Characterization of guided acoustic waves in an arbitrary direction with full-field instantaneous maps of the acoustic displacement

The full-field optical measurement of guided acoustic waves presents important advantages derived from the capability to map the acoustic field in a two dimensional region, so that acoustic information over a large area can be retrieved with each measurement. In this work, we introduce an extension of the well-established two dimensional spatio-temporal Fourier transform method to calculate the frequency spectrum of guided acoustic waves. For this, we take advantage of the unique capability of a self-developed double-pulsed television holography system to acquire the acoustic displacement field in two spatial dimensions and time. Then, the spatio-temporal Fourier transform method is expanded to three dimensions according to the nature of the experimental data, so that the frequency spectrum of the waves propagating in an arbitrary direction can be calculated. The method is tested experimentally by generating narrowband Lamb waves in an aluminium plate with a piezoelectric transducer. The good agreement be...