VDM Berebichez Ph. D. , D: Time-Reversal of Waves for Spatial Fo

Time-reversal is the process whereby a signal is emitted by a source and propagates in a medium with scatterers in it. The waves are recorded at a receiver over a time much larger than the original pulse duration so as to capture the multiple scattering. The receiver then reverses the waveform in time and re-emits it into the same channel. The waves back-propagate through the medium retracing the multiple scattering paths and ultimately refocus at the original source location in space and time. Time-reversal in an enclosure with a single detector achieves spatial focusing of the energy to within one half of a wavelength of the initial sound signal. A two- dimensional finite-difference time-domain code was calibrated to study the phenomena of time compression and spatial focusing of time-reversed acoustic signals in closed environments with strong reverberations. One of our main results is that time-reversal in enclosures with a keyhole aperture of size comparable to the width of the initial wave front achieves tight spatial focusing of the acoustic field at the source. We show that time-reversal can deliver trains of pulses to desired locations. ...