Knocking over LEGO minifigures with time reversal focused vibrations: Understanding the physics and developing a museum demonstration

Abstract

Time reversal (TR) of impulse responses can be used to focus wave energy, even in a reverberant environment. A demonstration of TR focusing was developed to knock over a targeted LEGO minifigure, standing among other minifigures (who remain standing), by focusing plate vibrations under the target minifigure\rquote s feet. In order to achieve a high degree of repeatability, the physics of the demonstration needed to be understood. A laser Doppler vibrometer was used to compare the motion of the minifigure and the plate directly beneath it. Speaker shakers are used to generate motion of the plate. If their induced vibration was too large the minifigure loses contact with the plate prematurely, causing it to be in the air when the main focused vibrations arrive. If the minifigure is in contact with the plate when the focused vibrations arrive then the minifigure is launched into the air before toppling on its side. This demonstration was optimized to be a two-player-game museum exhibit in a wave propagation museum hosted by ETH Zurich in Switzerland. This demonstration illustrates the power of focused vibrations.