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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Sound in the car: theory and practice. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers 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 driving for several hours a 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, first of all, 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?". The most common mistake is placing powerful and high quality speaker drivers on the rear shelf. In front, they establish that it will be necessary or nothing at all. Does the owner of such a car prefer to sit 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 creating a 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 in the cabin, 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 (see Fig. 1). The constructive implementation of such a solution is rather complicated and time-consuming, but the result is impressive. However, not everyone will decide to cut the floor to install the heads. Therefore, in reality, there 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. 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 emitters on racks along the side edges of the windshield, but in reality only tweeters (tweeters) can fit 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 the bass register instruments in the center of the sound stage, i.e. the audio 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 sound wave front 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. 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. Besides, this variant has the same effect of "quenching" the bass attack, though to a lesser extent, as 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 for installing 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 raises 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 meaning is "creating the effect of the hall" by simulating the reflected sound. For this, the signal spectrum of the rear channels (Fig. 2) must be limited [2], and the signal itself can be either a traditional (“left-right” channel), or a sum or difference, or a combination of them. The rear channel power is not a fork (no more than 10% of the total system power), and in some cases additional amplifying devices are not required to implement it. The simplest case is the back-to-back switching of the rear heads between the outputs of the amplifiers 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 ... 0,2 V. Damping of parasitic resonances of the elements and sound insulation of the cabin are also of great importance. The sound deadening material should be applied to all accessible panels and plastic bodywork, 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, cover half the cabin, the fireproof bulkhead, and the wheel area with noise-reducing material. Author: tolik777 (aka Viper); Publication: cxem.net
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