sound image. The main area of application is broadband and low-frequency heads. Carbon fiber fabric composites have a unique combination of low specific gravity with very high stiffness. However, due to insufficient internal damping and the complex anisotropic structure of the material, the transition to the zone regime is accompanied by numerous peaks and dips in the frequency response near the upper edge of the operating range. To successfully suppress unwanted overtones, crossover filters with a large slope of the decay are required, sometimes the use of selective corrective chains or special correctors is required. This greatly complicates the design of the system and creates problems with phase distortion. The main area of application is subwoofers. Kevlar is known, in particular, as a material for bulletproof vests. The first Kevlar heads were produced in the mid-80s by the French company Focal and the German Eton. The rigidity of Kevlar cones is unusually high, so the problems that are typical for cones of high rigidity manifest themselves in full force. At frequencies of 3 ... 4 kHz and above, a characteristic "Kevlar" sound appears - a jagged frequency response, a consequence of a sharp transition of a super-rigid cone to zone mode. By ear, this is perceived as a hard, aggressive sound, clearly dissonant with the sound of the same head in the lower part of the mid-frequency range. The designers of such systems are forced to install rather complex fourth-order crossover filters (24 dB/oct.), supplemented by a corrective chain tuned to the "Kevlar" resonance frequency - usually in the range of 5...7 kHz. The effect of "Kevlar" sound is a consequence of the combination of high rigidity with low internal losses. To improve damping, Eton developed a three-layer material consisting of two layers of Kevlar composite and a rigid "honeycomb" layer glued between them. A similar material is used by Focal under the name Aerogel. Other manufacturers use a damping rubber coating on the underside of the cone or a wide suspension collar to suppress unwanted resonances. The main scope - low-frequency heads and subwoofers. Attempts to use metal diffusers cannot be considered successful, since their considerable mass reduces the sensitivity of the heads to 84...87 dB. The absence of internal damping leads to the appearance of pronounced peaks at frequencies of 5...10 kHz. The piercing hoarse sound of horn "bells" installed in parks or squares is a music lover's nightmare. Metal diffusers are used only in certain models of subwoofers and dome tweeters. Rigid three-dimensional structures with a flat radiating surface and an internal filler in the form of honeycombs or foamed polymer have been known since the early 70s. They were often given a rectangular or polyhedral shape with rounded corners. Low-frequency dynamic heads with flat radiators were used in one of the variants of the S-90 speakers. The high mass of the diffuser in this case also greatly reduces the sensitivity of the head, and the bending vibrations of conventional diffusers in the zone range of radiation give way to volumetric vibrations and transverse buildup of a heavy diffuser. Damping the latter is very difficult. Tweeters with soft domes made of silk or synthetic materials have now practically supplanted diffuser HF radiators. The design feature of dome heads is that the entire radiating surface is inside the voice coil, and not outside, as with cone heads. The advantage of soft domes - excellent internal damping creates the prerequisites for obtaining a smooth frequency response with a smooth decay at the upper end of the operating range and a good transient response. Their disadvantage is the limited overload capacity, which imposes increased requirements on the frequency and/or slope of the crossover filter (crossover). A high dome profile (for reasons of rigidity) worsens the radiation pattern compared to flatter metal domes and often requires designers to use divergent acoustic lenses, a potential source of diffractive frequency response distortion. With the advent of dome tweeters, attempts were made to implement the hard dome concept. After experimenting with polymers, the designers settled on metal. Ultra-thin domes made of titanium and aluminum began to be introduced in the mid-80s; for their manufacture, methods of precision electrolysis and vacuum deposition were used. As befits heads with rigid diffusers, "tweeters" with metal domes have a characteristic frequency response peak at frequencies of 25...30 kHz up to 3...12 dB. Under certain conditions, conditions may arise for these components to intermodulate with others in the audio range. By ear, this can be perceived as a "metallic" timbre of sound. It should be noted that the sound of the best examples of metal domes is transparent, clear, approaching the sound of electrostatic emitters. The advantage of a hard dome is that it operates without deformation over the entire operating frequency range, providing high detail and transparency of sound. The directivity characteristic due to the low profile of such a dome is much better than that of soft domes, however, the characteristic ultrasonic peak in the frequency response can lead to unpleasant coloration in the sound. Unfortunately, the range of existing high-frequency