2019_programme: A METHODOLOGY FOR MEASURING THE PRESSURE LEVELS OF SOUND EMITTED BY TARGET MODES DRIVEN BY POINT-LIKE FORCES IN SMALL TANKS

• Session: 10. Design of new experimental facilities to address future problems in underwater acoustics
  Organiser(s): Sessarego Jean-Pierre, Fattaccioli Dominique, Real Gaultier
• Lecture: A METHODOLOGY FOR MEASURING THE PRESSURE LEVELS OF SOUND EMITTED BY TARGET MODES DRIVEN BY POINT-LIKE FORCES IN SMALL TANKS
  Paper ID: 800
  Author(s): Abawi Ahmad, Kirsteins Ivars, Marston Phil, Daniel Timothy
  Presenter: Abawi Ahmad
  Presentation type: oral
  Abstract: In earlier work [T. Daniel et al., JASA 140, 3123 (2016)], we experimentally demonstrated in a tank that the modes of elastic targets in water could be excited using modulated radiation pressure (MRP), generated by amplitude modulated focused ultrasound to create detectable sound emissions. Acoustic radiation pressure is linearly proportional to acoustic intensity, caused by the rate of change of momentum of an incident field scattered by an obstacle. A potential advantage of the MRP approach is that the narrow width of the ultrasound beam permits the generation of point-like forces on the target’s surface with surgical precision. \n\nAn important challenge that arose in this work was how to accurately measure the levels of the sound emitted by targets in a tank that was small in size relative to the sound wavelengths and the length of the point force tone burst excitation. When a hydrophone is used, the sound pressure measurements will generally be corrupted by reverberation from the tank boundaries and the water surface. Due to the small size of the tank, this reverberation cannot be eliminated by time gating. Here we develop a new multistep approach that is based on first using a laser vibrometer to measure the target’s surface velocity when it is driven at a mode. Then, the normal surface velocity and pressure for the particular mode are computed using a finite element model (FEM). These quantities are then scaled by the measured value of the surface velocity and are used in the Helmholtz-Kirchhoff equation to obtain the radiated pressure. \n\nSpecific examples are presented for circular plates, spheres, and cylindrically-shaped targets from MRP tank experiments conducted at Washington State University. We will also provide some rationale as to why tank reverberation should have little effect on this approach.
• Corresponding author: Dr Abawi Ahmad
  Affiliation: HLS Research
  Country: United States
  e-mail: