[Richard Crowley]

David Nebenzahl wrote:
Mr.T spake thus:
and reinject a perfectly matched inverted copy in exactly the same
spot,

Trivially easy, no? A simple phase inverter oughta do the trick.

No. Because it will almost instantly turn into a closed-loop
feedback oscillator and likely deafen the wearer. If it were
as easy as simple signal inversion, we would have had noise
cancelling headphones decades sooner.