ANC listens before it cancels
Active noise cancellation uses microphones to sample outside sound, then generates an inverse signal to reduce what reaches your ear. It is most effective against low, continuous sounds like engine rumble, HVAC systems, or train noise.
Why voices still slip through
Human speech changes rapidly and contains a broad range of frequencies, so it is harder to cancel cleanly in real time. Good passive isolation from pads or tips still matters. ANC should be seen as a layer on top of physical sealing, not a replacement for it.
Processing always has tradeoffs
Some ANC headphones change their tuning slightly when cancellation is enabled. Others introduce gentle pressure, hiss, or shifts in stereo image. That is one reason it helps to compare modes using a simple reference like the phase check or stereo image test.
Strong ANC is valuable, but it is best judged by comfort and consistency, not marketing numbers alone.