Wavefront Engineering and Computational Sensing with Acoustic Metamaterials

Acoustic metamaterials are a family of engineered materials that can be designed to possess flexible acoustic properties. They are composed of subwavelength periodic structures that can be homogenized as effective materials within the designed frequency bands. Acoustic wave controlling devices with spatially inhomogeneous or/and anisotropic acoustic properties can be designed with metamaterials. The early versions of acoustic metamaterials generally share several drawbacks that limit their applications: relatively high loss, narrow bandwidth, as well as difficulty in fabricating multiple samples with uniform properties. In this work, we approach these issues with a family of geometry-based acoustic metamaterials and demonstrate several devices based on these building blocks with various wave manipulation functionalities. The presented acoustic metamaterial-based devices are categorized into two kinds. The first kind of devices, including negative refraction prism, planar acoustic lenses, beam-steering metasurfaces and phase acoustic holograms, control the propagation or the states of existence of acoustic waves. The second kind focuses on a reciprocal process—instead of controlling the forward propagation, the sensing signals are modulated with randomized resonant metamaterials to realize computation sensing.

Our research approach is summarized as follows: firstly, we designed various metamaterial unit cells as the building blocks, adding to the existing unit cell library. Particularly, a family of labyrinthine or space-coiling unit cells provide access to a broader materials parameters space previously inaccessible by conventional spring-mass model-based unit cell designs. Second, with the extended unit cell library, we designed thin planar wave modulation devices, including acoustic lenses and metasurfaces that can bend the acoustic beam as predicted by the Generalized Snell’s Law. Third, we extend the spatially inhomogeneous modulation from 1D to 2D by designing computer generated phase holograms. Last but not least, a metamaterial-based compressive sensor is designed and demonstrated for the localization of multiple audio sources and the separation of overlapping audio signals.