Modes of targets in water excited and identified using the radiation pressure of modulated focused ultrasound

Abstract

Previously, we excited low frequency flexural modes of a circular plate in water using modulated radiation pressure generated by focused ultrasound [J. Acoust. Soc. Am. 137, 2439 (2015)]. This effect is distinct from excitations associated with parametric array sonar in that static deformation can be produced by radiation pressure as has been measured and analyzed for bubbles [T. J. Asaki and P. L. Marston, J. Acoust. Soc. Am. 97, 2138-2143 (1995)]. In our experiments, solid targets are suspended by lines or supported on sand and the modulated ultrasound is focused on the target’s surface. Target sound emissions were recorded and a laser vibrometer was used to measure the surface velocity of the target to give the magnitude of the target response. An improved high-power focused transducer allows us to drive modes of larger more-realistic target models. By varying the modulation frequency and monitoring the target response, resonant frequencies of the target can be found. By scanning the point-like driving force, the target mode shapes can be measured and compared to finite element models. [Work supported by the Office of Naval Research.]