A crossover is a frequency divider. Crossovers are used to route the various frequency ranges to the appropriate speakers. A 3-way crossover divides the audio spectrum into three frequency bands. It then sends the high frequencies to the tweeter, the mid-range frequencies to the mid-range speaker, and the low frequencies to the woofer or subwoofer. Similarly, a 2-way crossover divides the audio spectrum into two bands, sending the highs to the tweeter and the lows to the woofer. High-end pro audio systems sometimes employ 4-way, or even 5-way crossovers for more precise reproduction of sound. The frequency at which the signal is divided is called a crossover point.

Crossovers come in two varieties, active and passive. Passive crossovers are typically built into speaker cabinets and are designed to modify the amplified signal to the speakers. The crossover frequency on a passive crossover is optimized for the speakers in the cabinet and is not adjustable.

Active crossovers are external units that modify the signal before it is amplified. Active crossovers require separate amplifiers for each frequency band. There are several benefits to using active crossovers. Active crossovers allow you to set the frequencies at which to split the signal. This allows you to set the frequencies at the ideal points for your sound system. Since you are not sending extraneous frequencies to the power amp, an active crossover increases the efficiency and headroom of your power amps. Many active crossovers can be set up for operation in either mono or stereo mode.