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BOOKS AND ARTICLES
sound in the car
It is known that for many motorists the car has become something more than just a means of transportation. After all, a person who spends several hours behind the wheel every day wants to listen not only to local news but also to good music. However, when equipping the salon with a high-quality car sound-reproducing installation, few people manage to get by with only industrial products. In recent years, many publications have appeared in amateur radio literature devoted to the independent manufacture of automotive audio equipment, mainly amplifiers. But even a good amplifier does not solve all the problems facing the creator of a Hi-Fi class audio system for his car. The skepticism of many motorists towards such systems is caused, in my opinion, primarily by an illiterate approach to installing speakers in the cabin. Unfortunately, in most cases they are placed in the car according to the principle "where it fits", and not "where it is necessary". True, there is no unequivocal answer to the question - "where do you need it?" - does not exist. None of the known solutions gives a guaranteed result. It is much easier to answer the question "where not to?". The most common mistake is installing powerful and high quality speaker drivers on the rear shelf. In front, they install what is necessary or nothing at all. Does the owner of such a machine prefer to sit at a concert with his back to the stage? Contrary to popular belief, when designing a car audio system, the main thing is not to achieve high power, low distortion, or even good frequency response. The main problem is the wide sound picture for listeners sitting in the front seats of the car. Its decision is directly related to the installation location of the front heads. With any reasonable variant of their placement, the difference in the signal path from the left and right emitters to the listener reaches unacceptably large values. To reduce this difference, you can use the reflection of all or part of the signal from the windshield. This is how installations with floor front acoustics appeared (Fig. 1) [1].
The constructive implementation of such a solution is difficult and time-consuming, but the result is impressive. However, not everyone will decide to cut the floor to install the heads. Therefore, there really are not so many places left for installing front radiators in a passenger car: dashboard, windshield pillars, doors, vertical panels in the lower part of the cabin near the front wheels of the car (kick panels). The dashboard allows you to install the emitters at a sufficient height, but the size of the heads is usually limited to 10 ... 13 cm, there is practically no acoustic design, so low frequencies are not reproduced effectively in this case. To further raise the sound picture, you can install radiators on racks along the side edges of the windshield, but in reality only "tweeters" can be accommodated there. Popular due to its simplicity, the installation of low-frequency and coaxial heads in the doors of a car is usually argued by an increase in the efficiency of low-frequency reproduction by a ready-made acoustic design. The result is exactly the opposite of what you want. When mixing phonograms, most modern sound engineers place bass instruments in the center of the sound stage, that is, the sound signals of the left and right channels in this frequency range are in phase and have almost the same intensity. Therefore, when emitters are installed in the doors, the front of the sound wave at frequencies of 100 ... 150 Hz, which is critical for the subjective perception of a bass attack, reaches the opposite head in antiphase (which is determined by the width of the cabin) and is compensated [1]. Hence - a dull, lifeless sound that cannot be corrected by any equalizer. Installing heads in kickpanels reduces the difference in the signal path from the left and right emitters, but the sound picture falls unacceptably low. In addition, this variant has the same effect of "quenching" the bass attack, although to a lesser extent than when installing the heads in the car doors, and it is not easy to organize a decent acoustic design. From the foregoing, it follows that the best result can be achieved by using a multi-band dispersed front speaker. Strip radiators must be placed in those places where they will work with maximum efficiency. According to the author, the best place to install low-frequency radiators is in cases under the front seats with sound emission forward and upward. The midrange drivers are best placed on the front panel or in the upper front corner of the door, and the tweeters on the windshield pillars (this will raise the sound picture). Full reproduction of low frequencies is possible only when using acoustic design of a significant size, therefore, in almost all car audio installations, the frequency range of the main channels is limited to 100 ... 120 Hz, and lower frequencies are emitted by the subwoofer in the form of a total signal. Since the loudspeaker has a circular radiation pattern at the lowest frequencies, the choice of the location of the subwoofer is a matter of system layout. Most often it is placed in the trunk. The problem is that the frequency response of the interior, which plays the role of acoustic design here, has a rise in the lowest frequency region, which is individual for each body model. Therefore, in order to obtain a uniform overall frequency response of the system, it is necessary to be able to adjust not only the level, but also the frequency response in the subwoofer channel. The sound image formed without the use of rear radiators will certainly be incomplete. Their main purpose is to create a "hall effect" by simulating the reflected sound. The signal spectrum of the rear channels (Fig. 2) for this must be limited [2], and the signal itself can be either a traditional ("left-right" channel), or a sum or difference signal, or a combination of them. The power of the rear channel is small (no more than 10% of the total power of the system) and in some cases additional amplifying channels are not required for its implementation. The simplest case is the back-to-back connection of the rear speakers between the amplifier outputs of the left and right channels through a simple filter.
