Negative refraction in conventional and additively manufactured phononic crystals

Phononic crystals are acoustic metamaterials designed to manipulate sound when its wavelength is in the order of magnitude of the crystal lattice constant. Metallic phononic crystals for use in water were assembled using commercially available stainless steel rods with an average superficial roughness R-a=0.5 +/- 0.5 mu m, while Laser Powder Bed Fabrication, an additive manufacturing technique, was employed to produce similar Inconel 718 structures with R-a=20 +/- 6 mu m. Experiments in the 150 - 500 kHz frequency range indicated that acoustic band gaps and negative refraction were present in both cases, with similar features. This indicates that Laser Powder Bed Fabrication is a promising method for realising such phononic crystals.