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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Equalizer for car audio system. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers A car audio system is a very complex and controversial structure. Like it or not, but the closed space of the vehicle interior with the resonances that arise every now and then is not at all suitable for sound reproduction. The sound is affected by the shape of the body, the materials used to decorate the interior, the positions reserved by the manufacturers for acoustics, and much more in detail. Of course, no one claims that the creation of a solid mobile audio complex is an absolutely hopeless undertaking. You can experiment with crossover frequencies, find an acceptable location for the speakers, set the desired levels on the amplifier. But even these efforts are often unable to compensate for the conditions for the existence of components, prepared by the automakers. This is where the equalizer comes in.
Although there are opinions on this. Many high-profile installers are reluctant to add an additional electronic device to the signal path, as they follow the purist "less is more" philosophy that is fairly common in car audio. And in this they are, in general, right: the fewer components, the less reason for noise and distortion to penetrate the system. And feeling that they are right, they are ready to make great sacrifices, designing new optimal sockets for acoustics for days. In this righteous, in essence, impulse, they carry the client with them: he still has to pay for the installation delights, and the acoustics must be of the appropriate level, not 20 "conditional" per set. On the other hand, it happens that the client does not want to wait until the installer's creative torments are over. He needs three days, within the budget, and to play "as he likes, but at the level." Here, without the introduction of an equalizer, it is rarely when anyone does. This corrective device can be considered from two positions. On the one hand, it is a means of correcting the amplitude-frequency characteristic with little blood. That is, even with non-ideal orientation of acoustics and in an absolutely bad, from a musical point of view, salon, an equalizer (EQ) can often provide quite a decent result. In other words, to bring the sound in the car closer, if not to the ideal, - something at least to, so to speak, the individual requests of the customer. In addition, solid professional devices are often used to correct the frequency response in preparation for competitions. Equalizers themselves, as you know, are divided into two types - graphic and parametric. At their core, both varieties are signal processors. The function of the device is to receive a signal from the head unit, correct it and send it to the amplifier. Both types of EQ differ mainly in the number of frequency spectrum bands to be corrected (typically 20 Hz to 20 kHz), of which a device can have as little as one and as many as 30 or more. Each such band in graphic and parametric equalizers is separated from the frequency spectrum by high-pass and low-pass filters, after which “corrective work” begins with it: signal level adjustment. Thus, the corrective device itself is a kind of set of bandpass crossovers (by the way, many equalizers simultaneously perform the function of electronic crossovers). Graphic equalizers So they were named for clarity. The controls of many graphic equalizers are made in the form of sliders, and the graph of the desired frequency response curve can be observed directly on the front or top panel. Therefore, it is usually not too difficult to use them, especially those that are used for operational adjustments. Graphic EQs have fixed, adjustable bands and the center frequency does not change. These devices differ in the width of the bands: one, half and one third of an octave. There are also those in which part of the bands is distributed in breadth by one octave, and the rest - by half an octave or one third. It is clear that the more bands, the more precise the adjustments. Frequencies above and below the center frequency of an adjustable band determine its width or quality factor. At the same time, the greater the signal level set in the adjustable band, the narrower it is, the more "rocky" the frequency response will be. Conversely, a lower level affects a wider range of the frequency band, resulting in a flatter frequency response. There are, however, devices whose bandwidth (quality factor) does not depend on the rise in the frequency response (the so-called "Constant Q"). When setting the graphic equalizer, the "problem" frequency must match (or be close to) the center frequency of the band so that proper equalization can be performed. Parametric equalizers Again, the name of this EQ type speaks for itself. In parametric equalizers, you can adjust three parameters: the center frequency, the width of the adjustable band and, of course, the gain. What makes these devices an order of magnitude more flexible in comparison with the graphic devices discussed above, although they have an order of magnitude fewer adjustable bands, and they rarely cover the entire frequency range. But in the "coverage zone" of the parametric equalizer, you can achieve impressive results in the fight against resonances. The user himself, however, within certain limits, can set the center frequency. The bandwidth in the parameters depends on the quality factor (Q) and is set depending on the size of the problem area of the frequency spectrum. The higher the quality factor, the narrower the band, and vice versa. Setting Still, as we have already said, one should distinguish between equalizers for operational adjustments and those used for one-time adjustments. The first one is clear. Recently, a lot of manufacturers have adapted to stamping semi-din devices for installations directly into the dashboard or somewhere else nearby. This is actually convenient: the equalizer is right at your fingertips, and the number of bands (5-10) is enough for the listener to adjust the timbre at his own discretion at any time by turning (moving) a couple of adjustments. Simply put, everything is done by ear, without any measuring equipment. The device for one-time frequency response settings is another matter. Not entirely, of course, disposable (if desired, everything can be replayed), but, as a rule, the parameters are set for a long time. This process is quite laborious, and one cannot do without the help of special equipment. The minimum required is a real-time spectrum analyzer (RTA) with a source of pink noise (a test signal with an extremely uniform distribution of energy over octave bands. It sounds like static in a radio receiver). You will also need a fair amount of patience, since the presence of a measuring complex does not guarantee the result. For example, when playing pink noise, we will place the microphone in the area where the listener's head would normally be and plot the frequency response. Then move the microphone twenty centimeters to the left. And what do we notice? And the fact that in the closed space of the cabin, the frequency response curve has changed quite a lot compared to the original one. This is explained by the fact that the direct and reflected sound in the cabin is summed up, and the amplitude-frequency response is very sensitive to any displacement of the microphone. Therefore, it is impossible to equalize the frequency with an equalizer only based on the results of one measurement taken from one point. How is it possible?