So much has been written recently about the influence of connecting cables on sound quality that this topic can not be touched upon here. The cross section of the power wires must correspond to the current consumed and the length of the wires themselves. In any case, the voltage drop on the power wire at maximum power should be no more than 0,1-0,2V. Damping of parasitic resonances of the elements and sound insulation of the cabin is also of great importance. The sound absorbing material should be applied to all accessible panels and plastic body parts, with particular attention to panels adjacent to the loudspeaker heads or those used as part of the loudspeaker enclosure. Resonances usually disappear when 25 percent or more of the area is covered. To absorb road noise, sound-absorbing material should cover the floor of the passenger compartment, the fire-resistant bulkhead and the wheel area. The next most important influence on the sound quality of a car audio system is AF amplifiers. However, traditional amplifying devices with frequency division at the output by passive filters do not work well in a car, as they have a number of disadvantages. In addition to power losses, passive filters do not allow you to adjust the cutoff frequency of the frequency response (and this is often required when setting up an audio system) and are very sensitive to changes in the impedance of the heads. For the above reasons, when creating a car audio system, it is more convenient to use multiband amplification, and to separate the frequencies with active or passive filters installed at the amplifier input. The advantages of this approach are the exclusion of power losses and the possibility of using optimal circuit design solutions for amplifiers and bandpass filters. The graph shown in Fig. 3 shows the dependence of the ratio of the power of the low-frequency channel (in percent) to the total power on the crossover frequency. For example, at a crossover frequency of 500 Hz, the power of the low-frequency channel is 60%, and that of the high-frequency channel is 40%. (with the same sensitivity of the heads).
I propose a description of the stereo audio system installed in the VAZ2107 car, which is made in accordance with the principles set forth. The system uses two-band amplifying devices operating on eight dynamic heads and two piezoceramic emitters. The Philips 410 radio tape recorder with minor design modifications was used as a signal source. The node for adjusting the volume and tone has been redone in accordance with those shown in Fig. 3 of the article [3]. As a result, the tone control, which provides a decrease in the frequency response at higher sound frequencies, has been replaced by a tone control at lower frequencies within + 10 ... -4 dB (Fig. 4). The reproducing head of the tape deck of the radio tape recorder was replaced with a domestic 3D24N, which has better technical characteristics.
The radio tape recorder is installed on top of the instrument panel in a special container, which has a number of advantages over placing it in a regular place. In this case, the device is in the driver's field of vision, without obstructing the view of the road and without diverting his attention to controlling the radio. Also excluded is the heating of the radio from the stove of the car. The speakers are formed by three groups of loudspeakers: front, floor and rear (Fig. 5).
The front speakers consist of full range and high frequency drivers and operate in the frequency band above 260 Hz. To stabilize the load of the amplifier and reduce intermodulation distortion at medium frequencies, the front speakers use current-stabilizing resistors R1, R2 [4]. After their introduction, objective and subjective assessments of sound quality have improved. Wideband dynamic heads BA2, BA7 with an additional diffuser abhvs PIONEER TS-G1010 are installed on the front panel in regular places. The electrical resistance of these heads is 4 ohms, the characteristic sensitivity is 90 dB / W / m, the reproducible audio frequency band is 45 ... 20000 Hz. The frequency response of the heads has a significant drop at frequencies below 150Hz and above 11kHz. When installing the heads in the passenger compartment, spacers (sleeves) of various heights were used to turn them up and away from the standard mounting plane. Along the perimeter of the head are acoustically insulated with foam rubber. Since the front speakers do not work at low frequencies, no additional acoustic design was required. Piezoceramic diffuser emitters W-05 (BA1, BA7) are fixed on the windshield pillars of the car, providing at a distance of 50 cm a sound pressure of at least 95 dB (at a voltage of 8 V). The range of frequencies reproduced by them is 2000 to 30 Hz, the resonant frequency is 000 Hz. To equalize the frequency response at the highest frequencies of the range and obtain a normal sound picture, high-frequency emitters are deployed towards the windshield and are phased accordingly. Since the frequency response of the radiators