emitters with ceramic diffusers is insufficient. Compact automotive ceramic "tweeters" were the first to be released by Infinity. In fact, they are ceramic-metal: an even thinner (5...10 microns) layer of pure oxide ceramics, which has exceptional hardness, is deposited on a thin metal base. The stiffness of the dome due to the small thickness of the coating increases slightly, but the absence of "metallic" overtones contributes to the most accurate sound reproduction of high frequencies. Car heads have several standard sizes based on the inch system: 7,5 cm (3"), 8,7 cm (3,5"), 10 cm (4"), 13 cm (5"), 16 cm (6 "), 20 cm (8"), 25 cm (10"), 30 cm (12"). In addition to round heads, elliptical 4x6, 5x7 and especially 6x9 inches are widespread (they are also called "burdocks"). This design does not have any special advantages, except for layout. Most manufacturers include the head size in inches or centimeters in the model designation, which somewhat facilitates their "correspondence" choice. The delivery set includes protective nets for the head and fasteners. Heads designed to replace the factory ones in the regular places of the car are supplied without nets ("custom fit"). Loudspeakers used in automobiles can be divided into several groups according to their functions and design features. Broadband loudspeakers are built on the basis of electrodynamic heads with a single cone or with an additional cone cone glued to a common voice coil. In addition, broadband loudspeakers use heads with coaxial radiators or additional high-frequency radiators mounted on a common diffuser holder. In more expensive car audio systems, component (separate) loudspeakers are used: low-frequency, mid-frequency, and sometimes combined in two bands - LF-MF, high-frequency "tweeters". In the most broadband systems, subwoofers (subwoofers) are also used. The acoustic design of the heads involves their embedding in the elements of the car body or their implementation in separate cases. Now more specifically about the features of the loudspeakers in different audio frequency bands. Due to the transition of the diffuser from the piston mode of operation to the zone mode, the radiation pattern of conventional wideband heads narrows with increasing frequency, and the return decreases. To compensate for this phenomenon, an additional conical diffuser with a smaller opening angle is introduced into the design. The effect of its introduction is most noticeable in heads with a large diffuser. The material of the additional diffuser is paper or aluminum foil. The main diffuser of broadband heads is usually made of paper or polypropylene. Most automotive broadband heads are represented by models with round diffusers with a diameter of 7,5 ... 10 cm, there are also heads with elliptical diffusers. The reproducible frequency band of simple wideband heads is really limited from above by values of 8...12 kHz, heads with an additional cone - 12...16 kHz. The lower limit of reproducible frequencies, depending on the size of the head, varies from 100 ... 120 Hz for small-sized ones to 40 ... 60 for the most low-frequency ones. To reduce various distortions, additional MF-HF emitters (up to four) are introduced into automotive broadband heads. Both manufacturers and sellers completely incorrectly call such heads multi-band. In reality, the frequency band of the main radiator is not limited by anything, and additional radiators are connected through the simplest first-order filters (often these are oxide capacitors). To avoid overloading additional radiators with a powerful signal, the cutoff frequency of such a "filter" is relatively high (6...10 kHz). The bulk of heads of this type are represented by models with a round diffuser (diameter 10...16 cm) or elliptical (approximately 15x23 cm). The frequency band reproduced by the loudspeakers of this group is extended to 18...25 kHz. The lower limit of the reproducible frequency band is the same as for similar heads with a single cone. As additional midrange emitters, small-sized dynamic heads and diffuser piezoelectric emitters are used. High-frequency emitters are usually made on the basis of small-sized dome dynamic heads or piezoceramic plates (in inexpensive models). Since the additional radiator is installed inside the diffuser of the main head near its axis or coaxially with it, heads of this type are called "coaxial". Structurally, these radiators are mounted on a "bridge" mounted on a diffuser holder, or on a stand attached to the core of the magnetic system. All automotive broadband heads require a fairly large volume behind the cone to function properly. If this condition is violated, the frequency response unevenness in the low-frequency region sharply increases. The loudspeakers of this group are applicable as the main ones only in entry-level car audio systems. In high-quality systems, broadband heads are used as rear ones with the bandwidth of the frequencies supplied to them limited to 400 ... 