Accuracy assesment of planar nea-field acoustical holography applied on non-resonant vibration imaging

A numerical study is presented on the accuracy of Planar Near-field Acoustical Holography (PNAH) for measuring non-resonant vibration of plate-like structures. Lately the PNAH tech- nique is applied to detect vibrations, but in most cases the measurement are limited to the in- vestigation at resonance. However, in many practical situations the non-resonant behaviour of the structure is also important. Simulations are performed on a thin simply-supported baffled plate for which sufficient modes contribute to the vibration by applying modal superposition. The distribution in modal contribution as well as the radiation frequency is varied. The acous- tic pressure is calculated by means of the first Rayleigh integral, given the normal vibration velocity over the surface of the plate. The reconstruction of the velocity is performed by the PNAH algorithm. From the numerical study, it is found that the accuracy of the reconstruction depends on the radiation efficiency of the modes and the distribution in the contribution of the single modes to the total vibration.