Speakers and headphones have drivers. They're the vibrating parts that create the sound you hear.
Some speakers have three types of drivers -- a tweeter for high frequencies, a midrange for middle tones like voice, and a woofer for bass -- but most speakers have just a tweeter and a combination midrange-woofer.
Dynamic drivers are by far the most common type of driver for speakers and headphones; dynamic drivers are dome- or cone-shaped devices. Some high-end speakers and headphones have perfectly flat round, square or rectangular thin-film planar magnetic or electrostatic drivers. In any case, all drivers make sound by moving air.
Almost all full-size headphones have just one driver per earpiece, but some high-end in-ear headphones have more. For example, the Jerry Harvey Roxanne headphone has a total of 12 drivers per ear piece (four bass, four midrange, four tweeters).
Speakers and headphones with more than one driver are referred to as "two-way" or "three-way" designs and use crossover networks to route the corresponding frequencies to the woofer(s), midrange(s) and tweeter(s) drivers.
A speaker or headphone with a midrange-woofer and a tweeter is always a two-way design; even when the speaker or headphone has two or three or more combination midrange-woofer drivers and a single tweeter, it's still a two-way design. Only when the speaker or headphone has separate bass, midrange and tweeter drivers is it classified as a three-way design.
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