2500 Hz. It is also possible to use simple broadband heads as mid-frequency radiators in three-way systems. In high-level audio systems, several heads are used to separately reproduce low, medium and high frequencies. This allows you to place them in the most suitable places in the car interior for the best transmission of the sound picture. A separate crossover provides optimum crossover frequency selection in multi-band systems. Note that head kits are also sold as a ready-made kit containing components for separation filters. These kits are designed for mid-level audio systems. However, the quality of the crossover elements can be very different. Oxide capacitors and coils with a magnetic circuit are now not uncommon even in expensive kits, but in equipment of the highest level only high-quality crossover filters are used or two- or three-band amplification is used. Low-frequency and MF-LF heads, as a rule, have a diameter of 13 ... 20 cm and, like broadband ones, are also designed to work in a relatively large volume housing. It is difficult to draw a clear line between them: it all depends on whether the heads should work in a two- or three-way speaker. Some of them work well in closed cases and phase inverters. The diffuser material can be very different - from paper to Kevlar, so the upper limit of the reproducible frequency band is very individual for each model - from 2 ... 3 to 5 ... 8 kHz. The lower limit of the best models actually drops to 30...40 Hz, which allows, with a certain amount of ingenuity, to create a high fidelity car audio system without a separate subwoofer. Low-frequency subwoofer heads have a diameter of more than 16 cm and require a special acoustic design for normal operation (for example, a closed case, a phase inverter), if you make it yourself, you must either trust the manufacturer's recommendations, or choose the design and calculate it yourself [1]. To do this, you can also use the calculation programs provided by large manufacturers on the Internet [2-4]. The Thiel-Small parameters necessary for this are often available in the accompanying documentation for the heads. As a rule, a subwoofer reproduces the frequency band below 80-90 Hz in an automotive installation, although other frequency distributions are known. Subwoofer designs are not covered here. As high-frequency emitters in car audio systems, heads with soft textile or hard metal domes are used. According to the subjective assessment, the sound of these emitters differs significantly, and both types of heads have their adherents. As the saying goes, "the taste and color ...". The diameter of the dome emitters of the "tweeters" varies markedly - from 15 to 50 mm. Most manufacturers provide the ability to orient the heads using special mounting parts included in the kit. There are some features in the design of high-frequency emitters installed in car audio systems. Due to their small size, they can be placed almost anywhere, making them ideal for setting up your soundstage. To increase the efficiency of this method, the cutoff frequency of the high-pass filter is sometimes lowered to 1,5 ... 2 kHz, while the power supplied to the emitters increases to 30 ... 40% of the total system power. In such cases, the filling of the magnetic gap with a ferromagnetic "liquid" protects against overheating of the coil. Overloading of the heads is eliminated with a more sophisticated crossover filter and barretter-based current limiter. In amateur conditions, for this purpose, an incandescent lamp for a voltage of 6 ... 12 V is used, turning it on in series with the head. Horn radiators for midrange and high frequencies in car audio systems are exotic, but interest in them is gradually increasing. The sensitivity of the horn heads can reach 97...105 dB/W1/2m, which makes it possible to reduce the power of the amplifier. The horn is a special type of acoustic design and can be made independently [5]. At the turn of the 90s, off-the-shelf high-quality multi-band speakers were widely used in cars, but by now they have practically disappeared from the scene, giving way to coaxial and component loudspeakers. The so-called "car speakers" now commercially available - thin-walled plastic boxes with tiny heads - are nothing more than a toy. Mass models of automotive dynamic heads offered by Kenwood, Pioneer, Sony, Clarion, Panasonic, Philips, Prology, Pyramid are now widely represented on sale. Higher-end models are produced by Focal, Infinity, Kicker, Precision Power, Rockford Fosgate, MTX, Phoenix Gold, Jensen and others. The high cost of these products forces them to pay attention to domestic heads. Dynamic heads of domestic production for automotive speakers appeared relatively recently, and if it is impossible to purchase such, radio amateurs will have to focus on heads for general use. At the end of this article - a list of domestically produced dynamic heads that are quite suitable for use in automotive speakers. Since hams may have obsolete types of dynamic heads, they are also included in the table here.
Information about the parameters was taken by the author from many sources, in particular [1, 5]. However, they were not always exhaustive, only this explains the "blank spots" in the table. Unfortunately, for domestic dynamic heads, the Thiel-Small parameters are not given, so some of the parameters were obtained empirically. Alternative values (in cases of discrepancies in various sources) are indicated in brackets. The author would like to thank everyone who assisted in compiling the table. Literature
Author: A. Shikhatov, Moscow
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