Optical detection of mach shock wave cones in water using refracto-vibrometry

Recently, a hydrophone array was used to demonstrate Mach shock wave cone formation in water when 1m diameter steel pile rods for civil structures were driven into the ground by massive impact hammers. These shock wavefronts are of concern for marine mammals and fish. In the current project, Mach shockwave cones were directly imaged using refracto-vibrometry. A Polytec PSV-400 laser Doppler vibrometer was directed through a water tank towards a stationary retroreflective surface. The density variations of acoustic wavefronts which pass through the laser cause variations in the optical path length between the laser and retroreflector. This results in a time-varying modulation of the laser signal returning to the vibrometer, enabling optical detection of the acoustic wavefronts. A 35 mm diameter rod (steel or other metal) immersed in a water tank was repeatedly “impacted” by narrow pulses from a 1 MHz ultrasound transducer. The vibrometer sampled numerous scan points to generate videos of the time evolution...