One way is to calculate the so-called "spatial average" from the frequency response graphs obtained from six to eight positions in the cabin. That is, the average curve taken from 6-8 "strategically important" areas of the cabin. But, firstly, this is a rather dreary task, and secondly, one should take into account the fact that the human ear has the ability to separate direct sound waves from those reflected from the skin and glass. A real-time analyzer mixes everything together, while the optimal, from the point of view of subjective perception, is a flat curve, located somewhere in the middle of the frequency response of the signal that affects our ears directly from the speakers and the frequency response of the reflected sound. You can try to find a middle ground using the method published by the famous American car audio specialist Mark Rumreich. Moreover, he warns that this method of correcting the frequency response is not addressed to participants in autosound competitions "to the flattest curve", but is intended, let's say, to achieve optimal sound reproduction and, accordingly, sound perception (that a perfectly even frequency response and optimal sound are two big differences , we already wrote in the last issue). So, Mark recommends to start by setting the bass and treble controls on the head unit to zero, the equalizer controls to the center positions, and the fader and volume control to the "normal listening position". The pink noise from the RTA outputs is then fed to the EQ inputs. When adjusting the mid and mid-bass frequencies, the balance control should be turned to the extreme left position so that only the left speakers can be heard. The microphone in this case is located directly in front of the left speaker at a distance of 20-30 centimeters and is directed strictly to its center. In this case, the microphone picks up only direct sound waves and practically does not "hear" the reflected ones. After that, you can move (turn) the equalizer controls (the ones that are responsible for the bands from 150 Hz to 1,5 kHz) in search of a flat frequency response curve. In this case, it is recommended not to deviate too much from the central (usually fixed) position of the engines or "twists". When working with bass, the microphone is placed in the place where the head of the "main" listener - the driver - is usually located. The microphone must point upwards. Bands are adjustable from 45 to 150 Hz. The rest, below 45 Hz, should not be touched, but left in a fixed (central) position. Few speakers can reproduce these ultra-bass frequencies, so boosting these bands will only overdrive the amp and introduce distortion into the deep bass passages. At frequencies between 40 and 100 Hz, the level should be raised by about 5 decibels to overcome the effect of road noise masking (desensitization of the hearing organs in response to road noise), which especially affects the perception of lower bass. It remains to correct the medium-high and high-frequency components - bands from 1,5 kHz and above. The microphone is again installed in the area of \uXNUMXb\uXNUMXbthe headrest of the driver's seat, being directed towards the left front speakers. After correcting the frequency response, the microphone shifts to 3-0-35 centimeters to the right, adjustments are made to obtain an average frequency response curve. To be safe, you can try a few more microphone positions: high frequencies, as you know, usually require fine tuning. But measurements are measurements, and you still have to listen with your ears. In other words, whatever the objective reality is, a little bit of subjectivity won't hurt here. Therefore, the final chord of setting up the equalizer requires the involvement of auditory organs and test disks in the process. You can even - not test, but those that the client listens to, but it is very desirable that the musical material covers the entire frequency spectrum. The preference for normal, live instruments (piano, saxophone, drums, etc.) over synthesizers is also obvious. Particular attention here should be paid to "suspicious" settings. For example, if at frequencies above 150 Hz the signal gain in adjacent bands exceeds 6 dB, try lowering the level by 3 dB and listen to the test song again. If the second option sounds more convincing, then you can safely trust your perception of musical reality. Author: G. Samoilov, 12 Volt; Publication: 12voltsmagazine.com
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