Negative-dynamic-mass response without localized resonance

We propose a new mechanism to achieve negative-dynamic-mass response. The system consists of a water-immersed soft solid plate patterned with periodical heavy gratings. The negative dynamic mass stems from collective excitations of the coupled acoustic surface waves in the thin plate, but not from local resonances as usual. A simple spring-mass model is further applied to understand this exotic phenomenon. Extensive applications of the new mechanism can be anticipated, such as in designing acoustic metamaterials with negative indices.