A crossover is a very handy device that helps you take charge of the frequencies that are going to your various speakers. For example, it lets you send low frequencies through to your subwoofer or bass shaker amp, and it lets you send high frequencies to your other speakers.
Crossovers can be purchased as separate devices, but be aware that often come built-in to your amps and receivers.
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If you have no control over your frequencies, you could find your bass shakers vibrate at frequencies that are too high for your liking. For example, when you are watching Star Wars, you want the shakers to rumble with the low frequency SFX of a space craft, but you do NOT want them to vibrate whenever Darth Vadar opens his mouth. If you have a crossover, you can stop this happening.
Furthermore, it's kinda pointless sending high frequencies through to your bass shakers because they can't do anything with them. Bass shakers only "hear" low sounds. The same with subwoofers: high frequencies can even injure your subwoofer.
You possibly WON'T need a crossover if:
You possibly WILL need a crossover if: