Phase or polarity in loudspeakers

Phase or polarity

All speakers have two wires that must connected from the source of the signal (the amplifier or receiver) to the speaker's input terminals in correct polarity, or phase. If both sets of wires for left and right (in a stereo setup) are not connected in phase, the speakers will be out of phase from each other. In this case, any motion one cone makes will be 180 degrees opposite the other. This type of wiring error creates inverse sound waves which cancel out (to a degree) the sound of the other speaker. This won't cause silence because reflections from surfaces diminish the effect somewhat but resulting in a major loss of sound quality. The most prominent effect to the untrained ear will be a loss of bass response. The second most noticed will be an unsettling feeling.

A similar effect is used in sound-cancelling headphones. The headphones produce the inverse sound waves of the external noise. The inverse sound waves and external noise cancel each other out and produce near silence.