Acoustic time-reversal is a well-established technique that focuses an ultrasonic wave on the location of its source. It is based on the time-reversal invariance of the wave equation and is usually implemented using time-reversal mirrors made up of a hundred of piezoelectric transducers. However, a time-reversal experiment can be performed in a closed cavity (a water-filled beaker) with only one transducer as a pulse-echo system. This easy-to-build and low cost experiment involves students in the general concept of the time-reversal invariance of the wave equation. We show that it also can be adapted to become an ultrasonic time-reversal thermometer. A careful study of the focal point shows a dependence of its position as a function of temperature variations.

Topics
Ultrasound, Ultrasonics, Acoustic signal processing, Acoustic waves, Thermal instruments, Transducers, Partial differential equations, Geometrical optics, Particle symmetry, Education
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© 2004 American Association of Physics Teachers.
2004
American Association of Physics Teachers
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