Legos are a beloved staple of educational science activities and have even proved useful in particle physics experiments at CERN to explore the properties of hadrons. For Brian Anderson, a physicist at Brigham Young University, Legos are an essential component of his acoustics research. At a meeting of the Acoustical Society of America in Seattle earlier this month, Anderson described how he figured out how to focus sound-wave energy precisely enough to knock over a single Lego minifig without disturbing other minifigs clustered around it.
The key is a signal-processing technique called “time reversal,” originally used by submarines in the 1960s to help focus signal transmission in the ocean. The name is a bit misleading, since it’s sound waves that are being reversed, not time. The technique involves playing a sound (impulse) from a sound source—Anderson uses speakers for playing music through a computer or laptop—and using a sensor (like a microphone or a laser) at a targeted location on a metal plate to record the response to the impulse there.
That recording essentially maps the acoustic wave as it bounces around. One can then use software to reverse that signal and play it back so the waves retrace their steps and constructively interfere with each other, enabling Anderson to precisely focus that acoustic energy on the targeted location. The spatial extent of the focusing depends on the frequencies being used. Higher frequencies typically have smaller wavelengths, enabling Anderson to focus the acoustic energy to a more narrow point in space.
“Time reversal is really like ventriloquism,” Anderson said. “But instead of throwing our voice to another place, we’re focusing vibrations at a target location that may be far from where the vibrations originated.”
He has also likened this effect to the “whispering gallery” phenomenon, usually observed in rooms with an elliptical-shaped ceiling, which produces a natural focusing effect. So someone standing in one location can whisper and be overheard clearly by another person standing somewhere else. (St. Paul’s Cathedral in London is the most famous example. It’s where Lord Rayleigh first discovered whispering-gallery waves around 1878.) Anderson’s time-reversal technique enables him to turn any room into a whispering gallery.
To make the aural effect more visual, Anderson borrowed his children’s Lego mini-figures and brought them to the lab. He set them up on a metal plate and used time-reversal forced vibration to target one specific minifig and knock it over. “I promise you, there’s nothing flicking the plate underneath,” Anderson said at an ASA press conference. “It’s because the waves from the two speakers are only converging and producing a large amplitude right beneath the [targeted] minifig.”
Those initial experiments were featured in a 2017 paper in the Journal of the Acoustical Society of America, and the Lego connection ensured that the paper generated substantial media coverage. “Initially, I was a little bit worried because I was playing with toys in the lab,” Anderson admitted. “But it turns out everybody loves Lego, especially children.”