Michurintsy CAR AUDIO. The art of audio

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Michurintsy CAR AUDIO, or What happens if you cross a crossover with an equalizer

The art of audio

For installations with three-way frontal acoustics, the most rational structure is with separate amplification of the LF and MF/HF bands. The separation of these bands is performed by an active crossover in front of the amplifier, and the separation of the midrange and HF bands is performed by a passive one. To eliminate frequency response defects, it is possible to tune the crossover frequency within certain limits, but the possibilities of this method are limited and an equalizer is always present in a high-level setting.

Correction using an equalizer is subject to sections with dips of no more than 6 ... 8 dB, and peaks up to 10 dB, a deeper correction can be noticeable by ear and indicates, first of all, serious miscalculations in the design of the system. Typically, peak suppression is less audible than dip pull-up, which also requires a power margin (every 3 dB corresponds to a doubling of the signal power in the correction band). The main criterion for setting up the system is to obtain not the most flat, but the most smooth frequency response.

What degree of frequency response unevenness can be considered acceptable? Dips and peaks in the region of upper-middle and high frequencies with a width of no more than 0,5 octaves and a value of up to 4 ... 5 dB are hardly noticeable by ear, a large unevenness is perceived as a change in timbre color. In most cases, "detailed" correction in this range is not required, usually you can get by with integral correction using the treble tone control. The permissible local unevenness of the frequency response in the region of the lower-middle and lower frequencies is less - 2 ... 3 dB, but the dips in the frequency response in this region are less noticeable by ear than the peaks. The unevenness of the frequency response in this area is perceived by ear as a different loudness of the sound of individual notes in the passages.

The average frequency response for the interior of a medium-sized car is shown in the figure. The graph is based on the frequency response of the interiors of cars VAZ2106, VAZ2108, VAZ21099, Skoda-Felicia, Ford-Sierra and Renault (Master 12 Volt, May 2000). The most significant deviations are in bands about an octave wide with average frequencies of 150 and 350 Hz.

Michurintsy CAR AUDIO, or What happens if you cross a crossover with an equalizer

Thus, if we confine ourselves to eliminating only the most noticeable specific frequency response errors that occur in the passenger compartment, the number of control bands can be reduced. Usually parametric equalizers are used in this case. This allows you to adjust only in the required frequency bands, without affecting the "sinless" areas, which will reduce signal distortion. From the point of view of minimal interference in the signal, an equalizer of this type is unrivaled, but it requires control of the frequency response during the tuning process. The structure of the system is as follows:

Michurintsy CAR AUDIO, or What happens if you cross a crossover with an equalizer

The disadvantage of this structure is that the equalizer adjustments also affect those frequency bands where adjustment is not required. For example, at a crossover frequency of 400 Hz, raising the gain at 300 Hz also affects the midrange, effectively changing its cutoff frequency. To eliminate this shortcoming, you can install equalizers after the crossover, which is sometimes practiced in expensive installations. However, part of the regulation bands remains "out of work".

It would be much more rational to divide the equalizer into two parts and structurally combine it with an active crossover, leaving only the necessary control bands. In addition, to prevent midbass heads from being overloaded by the lowest frequencies, it is desirable to limit them. With this in mind, the structure of a three-band system based on a combined crossover-equalizer takes the following form:

Michurintsy CAR AUDIO, or What happens if you cross a crossover with an equalizer

Link for midbass with a tunable band. The lower limit is formed by a third-order filter with a switchable cutoff frequency - 45/80 Hz. The first value is used to work with "full-sized" woofers, the second - for small ones. This will prevent the midbass from being overloaded with low frequencies. The upper limit of the cutoff frequency is formed by a first-order filter in the range of 250 Hz...6 kHz. For ease of adjustment, you can enter two tuning limits (0,25 .... 1,5 kHz and 1 ... 6 kHz). For adjustments in the "problem" area, two equalizer sections with average frequencies of 150 and 300 Hz are required.

A link for a midrange or a tweeter with a lower limit, tunable in the range of 300 Hz ... 7 kHz. In terms of the combination of impulse responses and deadband suppression, a second-order filter is optimal. Smooth tuning in this range is not necessary, you can use a set of fixed cutoff frequencies, selected by a switch or resistor matrices. To adjust the frequency response in this zone, it is enough to use one or two relatively broadband links.

Subwoofer link (not shown in the diagram) with an adder and a regulator of the upper cutoff frequency of 50 ... 80 Hz. To be able to work with other types of acoustic design, except for a closed one, you need a subsonic filter at a frequency of 25 Hz, it can be made non-switchable.

The proposed structure has great flexibility and can be used in both three-way and two-way systems with separate amplification. For the formation of bands, in addition to filters of the second or third order, variable slope filters are promising, which have the first order for forming the working frequency band, and a higher one beyond its limits. These filters are more musical than traditional ones. This is due to the fact that phase distortion does not occur in the inflection zone of the frequency response, and the impulse response is also much better.

Such a structure can be obtained by sequential inclusion of two filters. With regard to midrange heads, this can be a second-order filter at 200 Hz for reliable resonance suppression, and in the operating frequency band (above 300 Hz) a tunable first-order filter.

In fairness, it should be noted that elements of a similar structure are found in some amplifiers (for example, Lanzar 5.200), but one equalizer link is clearly not enough. The proliferation of universal crossover equalizers designed to work with any amplifier could solve a number of problems in building a car audio system using a minimum number of components.

Publication: www.bluesmobil.com/shikhman

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