Crossover Networks: How do they work?

I took one of my speakers apart and I saw the board with the crossover network on it. There was a coil of wire, a capacitor, and a resistor. What do each of these parts do to perform the crossing over?

– Ralph G.
Atlanta, Georgia

In laymen terms, the crossover network in a speaker is used to split the whole audio signal passing through it into signals in different frequency ranges. These splitted audio signals are then sent to the appropriate drivers in the speaker. The main purpose of this signal splitting is to ensure that each driver is subjected only to signal in the frequency range that it is comfortable of driving, to avoid distorted sound.

Note that a speaker can still work without a crossover network, but in such situation each driver in the speaker is subjected to the full-spectrum signal. Each driver has its own frequency response, which is the frequency range of the sound that can be produced with a good level of intensity. So even though it may be subjected to full-spectrum audio signal, the sound out of its frequency range is only produced with lower intensity and may not be audible when other higher intensity sounds are present. The point to note here is that the driver will try to drive whatever signal given to it even out of its frequency range, and thus it tends to be stressed out more compared to drivers that are given frequency-limited signal from a crossover network. The stressing out of the driver often results in sound distortion, which can destroy the fidelity of the sound reproduction. Also in a speaker with multiple drivers, there are usually some overlaps among the frequency responses of the drivers. When these drivers are subjected to the same full-spectrum signal, the sound level in those overlapping frequency ranges will get bumped and creates unevenness in the frequency response of the speaker.

The main elements of a crossover network are an inductor (the coil of wire that the questioner observes), a capacitor, and a resistor. These elements can be combined in various arrangements to achieve the desirable signal-splitting or crossing over effects. The effects can be estimated or calculated, but without going into the mathematical details, the functions of the main elements of the crossover network are described below.

Crossover Networks: How do they work?

By itself, the resistor in the crossover network does not have anything to do with the frequency-splitting of the signal, but rather it is used to adjust the signal level in relation to the driver sensitivity such that a balanced level of sound over the whole speaker’s frequency range, which is usually produced by multiple drivers, can be achieved. A resistor will attenuate the signal passing through it regardless of frequency. The amount of the signal attenuation depends on the amount of resistance that the resistor has.

The capacitor and inductor do not produce a static resistance like a resistor, but instead they have a level of resistance that is dependent on the signal frequency, called impedance. Therefore, they can be used to attenuate signals in specific frequency range, or in other words, to split signals based on their frequency contents.

An inductor has a lower impedance for low frequencies, therefore it tends to block high-frequency content of the signal and lets the low-frequency content passes through. Hence, the inductor functions as a low-pass filter in a crossover network, useful for filtering the audio signal to be sent to the bass driver of a speaker.

A capacitor, on the other hand, has a lower impedance for high frequencies, therefore it lets the high-frequency content of the signal to pass through and blocks the low-frequency content. For this reason, the capacitor serves as a high-pass filter in a crossover network, filtering out the low-frequency content of the signal. This kind of filtration is useful to prevent overloading the tweeter of a speaker.

A combination of the elements above is possible to get a desired crossing over effect. For example, an inductor and capacitor in a series configuration can be used to block both the high- and low-frequency content of the signal, allowing only the middle-frequency range to pass through. With such an arrangement, a bandpass filter is created, useful for example for filtering the audio signal to be sent to the mid-range driver of a speaker.

In a nutshell, those are the functions of the main elements of a crossover network. By cleverly networking these elements, their functions can be tailored to achieve desirable crossover effects.