What is a speaker crossover and its importance

What is a speaker crossover and its importance

About audio crossovers.

Crossovers are devices in sound systems that create the desired operating frequency ranges for speakers. Speakers are designed to operate within a specific frequency range. They do not accept frequencies outside these limits. If a low frequency is applied to a high-frequency speaker (tweeter), then the sound picture will deteriorate, and if the signal is also powerful, then the tweeter will “burn out”. Tweeters should only work with high frequencies, and woofers should receive only low-frequency range from the overall audio signal. The remaining middle band goes to midrange speakers (midwoofers). Therefore, the task of crossovers is to separate the audio signal into the desired (optimal) frequency bands for the respective types of speakers.

Simply put, a crossover is a pair of electrical filters. Let's say the crossover has a cutoff frequency of 1000 Hz. This means that one of its filters cuts all frequencies below 1000 Hz and only passes frequencies above 1000 Hz. Such a filter is called a high-pass filter. Another filter that passes frequencies below 1000 Hz is called low-pass, . Graphically, the operation of this crossover is shown in Figure 3. The intersection point of the two curves is the crossover cutoff frequency equal to 1000 Hz. Three-way crossovers also have a mid-range filter (band-pass), which passes only the middle frequency range (approximately from 600 Hz to 5000 Hz.) The figure shows the frequency response of a three-way crossover.

The order of sensitivity is the ratio of the output signal intensity (dB) of the crossover to the frequency of the input signal, assuming that the input signal intensity is constant. Typically, sensitivity (cutoff slope) is characterized as a dB/octave ratio. For many mathematical reasons, the sensitivity of crossovers is always a multiple of 6 decibels per octave (6 dB/octave). The first order crossover has a sensitivity of 6 dB/octave. A second order crossover has a sensitivity of 12 dB/octave, a third order crossover has a sensitivity of 18 dB/octave, and a fourth order crossover has a sensitivity of 24 dB/octave.

Consider a third-order low-pass filter with a cutoff frequency of 100 Hz. As mentioned above, this crossover will only pass frequencies below 100 Hz, and cut off frequencies above 100 Hz. Frequency cutting will occur as follows: all frequencies above 100 Hz will lose their intensity at the output of the filter by a multiple of 18 dB, depending on the octave they enter. That is, a frequency of 200 Hz (the first octave above the cutoff frequency) will lose its intensity by 18 dB, the intensity of a frequency of 400 Hz (the second octave) will drop by 36 dB, and the third octave (800 Hz) will weaken by 54 dB. And so on, all subsequent octaves will be attenuated by a multiple of 18 dB. A less sensitive first-order low-pass filter with a cutoff frequency of 100 Hz will do the same, only unnecessary octaves will be attenuated not by 18 dB, but by 6 dB.

As you can see, the filters that make up the crossovers cannot immediately cut off unnecessary frequencies, but do it gradually, with different sensitivities depending on their order.

First-order crossovers are the simplest passive crossover, which consists of one capacitor and one inductor. The capacitor acts as a high-pass filter to protect the tweeter from unwanted low and mid frequencies. The coil is used as a low-pass filter. The sensitivity of first-order crossovers is low - only 6 dB per octave. A positive feature of these crossovers is the absence of a phase shift between the tweeter and the other speaker.

Crossovers of the second order. They are also called Butterworth crossovers, after the creator of the mathematical model of these crossovers. Structurally, they consist of one capacitor and coil on the tweeter and one capacitor and coil on the woofer. They have a higher sensitivity, equal to 12 dB per octave, but they give a phase shift of 180 degrees, which means that the membranes of the tweeter and other driver are not synchronized. To fix this problem, you need to reverse the polarity of the wires on the tweeter.

Crossovers of the third order. In such crossovers, one coil and two capacitors are placed on the tweeter, while on the low-frequency speaker, the opposite is true. The sensitivity of such crossovers is 18 dB per octave, and they have good phase characteristics in any polarity. A negative feature of 3rd order crossovers is the unacceptability of using time delays to eliminate problems associated with speakers that do not radiate on the same vertical plane.

Crossovers of the fourth order. Fourth-order Butterworth crossovers have a high sensitivity of 24 dB per octave, which drastically reduces the mutual influence of the speakers in the frequency separation region. The phase shift is 360 degrees, which actually means there is no phase shift. However, the magnitude of the phase shift in this case is not constant and can lead to unstable operation of the crossover. These crossovers are practically not used in practice.

Linkwitz and Riley succeeded in optimizing the design of the fourth-order crossover. This crossover consists of two second-order Butterworth crossovers connected in series for the tweeter, and the same for the bass driver. They also have a sensitivity of 24 dB per octave, but the output level on each filter is 6 dB less than the output level of the crossover. The Linkwitz-Riely crossover has no phase shifts and allows time correction for drivers that do not operate in the same physical plane. These crossovers offer the best acoustic performance compared to other designs.

Designing Passive Crossovers

As mentioned above, a passive crossover consists of capacitors and inductors. In order to assemble a passive first-order crossover, it is necessary to have one capacitor and one inductor. The capacitor is installed in series on the tweeter (high-pass filter), and the coil is installed in series on the woofer (low-pass filter). Coil inductance ratings ((H - microhenries) and capacitances ((F - microfarads) are given in the table depending on the desired crossover frequency and speaker impedance.

