1. Department of Electrical and Computer Engineering Duke University Durham NC 27708 USA
2. Department of Mechanical Engineering Rowan University Glassboro NJ 08028 USA
3. Acoustics Division U.S. Naval Research Laboratory Code 7165 Washington DC 20375 USA
4. Applied Research Laboratories and Walker Department of Mechanical Engineering The University of Texas at Austin Austin TX 78713-8029 USA
5. Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge MA 02139 USA