While active noise cancellation was not in wide use in 2001, the technique was becoming a practical noise-fighting solution for some specialized applications. For example, several companies, such as the Bose Corporation of Framingham, Massachusetts, were marketing headphones with active noise cancellation for use by professionals working in loud environments, such as airline pilots.
Active noise cancellation works best when the relative positions of the listener, the noise source, and the antinoise source stay the same. That is because effective noise cancellation demands extremely precise positioning. The antinoise wave must be almost perfectly lined up with the noise wave in order to nullify it. If the two waves are not aligned, the system magnifies the unwanted sound rather than eliminating it. A sound wave with a frequency of 1,000 hertz is about 0.3 meter (1 foot) long. For active noise cancellation to work, the antinoise wave must overlap the noise wave to a precision of about half a wavelength, or about 15 centimeters (6 inches). For this reason, it would be difficult to devise an active noise-cancellation system that is effective when the sources of noise and the listener are moving relative to one another.
However, active noise cancellation does appear promising for applications such as quieting the interiors of cars and aircraft. Here the position of the noise source—the engine (or engines)—is well-known and the passengers are generally sitting in fixed locations and facing the same direction. Engineers strategically place scores of microphones and antinoise sources throughout the passenger areas. In aircraft systems, engineers have found that they can muffle aircraft engine noise by as much as 10 decibels.
In December 2000, Honda Motor Co., Ltd., of Tokyo, announced that it was fitting some of its vehicles sold in Japan with an active noise-cancellation system. The system was designed to reduce low-frequency sound below 100 hertz. The system reduces road noise in vehicles by 10 decibels. To prevent the system from reducing the volume of the car stereo, Honda's noise-control system was engineered to recognize and ignore music from the car stereo and cancel only unwanted sounds.
An active noise-cancellation system that stops unwanted sounds closer to their source was introduced in 2001 by Bombardier Aerospace, an aircraft manufacturer with headquarters in Dorval, Quebec. Rather than broadcasting antinoise waves within the cabin to negate the unwanted thrum of the aircraft's engines, the Bombardier system cancels the vibrations that transmit the noise from the engines to the interior of the plane. The system uses microphones to pick up the vibrations in the cabin walls. It then analyzes the signals and generates countervibrations in the walls to produce a net result of zero vibrations.
(Source: How stuff works?)