Metamaterial ring for omnidirectional shear-horizontal elastic waves: from working mechanism to experiment

High transduction efficiency is essential in developing any transducer. This efficiency issue becomes more critical with omnidirectional waves, such as omnidirectional SH (shear-horizontal) waves in a plate, because the generated power decreases as an inverse square function of the radial coordinate. To substantially enhance the efficiency, we propose a metamaterial ring ('meta-ring') that can be attached to a target plate without directly modifying the transducer. We establish a rigorous theory to explain the transduction efficiency boosting mechanism for an omnidirectional SH wave that typical ultrasonic transducers can generate. Our meta-ring concepts are based on adjusting the effective characteristic impedance of the plate carrying the wave and an elaborate utilization of the Fabry-Perrot resonance. We use an equivalent mass-spring model for the theoretical analysis and employ the effective medium concept. The transduction efficiency enhancement by the proposed meta-ring, as confirmed by wave experiments performed in an aluminum plate, was over 250%. The proposed meta-ring method does not require any active element, only requiring external attachment of a T-shaped ring element. Therefore, it can be a new way to enhance the transducer in various applications for non-destructive testing, supported by a theoretical foundation.