Tweeters, woofers, subwoofers -- each type of speaker has a specific role to play in an audio setup. The subwoofer isn't designed to output Robert Plant's high-pitched wails any more than the tweeter is designed to handle the low-pitch rhythms of an upright bass. How do we make sure each speaker is doing its job correctly? The answer lies in the crossover.
A crossover directs audio frequencies to the proper targets, making sure only high-frequency sound reaches the tweeters and low-frequency sound finds its way to the subwoofer. Simple coaxial speaker setups -- like the kind that come standard with a new car -- have passive crossover circuitry already in place. As we mentioned previously in the section on amplifiers, you won't need to add anything special to one of these systems. But if you plan an upgrade to component speakers with an amplifier and a subwoofer, the crossover is an important part of the package.
Passive crossovers tie into the wired connection between an amplifier and the speakers and block unwanted frequencies. Active or electronic crossovers stand between the receiver and the amplifier, where they can direct frequencies to the proper speakers before the amplification process. Active crossovers are more efficient because they handle the sound before it goes through the amplifier, and they're more customizable than passive crossovers [source: Crutchfield].