Active control of sound transmission using structural error sensing

The active minimization of harmonic sound transmission into an arbitrarily shaped enclosure using error signals derived from structural vibration sensors is investigated numerically. It is shown that by considering the dynamics of the coupled system, it is possible to derive a set of “structural radiation” modes which are orthogonal with respect to the global potential energy of the coupled acoustic space and which can be sensed by structural vibration sensors. Minimization of the amplitudes of the “radiation modes” is thus guaranteed to minimize the interior acoustic potential energy. The coupled vibro-acoustic system under investigation is modelled using finite element analysis which allows systems with complex geometries to be investigated rather than limiting the analysis to simple analytically tractable systems. Issues regarding the practical implementation of sensing the orthonormal sets of structural radiation modes are discussed. Specific examples relating to the minimization of the total acoustic potential energy within a longitudinally stiffened cylinder are given, comparing the performance offered using error sensing of the radiation modes on the structure against the more traditional error criteria; namely, the discrete sensing of the structural kinetic energy on the boundary and the acoustic potential energy in the enclosed space.