Generally speaking, ANC works best on consistent, ambient noises. This refers to things like the murmur of a crowd on mass transit, a nearby AC unit humming away, or distant construction noises. While more adept applications of the technology can significantly reduce the volume of loud, sudden noises, they are generally still audible, though typically far less obtrusive or annoying. In short, you may still occasionally hear that screaming kid or constantly slamming door, but it might not give you a headache quite so fast.