Super Enhancement of Elastic Wave Emission Using Extremely Low Effective Impedance Medium Realized by Paired Resonators

Abstract

This work is mainly dedicated about the development of passive and non-destructive super enhancement of elastic wave emission, and revealing the reason of enhancement by setting up a new effective medium theory. Ultrasonic transducers have been designed and improved to enhance its wave emission performance, but yet manipulation on the medium where the transducers are attached has not been considered. However, conventional non-destructive manipulation on the medium leads to higher impedance, which is not desirable for the super enhancement. Inversely, digging out the material lowers the mechanical impedance but it is also not desirable in practice. Therefore, a method to lower the impedance without damaging the medium should be introduced. <br/> <br/> In order to achieve the goal, we use a pair of resonators to be attached on the base medium. Since just the pair of resonators is not in a periodic arrangement, it requires a new tool to analyze the effective properties of the system. With this unique arrangement of resonators, the corresponding effective medium theory is newly established. Furthermore, since size of the paired resonator system is not in a subwavelength range, the existing methods for characterizing the effective medium cannot be applied directly. <br/> <br/> With the newly derived analysis, the equivalent effective medium is found and the effective impedance of the medium surrounded by the resonators is found to be extremely lowered than that of nominal medium. As a usage of the lowered impedance, ultrasonic transducers are attached on the region where the impedance is lowered to enhance their wave emissions. However, the high contrast of impedance does not assure the high transmission of wave generated by the transducers. This work reveals the condition which the wave can be highly generated and fully transmitted through the outer medium. As examples of the theory, super enhancements of three modes in plate, S0, SH0 and A0, are shown. <br/> <br/> To investigate the extreme low impedance and the super enhancement, we show a detailed theoretical analysis, Numerical simulations using finite element method and experimental results.