Topological edge states, immune to backscattering, present challenges in confining them to specific positions within a waveguide. Topological photonic cavities have been shown to localize edge states along their propagation path, opening unique opportunities for manipulating and collecting electromagnetic waves. In this Letter, we realize a geometry-dependent underwater topological ultrasound cavity, created using a valley sonic crystal waveguide terminated by a rigid reflector. The strong confinement of sound wave energy in the oblique reflector surface is experimentally observed through ultrasound frequency- and time-domain measurements, which is due to the extended time delay of the valley polarization flipping. We further demonstrate that the valley-flipping time at the reflector surface is dependent on the edge configuration of the valley sonic crystals and the orientation of the reflector. Our Letter introduces a strategy for designing topological acoustic cavities and achieving ultrasonic wave confinement in underwater environments.

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• Acoustic modeling
• Photonics
• Topological defects
• Waveguides
• Ultrasound techniques