has a significant drop at frequencies below 9 kHz, they are connected directly to the amplifier, without crossover filters. Thus, the front speakers are two-way with a natural separation of the bands in the 10 kHz region. It should be noted that W-05 piezo emitters have an excellent impulse response. Floorstanding loudspeakers operate in the frequency band below 260Hz and above 6kHz. These homemade two-way speakers are installed under the front seats. They use domestically produced dynamic heads 20GD3 (BA4, BA8 low-frequency) and 2GD36 (BA3, BA9 high-frequency). The speaker cabinets have a volume of about 9 liters. They are made of 12 mm plywood sheets, fastened with bars. The inner walls of the hulls are lined with foam rubber. Dynamic heads are placed on the inclined front panel of the housings and radiate forward and upward. The loudspeakers are installed in such a way that the high-frequency radiators are closer to the doors. The design also includes built-in first-order crossover filters with a cutoff frequency of 5 kHz. Due to the high impedance and reduced sensitivity, the output of high-frequency drivers is small and they are intended mainly to create a diffuse sound field at higher frequencies. The rear loudspeakers operate in the frequency band 270...2500 Hz. They consist of two Chinese-made BA5, BA10 heads connected in anti-parallel connection, installed under the lining of the rear shelf of the car. The heads have an electrical resistance of 6 ohms and a characteristic sensitivity of about 84...86 dB/W/m. They are connected to the front channels in a similar way to the Huffler circuit. The bandpass filter is located on the left dynamic head. Domestic dynamic heads 3GDSH18 can be used as rear ones. In the described audio installation, a self-made two-channel two-band amplifier is used, which is structurally combined with crossover filters. In the front channel, a first-order high-frequency filter with a cutoff frequency of 260 Hz was used, in the floor channel, a first-order T-filter with cutoff frequencies of 260 Hz and 5 kHz. has the following technical characteristics (according to the microcircuit manufacturer and measurement results): sensitivity - 1554 V, input impedance - 0,5 ohms, nominal load resistance - 600 ohms, rated (maximum) output power 4x4 (15x4) W) with a coefficient of non-linear distortion 22 and 0,25%, respectively, the frequency response band is 10 ... 30 and 16000 ... 15 Hz with a frequency response unevenness of -25000 and -1 dB, respectively, supply voltage - 3 V, maximum current consumption - 14,4 A , quiescent current - 14 A, current consumption in standby mode - 0,3 A, readiness for operation when turned on - 0,001 s.
The diagram of the left channel of the amplifier is shown in Fig.6. The right channel is completely identical to it. Elements C1-C5 and R1-R5 form crossover filters. The amplifier is turned on when a control voltage of 12 V is supplied from the radio. When disconnected from the radio, the amplifier goes into standby mode. The main supply voltage is not switched, since the current consumed by the amplifier in standby mode is less than the self-discharge current of the car battery. R6C9 circuit provides turn-on delay. LC filters are used to filter noise in the power circuits of microcircuits. The high-capacity C10 capacitor in the power filter prevents voltage drop at power peaks and is installed directly in the amplifier case. The input signal is supplied via a shielded cable with a BNC bayonet connector. The power amplifier is assembled on a printed circuit board, the filters are surface-mounted. Microcircuits and the board are placed on a duralumin corner - a heat sink. Cooling of the amplifier microcircuits is forced by means of a fan from the computer's power supply. The amplifier is installed in the front of the cabin on a shelf under the glove box. If the amplifier will be connected directly instead of the dynamic heads of the radio, its signal level should be adjusted very carefully, starting from zero, so as not to overload the inputs of the microcircuit. In the event that the device with which the amplifier is supposed to be used has an output stage made according to the bridge circuit, oxide capacitors with a capacity of 10 μF must be connected between its output and the filter board, and their positive terminals must be connected to the input jacks. When mounting the amplifier, fixed resistors MLT-0,25 and variables SP3-12a can be used. Amplifier oxide capacitors K50-18 (C10) and K50-24 (C7-C9), the rest are any ceramic. The power filter coil L1 is wound on an annular magnetic circuit with dimensions of 20x10x8 mm made of 2000NN ferrite and contains 5 turns of a mounting wire with an inner core cross section (without insulation) of 1 ... 1,5 mm2. The speaker coil L1 is wound on a 2000NN ferrite rod with a diameter of 8 and a length of 20 mm and contains 15 turns of PEV-1 1,0 wire. Capacitors C1-C2 - KBG-MN, C3.C4 - K50-24, PEV resistors - 5 watts. Literature
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