2D pseudo-array using an ultrasonic one channel time-reversal mirror

Focusing and beam forming is achieved using a time-reversal process and a unique transducer coupled to a solid cavity. This technique allows one to focus acoustic energy anywhere on a 3D domain with a spatio-temporal resolution comparable to a multiple transducer array. We first record the signal emitted by the transducer and detected by a hydrophone needle at a reference point. The signal received is then time-reversed and remitted using the same transducer. At the reference point, one can observe a spatio-temporal recompression. Moreover, it is shown how the experimental Green´s functions at the surface of the cavity can be used to control the emitting ultrasonic field. The solid cavity becomes a 2D pseudo-array. A careful study of this phenomenon leads to a better understanding of the resolution of the focusing system; according to diffraction theory, the focal width is no longer dependent on the transducer aperture but on the dimensions of the solid cavity. The signal-to-noise ratio of such a system is then explained by a modal theory and leads to four influencing parameters being defined: frequency bandwidth; number of transducers; cavity geometry; time reversed signal duration. Finally, a one channel inverse filter based on the frequency analysis is defined; its efficiency is compared to the time reversal one and linked to the modal theory.