No, active noise cancelling isn't about masking noise with sound, active noise cancelling takes the original noise and mixes it back 180 degrees out of phase. The net result is zero noise, when done perfectly. In practice it's almost like that (for the frequencies covered by the noise cancelling equipment - in an airplane it's like the background noise just.. turns off. What's left is typically people talking. It's easier to hear the flight attendants).

If instead it was by adding sound then it would defy the purpose. I use active noise cancelling because if I don't I'll hear a noise in my head for weeks after a long-haul flight (I have tinnitus in one ear which makes it even worse).