Preliminary results of numerical analysis of sound field converged by convex acoustic lens with solid-liquid compound structure

We already designed and made a concave aspherical lens for Ambient Noise Imaging (ANI). For surveying underwater objects in a vast expanse of ocean, it is necessary to mount an ANI system into a movable vessel such as an autonomous underwater vehicle (AUV). However, the concave lens is not suitable to mount it on the bow of AUV. Because the concave lens does not fit to the bow shape, its water resistance is large. Our group has studied a convex lens to mount on an AUV’s bow. This lens is composed with solid lens faced to sea water, and inner liquid placed in the AUV’s bow. Recently, we measured the sound velocities and refractive indexes for some materials suitable to the lens, and the optimized aspherical surfaces of the lens were determined by the ray tracing method. In this study, the sound pressure field converged by the designed lens is calculated by the 2-D finite difference time domain method. The results showed that the designed lens has a potential, in which the directional resolution of this lens is less than 1 degree, for applying to an ANI system mounted in an AUV’s bow.