Time reversal acoustics uses recorded sound to send a field back toward the region that originally emitted it. An array of acoustic transducers records a pulse at multiple positions, reverses each recording in time, and emits the reversed signals from the corresponding positions. In a stationary reciprocal medium, the different paths can then contribute coherently near the original source.
A laboratory demonstration places a short-pulse source in a tank, records it at an array, removes or silences that source, and re-emits the reversed records. Mapping the returning pressure pulse tests the refocusing directly. Multiple propagation paths need not prevent focusing if they remain stable and are recorded; they can supply additional path diversity. Finite recording duration, limited bandwidth, incomplete spatial coverage and acoustic attenuation limit the reconstruction. Reversing recordings therefore implements a particular wave-control experiment, rather than undoing every dissipative process in the medium.
Multiple scattering can enlarge the angular diversity reaching a receiver array. Reversing the paths then makes the array act like a larger aperture, improving the focus in appropriate regimes of wave propagation in a random medium.
For a linear operator mapping transmitted signals to recorded signals, the positive operator describes adjoint back-propagation after recording. In a reciprocal array, physical time reversal acoustics implements this operation up to a final complex conjugation and the chosen signal conventions.
The DORT method uses eigenvectors of a time reversal operator to focus on individual point scatterers. A separate nonzero eigenvalue corresponds to each scatterer in the ideal weak-scattering, well-resolved limit; coherent overlap and multiple scattering can change this correspondence.
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