An experimental study of acoustically induced rocking motion of simple asymmetric geometries

Abstract

Otoliths are stone-like structures in the inner ear of fish that play a crucial role in fish hearing. The original object of this research was to determine if any rocking motion was present in an otolith suspended in tissue phantom when subjected to a plane acoustic wave. Measuring the motion of an actual otolith proved to be beyond the limits of project's resources, so an aluminum hemisphere suspended in water was studied instead. The hemisphere was chosen because it was the easiest shape to measure accurately, had the asymmetry necessary to investigate the relevant physics, and had been the subject of some theoretical modeling. A plane standing wave was generated in a short open ended thick-walled cylindrical-waveguide with the waveguide's axis perpendicular to the symmetry axis of the hemisphere. Measurements were taken along the hemisphere from top to bottom to determine if any rocking actually occurred. The expected vertical vibrational motion and symmetry-forbidden horizontal vibrational motion were also measured. The horizontal displacement of the hemisphere at each point was determined by using an ultrasonic vibrometer. The vertical motion was measured using alternative other sensors and methods, such as an accelerometer and Laser Doppler Vibrometer (LDV). The results from this experiment showed a small amount of rocking, but less than predicted. The vertical motion was around ten times greater in magnitude than the rocking motion at the edge, where it is largest. Additional follow-up experiments were then conducted to determine if any experimental artifacts, such as position in the tank and method of mounting, contributed to the overall result. Additional testing was then done on a series of semicircular cylinders to determine if their motion matched theoretical predictions. In this case, rocking was also present and was found to be on the order of the motion of the hemispheres. This motion was found to be smaller than published theoretical results. These results can ultimately be used to predict and understand the motion of more complex geometries, like otoliths.