Antiphase waveforms cancel when added together

Evidence

Active noise cancellation is based on the fact that if a waveform is added to an equal and opposite (antiphase) version of itself, cancellation results (see Figure 1.13). Active noise cancelation is designed into headphones which have a microphone on the outside of each earpiece to pick up the local acoustic background noise on each side of the head. This is essentially the acoustic noise that is reaching each ear. This microphone signal is phase reversed and added at the appropriate level to cancel the background noise. The wanted signal is also added in and the result is that the background noise is significantly reduced in level, and the overall volume of the wanted signal can be reduced to a more comfortable listening level. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2558

Categorization

• DEFINE: Explains the physical principle behind active noise cancellation: antiphase superposition.