Theoretical And Experimental Study On The Distributed Source Boundary Point Method Based Nearfield Acoustic Holography

Nearfield acoustic holography (NAH) is a powerful tool for identifying noise sources and visualizing acoustic field. By multiplying the inverse of the calculated vibro-acoustic transfer matrix and the measured acoustic pressure vector on the hologram surface, such acoustic quantities as acoustic pressure, particle velocity and acoustic intensity at any point in three-dimensional acoustic field and acoustic power on arbitrary surface can be reconstructed. In this technique, the hologram surface should be located as close to the source as possible in order to acquire precise field information including the non-propagating evanescent waves for an accurate source reconstruction. So far, several different methods, such as spatial Fast Fourier Transform (FFT), boundary element method (BEM), Helmholtz Equation Least Squares (HELS) method, etc. have been used to realize the NAH successfully, but none is the best.Considering the advantages of the distributed source boundary point method (DSBPM) compared with the BEM, it is proposed to be the transform algorithm of the NAH, and the source reconstruction and field prediction models based on it are established. Then, the influence of the measurement error to the reconstructed solutions is analyzed, and the truncated singular value decomposition (TSVD) method and the Tikhonov regularization method are proposed to stabilize the reconstruction procedure, restrict the influence of measurement error and insure the reconstructed and predicted precisions.Subsequently, the reconstruction and prediction methods in semi-free acoustic field and multi-source coherent acoustic field are put forward, and their validity and correctness are verified by numerical simulations for theoretical noise sources and experiments for actual noise sources.In order to overcome shortcomings of the routine distributed source boundary point that the locations of particular solution sources must be selected carefully but can only be determined by an experiential formula, the routine DSBPM is improved at first, and then a novel boundary point method, orthogonal spherical wave source boundary method (OSWBPM), is put forward. Here, all particular solution sources are located at only one point inside the vibrating body, so the problem of the locations of the particular solution sources is thoroughly solved, and such particular solution sources that are useless for the calculation results arediscarded. Subsequently, the method is proposed to realize the NAH. Two experiments in the semi-free acoustic field are investigated and validate the feasibility and correctness of the proposed method, and its superiority in realizing the NAH is proved by comparing with the HELS method based NAH.In addition, on the basis of the DSBPM based NAH, a similar method, the equivalent source method based NAH, is proposed and compared with the DSBPM based NAH in mathematics and by experiments. Here, the essence and superiority of the DSBPM are discussed again.Finally, the proposed DSBPM based NAH are demonstrated for reconstructing the vibro-acoustic field of a domestic roots vacuum pump. According to the reconstructed results, the noise sources at the dominant analysis frequencies can be identified and localized correctly, and the results show good agreement with those measured by sound intensity technique before.