This is over simplifying, but in effect ANC starts with the sound the mic picks up, and subtracts the "known" signal that is being played from the speakers. The result is (in a perfect world) the noise that should be canceled. Invert that noise and add it to the desired (music) sound, and viola! Tons of details make this more complicated, but that's the gist.Basically you need to measure the noise you want to cancel as close as possible to the speakers that will do the canceling. It wouldn't work to measure the sound a distance away from the speakers because the phase of the sound waves has to be aligned just right for them to cancel out and the phase is different at different points in space and also varies according to frequency. If they could put microphones and speakers right where the sources of noise were and produce sufficient volume they could theoretically cancel noise out at the source but that's probably not as practical as using speakers and microphones inside the car.