[andreareina]

ANC works on the level of physics, not just perception -- it attenuates the actual sound waves.

[novok]

Where does that energy go although? The waves go to 0-ish, but that air pressure / energy has to go somewhere correct?

[andreareina]

The energy goes into constructive interference in other parts of the wave (hopefully not traveling into your ear). Avery's beamlab[1] provides a nice playground with visual representation (use the anti-optimize mode). It's built around radio signals but the concept is very much the same.

[1] https://apenwarr.ca/beamlab/