A Method for Focusing Sound in Harbor Environments at Low Frequencies: Theory and Experiment

Open Access
Author:
Kankey, Andrew Thomas
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 03, 2008
Committee Members:
  • Gary Hugo Koopmann, Dissertation Advisor
  • Gary Hugo Koopmann, Committee Chair
  • Christopher Rahn, Committee Member
  • David Bradley, Committee Member
  • Kyle M Becker, Committee Member
Keywords:
  • acoustic array
  • harbor acoustics
  • underwater acoustics
  • sound focusing
Abstract:
A new method to focus low frequency acoustic energy in harbor environments was developed as a means of deterring swimmers with malicious intent. This method requires a priori calibration data from the harbor and picks the optimal phases which result in constructive interference at hydrophone locations. The phasing results from this Optimal Phase Search (OPS) method compare favorably with results from classic linear array theory in the experimental test harbor, Coddington Cove. Phasing and sound pressure level (SPL) data from two different source arrays are presented for the harbor at 100 and 200 Hz. An acoustic finite element method (FEM) code has also been developed to predict the sound level in the harbor and assist in understanding the phasing. The code is a simplified FEM code developed in MATLAB using linear shape functions and cuboid elements. The harbor geometry is voxelized and each voxel is turned into an element for the FEM program. The code is validated with numerous simple acoustic enclosures and a more complex underwater wedge. The results from the harbor model are compared to classic spreading theory and experimental data from Coddington Cove. The adaptability of the FEM code is discussed as well as areas of possible future work.