A SOLID IMPEDANCE-MATCHING MECHANISM FOR ACOUSTIC TRANSMISSION BETWEEN WATER AND AIR

Acoustic transmission between highly dissimilar media, particularly water and air, remains a significant challenge due to the large mismatch in acoustic impedance, which results in nearly total reflection of incident acoustic energy at the interface. In this study, a purely solid, periodically stiffened multi-plate mechanism is proposed and analyzed as an impedance transformer to enable efficient acoustic transmission between water and air. The proposed approach is based on a multi-plate structure connected by ribs, where the acoustic transmission is achieved through resonance of the fluid–structure interaction. The analysis begins with a two-dimensional two-plate system separating water, where it is shown that the low-frequency response of the system behaves as an equivalent spring, analogous to an air gap. This concept is then extended to the water–air interface, where explicit analytical expressions are derived to relate the system parameters, such as plate thickness and rib spacing, to the transmission frequency. The results demonstrate that the flex-layer can act as an impedance matching device when the parameters are chosen appropriately. To overcome the narrow bandwidth limitation of the two-plate system, a three-plate configuration is introduced. Theoretical analysis and numerical optimization show that the optimal design consists of a thick central plate separating two thinner plates, which can be interpreted as a mass-like impedance between two elastic elements. This configuration achieves broadband acoustic transmission. The approach is further extended to a three-dimensional structure using a rectangular array of pillar ribs, which makes the system manufacture-friendly while maintaining performance. Finally, the finite version of the structure is investigated using numerical analysis and finite element simulations. The results show that the performance of the finite system converges to that of the infinite periodic model as the number of ribs increases. The effects of boundary conditions, damping, and structural parameters on the transmission characteristics are also examined. Overall, the results prove that a simple, purely solid, periodically stiffened structure can serve as an effective impedance transformer between water and air. The proposed mechanism provides an analytically provable and a feasible approach for improving acoustic transmission across highly mismatched media.