Acoustic metamaterials are engineered materials with properties hard or impossible to find in natural materials (e.g. negative effective density and/or negative bulk modulus). Therefore, a myriad of novel applications could be imagined and some of them have already been theoretically and/or experimentally demonstrated. Gradient index acoustic lenses, acoustic cloaks or acoustic absorbing panels are some common examples. Here, we review the coordinate transformation approach (transformation acoustics) which provides the material parameters needed to precisely control the acoustic wave propagation. Then, we use this technique to design an acoustic black hole and a 3D acoustic ground cloak. We use numerical simulations to explore the practical feasibility of the material parameters required by these applications and design non-resonant, highly sub-wavelength unit cells that will implement them in practice.

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