Far-Field Acoustic Imaging Enhancement Using Compact Graded Refractive Index Metamaterials

Acoustic metamaterials (AMMs) are promising in high-performance acoustic detection and imaging; however, few can be combined with sensors to form a practical detector, and most imaging attempts use under near-field conditions. This article provides a systematic method of using a compact graded refractive index (GRIN) AMM with an embedded microelectromechanical systems (MEMS) microphone to enhance far-field acoustic sensing and imaging. The GRIN AMM device has frequency selective amplification capability and noticeable directivity; these attributes can increase the detection range, suppress the aliasing of the multitarget main and grating lobes, and support arrays with large element separation and high angular resolution. An imaging enhancement mechanism using the GRIN AMMs is proposed and elucidated. Experiments demonstrate that the improved phased array with compact AMMs can image a 315 mm pipe as far as 67 m away and has an angular resolution up to 0.76°. Finally, the improved acoustic imaging system with an AMM array is transplanted to an FPGA-based robot to demonstrate active acoustic target-tracking application in the field.