Double-negative acoustic metamaterial based on meta-molecule

This work presents a double-negative acoustic metamaterial (AM) constructed using unitary meta-molecule structure. Analogy to electromagnetic metamaterials, transmission and reflection parameters are obtained by experiments in free space. The refractive index, effective mass density, and effective bulk modulus are found to be negative in a specific frequency region by retrieving the effective parameter from experimental data. In contrast to the reference acoustic field distribution through sponge substrate, the slab focusing phenomenon of the fabricated metamaterial lens is clearly displayed within the range of double-negative frequency at 5.7 kHz. The negative refraction is demonstrated in a triangular sample with refraction index of -0.78, which agrees well with the derived results. The proposed meta-molecule model is an effective approach to preparing integrative AMs, and the method of studying AMs is extended to completely free space.