Design and characterization of broadband acoustic composite metamaterials

We demonstrate a method to design nonresonant acoustic composite metamaterials made of periodic arrangements of highly subwavelength unit cells composed of one or more inclusions embedded in a fluid. The inclusion geometry determines the degree of anisotropy and effective material parameters, and its nonresonant nature makes it suitable for large bandwidth applications. To characterize the resulting acoustic metamaterial, two sets of normal incidence plane wave reflection and transmission simulations on thin samples are performed. Applying this method, we show that samples as thin as one unit cell in the propagation direction are sufficient to characterize the composite. Different simple unit cell geometries were considered in order to bound the achievable material parameters of such composites. The large range of obtainable parameters, including strong anisotropy, makes this approach suitable for creating the materials needed for applications of transformation acoustics. As an example, we illustrate our method by designing and simulating a physically realizable implementation of a beam-bending flat lens that theoretically requires complex inhomogeneous materials not easily available in nature.