Near-field structure-borne sound holography [article]

Dietmar Greussing, Michael Möser, Technische Universität Berlin, Technische Universität Berlin
This thesis investigates the extrapolation of bending wave fields in homogenous plates. Using a number of structural response measurements, the continuous spatial field and the spectra of the response are reconstructed without knowledge of the mechanical boundaries or the excitation forces. It is clarified whether the deliberate inclusion of the structural near-fields close to sources and boundaries can be beneficial in the process. Two methods are therefore presented. The first one represents
more » ... rst one represents an application of Fourier transform-based Near-Field Acoustic Holography in a structure-borne sound setting. The use of a polar coordinate system allows analytical propagation functions to be convolved and deconvolved with boundary values measured in the structural near-field of the plate. Because this constitutes an inverse problem, numerical stabilization in the form of wavenumber filtering and regularization is applied. The second method utilizes the Kirchhoff-Helmholtz integral and requires far-field conditions at the measurement positions. It constitutes the solution of a direct problem in comparison and is not restricted to a specific coordinate system for measurement. The methods are evaluated through a series of simulations and experiments, considering three distinct cases: the infinite plate, the fully free and the simply supported plate, excited by normal point forces, respectively. In terms of the usually neglected influence of the near-field, the latter two can be seen as worst and best case scenarios for the wide range of possible boundary conditions in engineering practice. It is found that the average reconstruction error to be expected from holography ranges from 1 dB to 3 dB, depending on the boundary condition. The method is not very well suited to analyze very narrow frequencies in a spectrum, and the extrapolatable plate area is inherently limited. The Kirchhoff-Helmholtz integral-based method leads to an average reconstruction error between 2 dB and 3 dB. As with the holographic method, the observed de [...]
doi:10.14279/depositonce-4215 fatcat:bnfojug2vffnlm3lqavorg4m5a