How are capacitors used in speaker applications?

Capacitors are used to limit the frequencies sent to speaker units in multiple speaker applications.

As the frequency increases, so does the effective resistance of the capaitor. (known as reactance)

For instance, tweeters only work efficiently with high frequencies. Applying low frequencies to these devices is inefficient and could cause harm to the unit. Putting a capaitor in line with it will only allow high frequencies through. Using a combination of capacitors and coils, in parrallel to form filters, can make only desired frequencies pass to the correct driver unit. Each speaker can then work most effeciently with the desired frequency. This is a fine art, to make sure that the sound is pure and not coloured or distorted and has to also match the cabinet in which the drivers are mounted.