Noise cancelling cannot cancel out voices. It's designed to cancel out predictable lower frequency sounds such as fans, A/C system fans, engine rumbles and jet engine noise.
The noise cancelling system has an inherent delay in processing the sound it's listening to output an opposing waveform. Since voices are not predictable, outputting a waveform after a slight delay will just cause it to be out of sync and cancel out nothing. Maybe some ML system could do this in the future, but it would have to be absolutely perfect to not sound distractingly strange at times.
Fans on the other hand are predictable, so you can output an opposing waveform despite the processing delay because you can predict how the waveform will act after your processing delay and keep them in phase.
There's an interesting twist on the active cancelling idea for stopping voices. You embrace the delay, increase it to ~200 msec, and just play the normal waveform out loud. The result is uncomfortable and will make the speaker shut up.
In order to effectively cancel, it would have to be less than the time difference between the sound being detected/processed by the headphones and the sound being detected by your ears.
So, roughly the time it takes sound to travel at most an inch?
The noise cancelling system has an inherent delay in processing the sound it's listening to output an opposing waveform. Since voices are not predictable, outputting a waveform after a slight delay will just cause it to be out of sync and cancel out nothing. Maybe some ML system could do this in the future, but it would have to be absolutely perfect to not sound distractingly strange at times.
Fans on the other hand are predictable, so you can output an opposing waveform despite the processing delay because you can predict how the waveform will act after your processing delay and keep them in phase.