Active control of sound

The active control of sound waves has become an extraordinarily large and v igorous area of academic research and technological development. In this pa per we describe the physical principles underlying the control of sound and review their application in a wide range of contexts. One scenario involve s the control of noise from a primary source by the introduction of seconda ry sources, and this technique is described for fields in ducts, in free sp ace, in enclosures (with particular reference to aircraft cabins), and for turbomachinery. A second scenario involves the use of the active control of sound to eliminate large-scale oscillations in more complicated flows, in which part of an unstable feedback cycle is mediated via acoustic waves. Su ccessful applications of this idea include the control of combustion instab ilities and compressor surge.