Fig. 1. Attenuation of tones by phase-cancellation.

(a) Schematic of sound pressure variation produced by a sinusoidal waveform. Sound is a pressure wave with neighboring regions of high (peaks) and low (troughs) of air pressure. (b) Schematic of a computer whose two speakers emit tones in anti-phase: the right speaker (red) emits a high pressure peak (solid lines) while the left speaker (blue) emits a low-pressure trough (dashed lines). Some distance away from the computer, the relationship is reversed. (c) Waveforms of two sinusoids close to anti-phase, and their superposition. Pressure from two waves sum linearly, such that when the waves are close to anti-phase (as is the case when the distance to the right and left speakers is similar) their sum is of lower power than either of the constituents.