1st order crossover (6 dB/octave)

For example, we will select the capacitance and inductance for a crossover with a cutoff frequency of 4000 Hz with a speaker impedance of 4 ohms. From the above table, we find that the capacitance of the first order capacitor should be 10 mF, and the inductance of the coil should be 0.2 mH.

To determine the nominal values ​​​​of the components for a second-order crossover (12 dB / octave), it is necessary to multiply the values ​​\u200b\u200bfrom the same table for the capacitor by a factor equal to 0.7, and the value for the inductor multiplied by a factor of 1.414. It must be remembered that a second-order crossover requires two capacitors and two inductors. Let's make a second-order crossover for a cutoff frequency of 4000 Hz. To determine the values ​​for both capacitors, multiply the value from the table 10 mF by a factor of 0.7 and get 7 mF. Further, the value of the inductance of 0.2 mH is multiplied by a factor of 1.414 and we get the value of the inductance for each coil of 0.28 mH. One of these capacitors is installed in series on the tweeter, and the second in parallel on the woofer. One coil in parallel to the tweeter and the other in series to the woofer.

Passive and active crossovers

The difference between these two types of crossovers is very simple. An active crossover requires an external power supply, while a passive crossover does not. Because of this, the active crossover takes place in the sound system before the amplifier, processing the audio signal from the preamplifier of the head unit (for example, a car radio). Further, after the active crossover, two or three power amplifiers are installed. In this case, one amplifier is not installed, since it makes no sense to reduce the signals separated by an active crossover into a single signal in the amplifier. Separated signals must be amplified separately. As you can see, active crossovers are used in expensive high-quality sound systems.

Passive crossovers process the already amplified signal and are installed in front of the speakers. The possibilities of passive crossovers are limited compared to active crossovers, but their correct application can give good results at minimal financial cost. Passive crossovers perform well when requiring a sensitivity order of less than 18 dB per octave. Above this limit, only active crossovers work well.

Passive crossovers are mainly used to process the signal of tweeters and mid-range speakers. For low-frequency speakers, these crossovers can be used, but the demand for the quality of capacitors and inductors increases sharply, which leads to their rise in price and increase in size. Passive crossovers do not tolerate overloads well. Peak signal intensities coming from the amplifier can change the cutoff frequency of the filters. In addition, an overloaded filter attenuates the audio signal (damping). Therefore, when choosing passive crossovers, pay attention to their ability to withstand the peak loads created by the amplifier.

Active (or electronic) crossovers are a set of active filters that can be controlled and easily changed the cutoff frequency of any channel. The order of sensitivity of active crossovers can be anywhere from 6 dB to 72 dB per octave (and higher). Mostly active crossovers for car audio systems have a sensitivity of 24 dB per octave. With this sensitivity, the frequency exchange between the speakers is practically excluded. The sound picture is very high quality. The only drawback of active crossovers is their high cost compared to passive ones.

Phase shift

Now let's talk about the phase shifts that can occur in sound systems that use crossovers. Phase shift is an inevitable phenomenon resulting from the design features of high-pass, low-pass and band-pass filters.

Phase is the timing of two signals. The phase is measured in degrees from 0 to 360. If two identical speakers emit sound waves in the opposite phase (180 degrees phase shift), then the sound is attenuated. The problem is fixed by reversing the polarity on one of the speakers.

When the speaker system consists of different speakers operating in different frequency ranges (tweeter and mid-woofer), the elimination of phase shift is not always solved by simply changing "+" to "-". The wavelength from the tweeter is shorter than from the midwoofer. Therefore, the front of the high-frequency wave can reach the listener later (or earlier) than the front of the medium-frequency (or low-frequency) wave. This time delay is a consequence of the phase shift. In this case, you can optimize the sound picture by physically aligning the two speakers relative to each other in a vertical plane until the sound picture is improved. For example, at a wave frequency of 1000 Hz, a time delay of one millisecond is eliminated by shifting the speakers relative to each other by 30 cm.

Active Crossover Setting

The most important thing in setting up a crossover is choosing the right cutoff frequency. If we have a three-band active crossover, then we are faced with the task of determining two cutoff points (frequencies). The first point determines the cutoff frequency for the subwoofer (low-pass) and the beginning of the midrange for the mid-range (high-pass). The second point determines the end frequency of the middle range (low-pass) and the starting frequency of the high-frequency range for the tweeter (high-pass). Most importantly, when setting the crossover cutoff frequencies, remember the frequency characteristics of the speaker and in no case load the speaker with frequencies that are not included in its operating range.

For example, if the subwoofer rattles a little or emits a hum (unpleasant resonance of the car body), then it is overloaded with undesirable mid frequencies (above 100 Hz). Move the cutoff frequency (low-pass) to 75 Hz and/or set the sensitivity to 18 dB or 24 dB per octave if possible. Recall that an increase in the order of the crossover sensitivity (dB/octave value) cuts unwanted frequencies better, preventing them from leaking through the filter. The order of sensitivity of the high-pass filters for the midwoofer can be left at 12 dB / octave (for "soft" mid-range speakers). This setting of the active crossover is called asymmetric.