Active Control Of Acoustic Fields And The Reproduction Of Sound

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In recent studies of the active control of acoustic fields analytical methods and multi-channel signal processing techniques have been used that can be usefully applied to problems in sound reproduction. In this paper several aspects of the reproduction of sound are considered. First, the possibility is considered of the perfect reproduction of an acoustic field in both space and time. Results from classical acoustics suggest a means by which this could be achieved, but it is soon concluded that this is an unrealistic objective in practice. The reproduction of a sound field over a restricted spatial region is also considered. Some new results are presented which demonstrate that a field can be reproduced that closely approximates the original by first recording the acoustic signals at a finite number of positions in the original sound field. The signals are processed via a matrix of linear filters in order to produce the inputs to a number of sources used for reproduction. An analysis in the frequency domain shows that such a strategy could be useful, but its practicability at high frequencies appears to be limited by the need to provide adequate spatial samping of the original field. Another approach that is considered is to concentrate on ensuring that the direction of propagation of the waves in the original field are well approximated in the reproduced field. This approach appears to be a more practicable alternative, and offers the promise of successful operation over a wide frequency bandwidth. Some discussion is presented of the realizability of the optimal filter matrix and a practical, adaptive, filter design technique is presented which has already provded successful in some limited experiments. Finally, some further possibilities are suggested in which the same principles are used to improve the quality of existing stereophonic sound reproduction systems.