We report the numerical design and preliminary experiment of 2-D acoustic metamaterials composed of a planar network of subwavelength Helmholtz resonators. The considerably smaller size of the Helmholtz resonator to the corresponding resonant wavelength, allows a compact and light weight design for kilohertz frequency applications. Based on transmission line model to describe the acoustic wave propagation inside such ultrasonic metamaterials, we derived the acoustic parameters such as effective density and compressibility. Extremely large or even negative value of effective density and compressibility can be designed in this acoustic metamaterial. Our simulation demonstrates the focusing and imaging of sound sources through different lenses made of this novel acoustic metamaterial, which may have great potential application in ultrasound imaging. The influences of frequency and source position on the property of the focused image are also investigated.

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