AN EQUIVALENT SOURCE TECHNIQUE FOR CALCULATING THE SOUND FIELD INSIDEAN ENCLOSURE CONTAINING SCATTERING OBJECTS

The equivalent source method has previously been used to calculate the exterior sound field radiated or scattered from bodies in the free-he ld. In this paper the method is used to calculate the internal pressur e field for an enclosure which can have arbitrary boundary conditions and may include internal objects which scatter the sound. Some of the equivalent source positions are chosen to be the same as the first ord er images of the source inside the enclosure, some are positioned with in the scattering objects, and the remainder are positioned on a spher ical surface some distance outside the enclosure. The normal velocity on the surfaces of the scattering objects and the enclosure walls is e valuated at a larger number of positions than there are equivalent sou rces. The sum of the squared difference between this velocity and that expected because of the admittance of the boundary, is minimized by a djusting the strengths of the equivalent sources. The convergence of t he method is checked by evaluating the velocity at a larger number of monitoring positions. Example results are presented for the sound fiel d and frequency response inside a damped rectangular enclosure, which compare very well with the conventional modal model. The effect of hav ing rigid spheres inside the enclosure are then investigated, and it i s found that the effect is significant even some distance from the sph eres and at frequencies for which the size of the sphere is small comp ared to a wavelength. Finally the effect of a nonlocally reacting boun dary condition is illustrated by assuming that one of the walls of the enclosure is an elastic plate. (C) 1998 Acoustical Society of America . [S0001-4966(98)02206-1].