Yep, just push an inverted waveform back at the source, and you'll cancel it out.
Although in actual practice, accounting for every last echo, and transmission accelerated and decelerated medium in a given area is difficult, if not from a computational standpoint, but from a practical one to emit every last one of those waveforms in the proper frequency, power, and time to account for all the variable is very hard.
That's why the technology is most pronounced in contained and controlled circumstances, like in headphones where all that need be accounted for is the sounds between the headphones and the user's ears, or maybe in a car interior, where the acoustics are known, and the noises to be cancelled out are also known, like engine sounds at certain RPM's.
I'd also heard of active dampening/noise cancelling mufflers for cars. Although it's never gotten off the ground due to the expense, the muffler practically a wear part on most cars in snow/salt states, and because the engines and emission control systems now rely on the backpressure, that to use an noise cancelling active muffler would create a whole host of other expensive upstream changes in the car's functioning.
