by Marty McCann
What is a Crossover?
A crossover is a filtering device that separates the audio frequency spectrum and directs portions of that spectrum to a specific audio device. As an example, all sound reinforcement quality two-way loudspeaker enclosures have a passive crossover as part of the design that separates the audio spectrum into two bands of frequencies. This passive crossover is a bit like a traffic cop that directs the lows to the woofer and the highs to the horn. All of the frequencies below the designed crossover point (Low Pass) go to the woofer, and all of the frequencies above this crossover point or frequency (High Pass), are directed to the high frequency compression driver and the horn. This is accomplished via a network of inductors, capacitors, and resistors that direct or pass the proper information to the proper transducer.
How does the crossover work?
Inductors pass low frequencies and capacitors block low frequencies. Capacitors pass high frequencies and block low frequencies. You can have both a very high quality woofer and horn/driver combination, but it takes an equally high quality crossover circuit to make the units perform well as a system. The more complex the crossover network, the greater number of poles or orders you will find in the design. The more poles or the higher the order of crossover the more components you will have in the low and high pass filter stages of the design. The more filter stages, poles, or orders incorporated in the design, the more phase shift you will introduce to the audio signal. The phase of the output signal of the crossover can either lead or lag that of the original incoming signal. When signal passes through an inductor, the current flow is said to lag or be behind the voltage in regards to time. When an electrical signal passes through a capacitor, the current flow is said to lead or be ahead of the voltage in time. So in a simple single pole or first order crossover network, the voltage output of the high pass port will be shifted -90 degrees in phase, while the low pass will exhibit +90 degrees of phase shift, with the maximum of 90 degrees being at the extremities or farthest away from the crossover point.
Each crossover filter network stage (order or pole) will also introduce a specific rate of roll-off or attenuation of the signal, above and below the crossover frequency point. The rate of attenuation or roll-off is measured on the decibel scale in the form of so many dB per octave of attenuation (signal reduction) above and below the crossover frequency. For instance a 1st order filter network will attenuate at a rate of -6 dB per octave. Let's use the example of a 1600 Hz crossover design. The crossover frequency is designated by the -3 dB down point from flat, so in a two-way 1600 Hz crossover design, the Low pass output would be down -3 dB at 1600 Hz, as would the High pass. Above and below this crossover frequency the system would be flat (with the exception of CD horn attenuation and EQ, which will be addressed later).
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