Reconstruction of vibro-acoustic fields using hybrid nearfield acoustic holography

Hybrid nearfield acoustical holography is utilized to reconstruct the vibro-acoustic fields generated by an arbitrary structure vibrating in an unbounded fluid medium. This hybrid NAH is based on the modified Helmholtz equation least-squares method that expresses the field acoustic pressure in terms of expansions of outgoing and incoming waves. The expansion coefficients are determined by solving an over-determined linear system of equations obtained by matching the assumed-form solution to a finite number of acoustic pressures measured in the field via least squares. The measurements are taken over a conformal surface around the source at close range so that the evanescent waves can be captured. Once the expansion coefficients are specified, enough field acoustic pressures are regenerated on a conformal surface around the source and are taken as input to the Helmholtz integral formulation implemented numerically by the boundary element method. The acoustic pressure and normal velocity on the source surface are reconstructed using singular value decomposition, modified Tikhonov regularization and generalized cross-validation method. This hybrid NAH enables one to obtain a satisfactory reconstruction cost effectively because the majority of the input data are calculated but not measured. Numerical examples of reconstructing the vibro-acoustic quantities on the surfaces of an engine block and a highly elongated cylinder are demonstrated.