Acoustic impact localization in plates: properties and stability to temperature variation

Localizing an impact generated by a simple finger knock on plate-shaped solid objects is made possible in an acoustic time reversal experiment. It is shown that the technique works with a single accelerometer. To better understand the phenomenon and to know exactly the nature of the created waves, a two-dimensional (2-D) elastic simulation is used, showing that in a very good approximation the A₀ Lamb mode is the only propagating one. However, it is shown that, within one wavelength distance from the edges, evanescent waves must be taken into account. As a first consequence, the ability to distinguish two neighboring impacts improves when the plate thickness decreases and the frequency increases. As a second consequence, it is expected theoretically that temperature variations lead to a stretching or a contraction of acoustic signatures. The experimental demonstration used a heterodyne interferometer to measure the impulse responses created by a knock on a plate during the cooling. A simple algorithm is shown to perfectly compensate for temperature impacts, which demonstrates the feasibility of the technique for outdoor time reversal interactive experiments