Q. Antinoise Systems — Noise Cancellation?

Traditionally sound-absorbing materials have been used quite effectively to reduce noise levels in aircraft, amphitheaters, and other locations. An alternate way is to develop an electronic system that cancels the noise. Ear doctors and engineers have successfully developed ear devices that will nearly eliminate the bothersome and irritating noise (so-called tinnitus) experienced by patients suffering from M´ eni` ere's disease. For passengers in airplanes, helicopters, and other flying equipment, a proper headgear is being developed in order to eliminate the annoying noise.

Applications could conceivably extend to people residing near airports and bothered by airplane takeoffs and landings. For industrial workers who are likely to develop long-term ill effects due to various noises they may be subjected to in their workplace, and even for persons who are irritated by the pedestrian noise levels in certain locations, antinoise systems that nearly eliminate or nullify noise become very desirable.

Figure illustrates in block-diagram form the principle of noise cancellation as applied to an aircraft carrying passengers. The electric signal resulting after sampling the noise at the noise sources is passed through a filter whose transfer function is continuously adjusted by a special-purpose computer to match the transfer function of the sound path. An inverted version of the signal is finally applied to loudspeakers, which project the sound waves out of phase with those from the noise sources, nearly canceling the noise. Microphones on the headrests monitor the sound experienced by the airline passengers so that the computer can determine the proper filter adjustments.

Signal processing, which is concerned with manipulating signals to extract information and to use that information to generate other useful electric signals, is indeed an important and farreaching subject.