Active Control of Acoustic Fields and the Reproduction of Sound

Recent studies of the active control of acoustic fields have used analytical methods and multi-channel signal processing techniques that can be usefully applied to problems in sound reproduction.This paper considers several aspects of the reproduction of sound.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 ofa 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 amatrix 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 sampling 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 eld.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 realisability of the optimal filter matrix and a practical, adaptive, filter design technique is presented which has already proved 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.Kirchhoff-Helmholtz integral theorem, which shows that for field points it outside V, equation (2) holds with pix, 1) equal to zero.Finally, of course one has to assume that both pa and c, are identical in V and V'.