Two-Step Fabricated Metastructured Acoustic Windows for Gradient Acoustic-Impedance Matching in 40 kHz Sonicators

High-intensity sonication systems operating at tens of kilohertz deliver acoustic energy from a Langevin transducer into a liquid medium, where the severe impedance mismatch between the metallic head mass and the liquid causes substantial reflection at the vessel wall and limits the pressure available for cavitation. Here, we present a two-step fabricated metastructured acoustic window that realizes continuous gradient acoustic-impedance matching for 40 kHz Langevin-coupled sonicators. The window is fabricated by printing a micro-pyramid array in glass-filled photopolymer, which features a high acoustic impedance of 6.3 MRayl, using a micro-precision stereolithography (SLA) printer, and casting low-viscosity epoxy (3.1 MRayl) into the interstices of the pyramid array, followed by room-temperature curing. Since the resin fill fraction decreases monotonically along the thickness direction, the effective acoustic impedance transitions continuously from the resin-rich base toward the liquid, thereby enhancing pressure transmission into the working medium near the targeted frequency (40 kHz). The performance of the gradient matching window was evaluated by k-Wave simulations, finite element analysis (FEA), and experiments with a 40 kHz Langevin sonicator. Numerical simulations indicated higher local pressure near 40 kHz, while relative receiver-voltage measurements provided preliminary support for the predicted output trend. The proposed two-step fabrication technique combining SLA printing and low-viscosity epoxy casting provides a simple and cost-effective strategy for gradient impedance matching and is expected to facilitate rapid prototyping and design of 40 kHz high-power sonicator components.