Many folks called us recently asking about the application of ANC to domestic and commercial noise control problems. The conventional wisdom seems to be that any noise situation is amenable to an ANC solution. This is false, just as is the notion that a computer is the solution to all of your calculation and bookkeeping problems or that computers reduce the consumption of paper. Consider that most general noise problems are 3-dimensional in nature, typically a noisy area where one or more persons move about. This signal is amplified and fed to a loudspeaker which emits it precisely 180 degrees out of phase with the original wave.
Cancellation is created at that point.
In the case of two or three dimensional propagation into space or into a room, The noise may have been produced a distance away. The divergence of the loudspeaker sound does not match the pattern of the divergence of the noise field, so cancellation does not occur. It requires that the loudspeaker be precisely placed at the center of the source of noise, usually an impossible task. Fortunately, this is a useful capability since noise above 250 Hz is easily absorbed by duct linings while noise below 250 Hz is not, requiring some other means. ANC satisfies this need.