Acoustic wave focusing using a 2.5-D graded metamaterial lens

• Published in: The Journal of the Acoustical Society of America 2020-10, Vol.148 (4), p.2538-2538
• Main Authors: Zhao, Yuanyan, Subramanian, Sriram, Memoli, Gianluca
• Format: Article
• Language: English
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.5147047

The creation of a three-dimensional focal spot in air underpins applications in many fields. The literature presents many examples of focusing with acoustic metamaterials, but they are difficult to manufacture in large scale. Sonic crystals, are easier to make—since they are periodic arrangements of simple scatterers—but there are not many studies on achieving 3-D acoustic convergence through a 2-D sonic structure in air. In this study, we exploit the methods of sonic crystals to design the acoustic equivalent of a gradient index (GRIN) lens. We show that an extruded 2-D hexagonal lattice array of rigid cylinders with gradient diameters can be used to achieve the focusing of a 3-D shaped beam at audio frequencies i.e., a 2.5-D lens. We use finite-elements simulations with a commercial software to describe the device's performance in terms of band structure and equifrequency contours. We also present some preliminary measurements taken using a 3-D-printed model of our lens, designed for a focal length of 3.1 cm at 8 kHz. We find our 2.5-D GRIN lens works over a bandwidth of almost one octave. We discuss potential future uses, like in the correction of acoustical aberrations and in devices of flexible focal length.