Applications of active control of sound and vibration

The field of active control of sound and vibration has been growing very rapidly in the last two decades, and many applications have been developed. Most of this development has been driven by advances in control theory and the availability of low-cost electronics for digital signal processing. The idea of active control does, however, have a long history. The fundamental concept of sound reduction by interference of a primary and secondary wave was established by Lord Rayleigh. There was little activity on active control until a German physicist, Paul Lueg, obtained a United States patent in 1935 on reduction of sound through active control. In 1953, Olson and May reported on experiments with active control and showed applications in a number of areas. For a worker near a machine and a person seated, sound waves are picked up by a microphone and fed back out-of phase to produce a canceling wave. The system creates a "zone of silence" around the heads of the persons. The systems developed by Olson and May were analog in nature, and careful control of phase shift in both the microphone and loudspeaker were essential to demonstrate the possibilities of active control. Serious laboratory experiments on practical applications of sound reduction were initiated in the 1970s when digital control systems became feasible. Researchers quickly discovered that a substantial research effort would be needed to obtain sufficient insight into the properties of specific sound fields and elastic waves in structures to determine schemes to implement active control systems. One result of the past twenty-five years of active control research is the advancement of the general knowledge of the fundamental properties of acoustic fields, sound energy propagation and radiation, and structural vibration