On: MyNoise – Background Noise Generators

[wowokay]

And for some reason I thought it required triangulation, in that noise cancelation can't be done with sound in a space because the inverse of the sound you hear is relative compared to the person a foot or two away from you.

[hackerlight → wowokay]

It does only if the microphone is far away from the ear canal.

If it's close to the ear canal, all you need is low latency hardware (or hardware+software), responding in a few milliseconds thereabouts.