Earlier quoted context omitted.
I get a similar answer but through a wildly different path: I think you have to detect the sound with a microphone, then come up with the canceling waveform before the sound hits your headphone drivers (as they will be generating the canceling waveform). Given the typical size of headphones, it would be something like 2cm/(34300cm/s) ~ 60us Although, sound travels faster through solids, so might want to be a little q…
wonder if there's some kind of "virtual microphone" like device that let's you hear sounds further away than a mic in the headset. Google fu showed some ultrasonic virtual microphone but idk if it is useful here..
I imagine you want a very good approximation of the soundwave that will hit the ear (so you can cancel it out very accurately). The farther that your 'virtual microphone' is from the ear, I guess the harder it will be to figure out exactly how the sounds it is picking up will combine to hit the ear.
I mean, imagine the extreme case -- your 'virtual microphone' is (virtually) sitting in the path of some directed audio beam which won't hit the headphone wearer at all. Now you do the signal processing and generate a signal to cancel out that directed audio beam, which, because the user isn't in the path of it, causes them to hear it!