Measuring the modal shapes of elastic targets using modulated radiation pressure

Abstract

In earlier work [T. Daniel et al., JASA 140, 3123 (2016)], we had shown experimentally that the resonance modes of elastic targets in water could be excited using modulated radiation pressure (MRP) generated by focused ultrasound to create detectable sound emissions. A potential advantage of the MRP approach is that the narrow ultrasound beam permits the generation of point-like forces on the target’s surface with surgical precision. In addition to exciting target modes with this technique, we show that by physically scanning the surface of the target with the point-like force generated by MRP, the mode shape of the target itself can be determined. The mode shapes are determined from the variations of the amplitude and phase of the radiated sound or the target’s surface velocity obtained using a laser vibrometer as a function of force locations. Theory will be developed, and results will be presented from experiments conducted in a tank at Washington State University. In these experiments, the mode shapes of small plates, cylinders, and UXOs were measured with MRP using hydrophone and laser vibrometer measurements and compared against those computed using finite element models.