Acoustic metamaterials are large-scale materials with small-scale structures. These structures allow for unusual interaction with propagating sound and endow the large-scale material with exceptional acoustic properties not found in normal materials. However, their multi-scale nature means that the manufacture of these materials is not trivial, often requiring micron-scale resolution over centimetre length scales. In this review, we bring together a variety of acoustic metamaterial designs and separately discuss ways to create them using the latest trends in additive manufacturing. We highlight the advantages and disadvantages of different techniques that act as barriers towards the development of realisable acoustic metamaterials for practical audio and ultrasonic applications and speculate on potential future developments.

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2-D periodic lattices and finite grids of lumped L-C elements have been investigated to elucidate the mechanisms governing wave phenomena in this kind of metamaterial. Isofrequencies for a lattice unit cell and circuit simulations of finite meshes have shown that such a medium exhibits unusual properties of channeling power from a point source into pencil beams which can propagate only along specific trajectories. The beam direction varies with frequency, and at the resonance frequency ω=1/(LC) 1/2 , the phase and group velocities of the transmitted wave are orthogonal. It is shown that the channeled beams do not obey the laws of geometrical optics (Snell's law, Brewster angle, etc.) at the skewed interfaces of dissimilar meshes, and that beam scattering is governed by the local conditions of the phase synchronization and impedance matching. Finally a new physical planar L-C mesh unit cell arrangement has been designed and measured.Index Terms -anisotropic metamaterial, backward waves, negative refraction, resonance cone, power flow, RLC mesh.

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