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A humbucking pickup, humbucker, or double coil, is a type of electric guitar pickup that uses two coils to "buck the hum" (or cancel out the interference) picked up by coil pickups caused by electromagnetic interference, particularly mains hum. Most pickups use magnets to produce a magnetic field around the strings, and induce an electrical current in the surrounding coils as the strings vibrate (a notable exception is the piezoelectric pickup). Humbuckers work by pairing a coil that has the north poles of its magnets oriented "up" (toward the strings) with another coil right next to it with the south pole of its magnets oriented up. By connecting the coils together out of phase, the interference is significantly reduced via phase cancellation: the string signals from both coils add up instead of canceling, because the magnets are placed in opposite polarity. The coils can be connected in series or in parallel in order to achieve this hum-cancellation effect, although it is much more common for the coils of a humbucker pickup to be connected in series. In addition to electric guitar pickups, humbucking coils are sometimes used in dynamic microphones to cancel electromagnetic hum.
Mains hum is caused by the alternating magnetic fields created by standard AC power, either 60 hertz (most of the Americas) or 50 hertz (most other parts of the world). While playing a guitar without humbuckers, a musician would hear a hum through the pickups during quiet sections of music. Sources of studio and stage hum include high-power amps, processors, mixers, motors, power lines, and other equipment. Compared to single-coil pickups, especially unshielded ones, humbuckers dramatically reduce hum, and (especially when the coils are connected in series) produce a louder signal with more mid-range presence.