Time Reversal Acoustics
Time Reversal is a signal processing technique that can be used to focus wave energy to a selected point in space. It started in the 1960s as a technique for signal transmission in the ocean between ships. It has been used for underwater acoustic communications, biomedical lithotripsy using focused ultrasound, earthquake localization and characterization, detecting and imaging cracks and defects in structures (nondestructive evaluation), and focusing sound in rooms. At BYU, Brian Anderson and his students have extended the use of time reversal for nondestructive evaluation and have been exploring the use of time reversal to produce focused sound in rooms.
B. E. Anderson, M. C. Remillieux, P.-Y. Le Bas, and T. J. Ulrich, “Time reversal techniques,” Chapter 14 in Nonlinear Acoustic Techniques for Nondestructive Evaluation, 1st Edition, Editor Tribikram Kundu, ISBN: 978-3-319-94476-0 (Springer and Acoustical Society of America), pp. 547-581 (2018). http://dx.doi.org/10.1007/978-3-319-94476-0
LEGO Demonstration of Time Reversal Focusing
In order to demonstrate the power of time reversal focusing we have conducted some fun experiments to knock over selected LEGO minifigures using focused vibrations. Vibrations are generated in a plate using speaker shakers that are designed to play music through your desk or table top. An impulse response between each shaker and a target location is obtained and then these impulse responses are reversed in time and played simultaneously from the shaker(s) (sometimes we only use one!). Waves then constructively interfere at the target location, focusing energy momentarily at that location. We can then knock over a LEGO minifigure at that location without knocking over other nearby minifigures. Check out the video below showing experimental data that shows waves being focused and then some fun demos of LEGO minifigures being knocked over. BYU produced a nice video of the demonstration as well. This demonstration will be featured in a Wave Propagation Museum exhibit at ETH Zurich hopefully starting in the summer 2021.
More information about this demonstration can be found in a paper we wrote:
C. Heaton, B. E. Anderson, and S. M. Young, “Time reversal focusing of elastic waves in plates for educational demonstration purposes,” J. Acoust. Soc. Am. 141(2), 1084-1092 (2017). http://dx.doi.org/10.1121/1.4976070