What does a "crossover" do?
A crossover is an electronic circuit that splits the audio signal into two parts so they can each be sent to a dedicated speaker. Usually, the lower frequencies (bass) are sent to a speaker that is optimized to handle that part of the frequency spectrum, and the higher frequencies (midrange and treble) are sent to a speaker that is optimized for them. This so-called "two-way" design delivers superior sound quality, since each part of the frequency range is reproduced better than a single full-range speaker could do the job. The SE425 earphones, for example, utilize two speakers in each ear, fed by an inline crossover located in the cable.