Detection of plate vibration modes by means of Near-field Acoustic Holography in the presence of reflective surfaces

A numerical and experimental study is presented on the influence of reflections on the reconstruction accuracy of the surface velocity of vibrating plates by means of Fourier-based Planar Near-field Acoustic Holography (PNAH) and a single-layer microphone array. At low frequencies (well below coincidence), it is shown that the effect of the presence of the reflective surface on the reconstruction depends on the source. When measuring close to the source, the classical PNAH transformation, based on the free-field radiation assumption, provides a good quantitative reconstruction of the surface vibration velocity. The effectiveness of a modified propagator is investigated which includes the effect of reflections on the inverse propagation from measured pressure at the hologram plane to surface vibration velocity at the source plane. This is done by simulations and measurements for sources that differ in velocity profile and size and that are radiating well below coincidence. Measurements are performed for a free plate in a large baffle in front of a granite block which is less than one acoustic wavelength apart.