This presentation deals with the experimental characterisation of a thick sandwich plate with a honeycomb core. Measurements were conducted by means of a 3D laser Doppler Vibrometer, allowing the assessment of the out-of-plane and in-plane displacements of the sandwich structure due to piezo excitation. In addition, similar laser Doppler measurements were performed using a pump laser to excite the plate up to 300 kHz. Trough this experimental study, the complex dynamic behaviour of the structure (shearing of the core, coupling of the dispersion curves, dynamic evolution of the mechanical properties) is identified and analysed. Dispersion measurement results, obtained by using wave fitting methods (Hankel and IWC : Inhomogeneous Wave Correlation) as well as the CFAT (Corrected Force Analysis Technique) method, are shown. These results are compared to the Lamb wave theory and an equivalent model based on the work of Guyader (JSV 1978).