On the use of near-field acoustical holography to study the transient radiation of impacted plates

This paper discusses the application of near-field acoustical holography to visualize the sound pressure radiated by impacted plates as a function of time. It is known that when Fourier transform based near-field acoustical holography is applied to propagate sound fields away from the acoustic source, the use of direct and inverse time domain discrete Fourier transforms leads to significant aliasing errors in the reconstructed time signals. To avoid such errors, the time domain discrete Fourier transform can be replaced by a numerical Laplace transform as long as the Dirichlet κ-space Green's function is reformulated. In the present paper, the resulting transient acoustical holography method is applied experimentally to study the sound radiation of a free plexiglas plate impacted at its center by a steel pendulum. Forward propagated signals show good agreement with measured signals in propagation planes at distances up to 20 cm from the hologram, i.e. distances comparable to the plate dimensions. A time and space reconstruction of the sound pressure field is also computed from the measured hologram. Both airborne and structure-borne sound radiation can be observed as a function of time in a plane perpendicular to the plate. For instance, sound pressure wavefronts due to the reflection of flexural waves on the edges of the plate are clearly visible in this reconstruction.