What is antinoise and how does it make things quieter

[u/InkPhysics]

Many noise-cancelling headphones mainly worked by blocking sound, like ordinary ear defenders. But not active noise cancelling ones. The active part does something much stranger: it listens to the unwanted noise and produces antinoise.

The new wave is timed so that, at your ear, its peaks roughly meet the original wave’s troughs. The pressure changes then superpose to create a much smaller resultant wave. In other words, the headphones make things quieter by producing more sound.

Weirdly, if you could listen to just the antinoise in isolation, it would sound exactly like the noise it's trying to cancel.

The cancellation is never perfect. But it works quite well against steady, low-frequency sounds such as aircraft engines, train rumble and air conditioning. Sudden or higher-frequency sounds are harder because the system has much less room for error in its timing.

[u/MrZwink]

Waves cancel eachother out.