Super resolution focusing of sound among a Helmholtz resonator array: Nonlinear aspects

Time reversal (TR) is a technique used to focus wave energy to a selected location. High energy TR focusing has applications in biomedical ultrasound and nondestructive evaluation of cracks or defects in solids. These applications can benefit from the ability to narrow the spatial extent of the focused sound energy. Two-dimensional Helmholtz resonator arrays placed in the near field of TR focusing have been shown to produce a sub-diffraction limited spatial extent of the focused energy (when compared to the free-space wavelength). There is an apparent amplitude dependence on this focus and this presentation will discuss these nonlinear aspects. These observations were made by analyzing the experimental results of TR focusing on an array of soda cans at different sound levels. These nonlinear effects occur at much lower sound levels than is typical for nonlinear waveform steepening.