Passive and Active Graded-Index Acoustic Metamaterials: Spatial and Frequency Domain Multiplexing

摘要

Acoustic metamaterials, similar to their electromagnetic counterparts, are artificial subwavelength materials designed to manipulate sound waves. By tailoring the material's effective properties such as bulk modulus, mass density, and reflective index, these materials can be designed to achieve unprecedented acoustic waves control and realize functional devices of novel properties. Specifically, high-refractive-index acoustic metamaterials have an effective refractive index much larger than air, enabling wave compression in space and a strong concentration of wave energy. Another type of acoustic metamaterials closely related to high-index acoustic metamaterials is graded-index metamaterials, which can be obtained by gradually varying material compositions or geometry over a volume of high-index acoustic metamaterials.The overall goal of this dissertation is to achieve a fundamental understanding of passive and active graded-index acoustic metamaterials for spatial and frequency domain multiplexing and explore their applications in far-field acoustic imaging and sonar systems. Three research thrusts have been pursued. In the first thrust, the spatial domain multiplexing of passive graded-index acoustic metamaterials has been investigated for enhancing far-field acoustic imaging. An array of passive graded-index acoustic metamaterials has been designed and developed to achieve a far-field acoustic imaging system. Parametric studies have been carried out to facilitate the performance optimization of the imaging system. The performance of the metamaterial-based imaging system has been investigated and compared to the scenario without the metamaterials. In the second thrust, frequency-domain multiplexing with active graded-index acoustic metamaterials has been investigated. An active graded-index metamaterial system with a number of active unit cells has been designed and fabricated. A fundamental understanding of the frequency multiplexing properties of the metamaterials has been developed through numerical and experimental studies. In the third thrust, the capabilities of an acoustic sensing system with active graded-index metamaterials as an emitter for shape, size, and surface classification have been explored.