Acoustic radiation of sub-wavelength conical concentrators for local generation of ultrasonic waves

This study investigates the acoustic radiation of axisymmetric conical concentrators with a sub-wavelength radiating aperture designed for localized ultrasound generation. The system is composed of a piezoelectric disk coupled to a conical transition, radiating into the target medium through an aperture smaller than the acoustic wavelength. The originality of this work lies in the integration of the energetic formulation that quantifies, over the useful bandwidth, the injected electrical power, the transmitted mechanical power, and the radiated acoustic power. This enables a rigorous evaluation of the conversion efficiencies associated with geometric transitions. A multiphysics 2D axisymmetric finite element model (FEM) is used to investigate the influence of key geometric parameters. The results show that optimal acoustic amplification is obtained for conical concentrators with 45° conical transitions. Indeed, at this angle, the interactions between propagating pressure waves and internally reflected wavefronts within the cone result in constructive axial focusing. Increasing the diameter of the piezoelectric source significantly enhances the axial pressure and radiated acoustic power, but also increases internal mechanical losses, leading to a reduction in overall electroacoustic efficiency. Experimental validation performed in an immersion tank for two numerically identified efficient configurations operating at 3 MHz shows good agreement with the numerical trends. These findings highlight the importance of a joint optimization of the conical’s geometric parameters to maximize energy transfer and acoustic radiation in miniaturized ultrasonic concentrators.