Refers to the cancellation of one signal or frequency component of a signal by another signal of equal amplitude but opposite polarity. Sometimes this is called phase cancellation. It also is a phenomenon that is part of the sound of a phaser or flanger. As they sweep through their range various frequencies are accentuated or (nearly) cancelled producing their characteristic “whooshing” effect. Hum bucking, as in hum bucking guitar pickups, is the bucking of frequencies we associate with hum (60 Hz in the United States). In this case the cancellation is of EMI that is being picked up by the guitar’s pickup, which is acting as a transformer picking up various fields nearby.