A novel acoustic metamaterial (AMM) sandwich panel with enhanced sound transmission loss (STL) is proposed and designed by periodically distributing the local resonators in the stiffened core. The vibro-acoustic coupling is considered by the continuity of velocity at the fluid–structure interfaces. The effective dynamic mass density method is used to investigate the influences of the local resonators on the reinforced STL of the sandwich panel. The space harmonic expansion method and the virtual work principle are applied to derive the dynamic equations. An acoustic experiment is carried out and compared with the theoretical result. Experimental and theoretical results show that the STL curve has a peak near the objective frequency of the local resonator. The vibro-acoustic behaviors are further investigated using the finite element method (FEM). The peaks and dips in the STL curves are explained by the vibration modes and stress distributions obtained by the FEM. The proposed AMM sandwich panel possesses improved sound insulation performance around the mass-air-mass resonance frequency. For specific working conditions, the present method can be used to improve the STL of sandwich plates by adding resonators.

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