Acoustic time-reversal mirrors

Abstract

The objective of this paper is to show that time-reversal invariance can be exploited in acoustics to accurately control wave propagation through random propagating media as well as through waveguides or reverberant cavities. To illustrate these concepts, several experiments are presented. They show that, contrary to long-held beliefs, multiple scattering in random media and multi-pathing in waveguides and cavities enhances spatial resolution in time-reversal acoustics by making the effective size of time-reversal mirrors much larger than their physical size. Applications of time-reversal mirrors in pulse-echo detection are then described and, for a multi-target medium, the iterative time-reversal mode is presented. It will be shown how the study of the time-reversal operator allows us to select and to focus on each target through a distorting medium or inside a reverberant medium.