Protection

Very powerful low frequencies can damage the tweeters. A crossover circuit is used to isolate these frequencies and feed the sensitive tweeters with only safe, high-range frequencies.

Attenuation

On the other hand, high-range frequencies are almost inaudible when passed through woofers. A crossover is necessary to allow only low frequencies to go through woofers.

Three-way Crossover

To optimize the drivers, a three-way is used to divide low-, mid- and high-range frequencies. The 20Hz to 20kHz frequencies are diverted to the woofer, while frequencies above 35kHz are passed to the tweeter. Anything in between is passed through to midrange.

Impedance Matching

When chosen correctly, a crossover can match a speaker's impedance and offer optimal results in addition to providing protection from damaging frequencies either above or below the recommended range.

Fine-tuning

Crossovers enable for fine-tuning of each frequency band to the specific drivers in use.

Tip

Ideal crossovers can be very difficult to design. Computer simulation is used to accomplish this. Passive crossovers, which use passive components like nonreactive resistors as opposed to transistors, are generally bulky, cause power loss and should be avoided.

Source:

audio crossover

More Information:

equalizers & crossovers