(An) efficient acoustic holography of vibro-acoustic radiation and source fields using equivalent sources and boundary element method

Abstract

In this thesis, an efficient method of reconstructing vibro-acoustic radiation and source fields of a vibrating object is presented. The use of multi-point, multipole equivalent source method (ESM) as an alternative to the boundary element method (BEM) is attempted. The ESM allows us to express a vibrating source by a collection of acoustic multipole sources distributed within the noise source. One of the previous acoustical holography methods of using the ESM is the so-called Helmholtz equation least-squares method: this is conceptually equal to the ESM with a single-point acoustic multipole source in a series expansion of spherical Hankel functions and spherical harmonics. In this study, a generalized ESM based on multi-point acoustic multipole sources is employed to reconstruct the sound field. The reconstruction accuracy may not be satisfactory because the system matrix is in general ill-conditioned, mainly due to the radiated evanescent waves which are decaying at high rates away from the source. To ensure and improve the accuracy of reconstruction, the spatial coefficient filtering (SCF) method together with regularization technique is adopted for suppressing the adverse effect of high-order multipoles and wave-vectors. The optimal number of expansion terms is determined by the SCF method with the generalized cross validation (GCV) function without the aid of additional field measurements for constraints on the assumed solution. It is also attempted to utilize the effective independence method for the positioning of linearly independent measurement sensors as well as equivalent sources. For validating the effectiveness of the present method, numerical simulations of a pulsating sphere and a baffled plate on a parallelepiped box are demonstrated. As a result, it is thought that a number of simple equivalent sources should be used to regenerate the field pressures rather than using a small number of high-order equivalent sources. After obtaining the suffici...