Recently the presence of a Dirac cone within the band structure of graphene has inspired research on phononic crystals with Dirac-like behaviors — including structures mimicking zero refractive index materials. The interesting phenomena produced by these structures occur at fixed frequencies and cannot be adaptive to needs and environmental changes. To address this constraint, researchers have designed tunable phononic structures; however, the tunable frequency ranges from the studies reported to date are limited by geometric constraints. Using a reconfigurable origami structure to modulate between different classes of phononic Bravais lattices, this research numerically investigates the effects of phononic lattice perturbation to produce drastic changes in the frequency of useful accidental degeneracies.

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.

Significant research has recently been dedicated to reducing the interaction between acoustic waves and given objects. The coordinate transformation theory provides an elegant approach to the design of coatings capable of reaching this goal. First demonstrated for electromagnetic waves [1], this method has later been adapted to acoustic waves [2–5]. Its appeal stems from its generality: it can be applied to obtain coatings capable to conceal objects of arbitrary shapes and sizes. Moreover, its efficacy depends only on our ability to create materials having the right acoustic properties. However, these properties are quite restrictive. They include inhomogeneity, i.e. material parameters that vary continuously with position, and a high degree of anisotropy. In practice, one would relax these constraints but reduce the performance of the design. For example, in many instances an inhomogeneous material can be approximated by thin layers of homogeneous media. However, a large number of such layers are usually required to obtain acceptable results [6].