Real-time focusing using an ultrasonic one channel time-reversal mirror coupled to a solid cavity

Focusing and beam steering is achieved by using a time-reversal process and a single transducer coupled to a solid cavity that is immersed in water. This low-cost technique makes it possible to focus acoustic energy anywhere on a 3D domain with a spatio-temporal resolution comparable to that of multiple transducers array. A short pulse is emitted from a transducer stuck at the surface of the solid cavity. The multiple-scattered field is measured in front of the solid cavity using a hydrophone needle at a reference point. This signal is then time reversed and remitted by the transducer. Around the reference point, one can observe a spatio-temporal recompression. The sidelobe level as well as the focal width no longer depend on the transducer aperture but on the dimensions of the solid cavity and the multiple paths covered by the acoustic waves in the solid. Moreover, it is shown how the experimental impulse responses on the front face of the cavity can be used to control the emitting ultrasonic field. This “synthetic time-reversal” technique is shown to be as powerful as a real time-reversal process.