Hybrid NAH formulations for an arbitrary object in a nonfree field

Abstract

Previous nearfield acoustical holography (NAH) formulations based on the Helmholtz integral theory and boundary element method (BEM) are effective for an arbitrary object in a free field, although a true free field is nonexistent. While a modified Helmholtz equation least squares (HELS) method using expansions of both outgoing and incoming spherical waves can provide an approximate solution for such a scenario [Wu and Zhao, J. Acoust. Soc. Am. 111, 2409 (2002)], the reconstruction accuracy may be unsatisfactory when the source is of arbitrary geometry. In this paper, a hybrid NAH formulation that combines a modified HELS method, the Helmholtz integral theory, BEM, and regularization techniques (e.g., modified Tikhonov regularization, generalized cross validation) is developed for reconstructing acoustic radiation from an arbitrary object in a nonfree field. This hybrid formulation has the advantages of the modified HELS method and BEM‐based NAH, and represents a significant improvement over the combined HELS method [Wu and Zhao, J. Acoust. Soc. Am. 112, 179–188 (2002)]. The input data are collected on a conformal surface at close range, so the evanescent waves can be captured and the reconstruction accuracy can be improved. Moreover, the majority of the input data are calculated but not measured, and the reconstruction efficiency is enhanced. [Work supported by NSF.]