Time-Reversed Experiments with Acoustics

In many mechanical systems the propagation of elastic and acoustical waves is reversible in time. A time-reversed reconstruction of the original signal can be obtained by recording the evolving signal, than reversing the recorded signal, and sending the reversed signal again through the system. The reconstruction of the time-reversed signal occurs in time as well as in space. The process is very robust and works even in systems with multiple scattering of the waves due to complex boundaries or heterogeneous material properties. Examples of such complex signals are the ‘Coda ’ response in seismology and reverberation in room acoustics. In open systems where the waves can escape the region of interest, multiple transducers at the boundary of the region are required to perform an accurate reconstruction. The array of transducers acts as a time-reversal mirror refocusing the reversed signal to the original location of the source. In cavity like closed spaces a single transducer pair is sufficient to perform the time-reversed reconstruction. Any part of the reverberating source signal in a chaotic cavity reaches the receiving transducer after a while and contains enough information to reconstruct the original signal by time reversion. Braking of the time-reversal symmetry occurs for example by absorption, changes in acoustic velocity and dynamical changes of the structure. Such changes immediately influence the reconstruction quality. Time reversed reconstruction has many applications in non destructive materials testing and medical research. Laboratory experiments in a liquid filled bottle and initial audio experiments in a reverberating room are presented to illustrate some of the concepts. 1