Input impedance-matching metamaterial to enhance ultrasonic pulse transmission through an acoustic barrier

Ultrasonic pulse energy is poorly transmitted through acoustic barriers due to impedance mismatch with the surrounding medium. This mismatch significantly limits the performance of many ultrasound-based applications such as biomedical imaging, nondestructive testing, and therapeutic ultrasound. In this study, we design metamaterials to enhance pulse transmission through the acoustic barrier. By introducing a virtual intermediate layer, an original problem—designing a metamaterial—is decomposed into subproblems that are mathematically equivalent to the original problem, enabling a broader design space and improved metamaterial performance. The metamaterial matches the input impedance to the characteristic impedance of the incident medium, achieving a transmittance greater than 0.5 over a bandwidth of approximately 20% relative to the center frequency. The designed metamaterial is fabricated using wire electrical discharge machining (WEDM), and its pulse transmission performance is experimentally verified in a water tank. A comparison of transmitted signals with and without the metamaterial shows that the ultrasonic pulse energy is increased by a factor of 7.78. This enhancement greatly benefits signal processing in pulse-echo applications such as imaging and structural defect detection.