Research Projects

Research conducted by: Steven Tomsovic, Jian Huang, Eric J. Heller (Harvard University)

Introduction

In recent years, time-reversal (TR) mirrors have been developed to create TR waves for ultrasonic transient fields propagating through complex media. A TR wave back propagates and refocuses exactly at its initial source. One can verify the time-reversal invariance of the acoustic wave equation in a nondissipative medium by introducing a mirror of the time variable. In an image world such a mirror exists by taking a motion picture of the wave propagation and then running the film backwards. In the real world reversing the time variable means generating experimentally a back-propagated replica from a forward propagation field.

Our work

From Huygens principle, the mirror of the time variable can be replaced by a time-reversal operation performed on a closed surface that surrounds the initial source location. This active surface, called a time-reversal (TR) cavity, is covered with reversible transducers (that act both as microphones and loudspeakers). In the first step, a pointlike source emits a pulsed field that is measured by the transducers. In a second step, the recorded field is time reversed and then retransmitted back into the medium by the same transducers. As the wave is back propagated from all directions, the focusing is perfectly isotropic around the initial source position with a focal spot as narrow as one half wavelength. Time-reversal experiments have been performed through many different complex media including scatterer forests, acoustical waveguides and biological tissues for various purposes such as nondestructive testing, medical acoustic or ocean underwater acoustic.