Sound Is a Wave

Sound travels as pressure waves through air. Each wave has peaks of high pressure and troughs of low pressure, oscillating at a specific frequency. A 200 Hz hum means 200 complete pressure cycles per second.

When peaks of one wave align with troughs of another, they cancel — destructive interference. If you generate an exact mirror image of an incoming sound wave and play it simultaneously, the two cancel each other out. That is the entire principle behind noise-cancelling headphones.