Loosely coupled reflective impedance metasurfaces: Precise manipulation of waterborne sound by topology optimization

Abstract

Waterborne acoustic metasurfaces (WAMs), unlike airborne acoustic metasurfaces, generally suffer from nonlocal interaction induced by the strong fluid–solid coupling. Unpredictable parasitic scattering is usually caused by nonlocal vibration coupling between solid units, thereby interfering with the prescribed sound control. In this work, we propose a loosely coupled reflective impedance WAM to realize precise manipulation of waterborne sound. A function–structure integrated topology optimization framework involving surface impedance model and vibration coupling restriction is systematically established to inversely design WAMs. Successful individual design of lossy units with multiple inclusions is realized by weakening vibration coupling of solids through local resonance. Various customized waterborne sound fields, including extraordinary beam steering with steep deflection, propagating-to-evanescent mode conversion and super-resolution near-field focusing, have been successfully implemented and demonstrated. Good agreements between the theoretical and simulated results are observed, thereby validating the feasibility and effectiveness of the design strategy. In addition, inverse-design WAMs show very good robustness to the operating frequency, incident angle and geometrical shape. Deep-subwavelength precise manipulation of waterborne sound is further achieved by the WAM with the thickness of λ/40. The excellent performances of WAMs result from the precisely customized surface impedance distribution. The proposed strategy opens the door to practical applications of metasurfaces in the delicate and complex waterborne sound control.