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."
The people in charge of an interactive science outreach museum on wave propagation ("Waves: Dive In!") at ETH Zurich in Switzerland were so impressed by that initial demonstration that they invited Anderson to design a version suitable for inclusion in the museum. That required figuring out how to improve the repeatability of the demonstration, since even in the highly controlled conditions of the laboratory, the demo only worked roughly one-third of the time.
The issue turned out to be too much amplitude. One of Anderson's students decided to turn the laser used to measure vibrations onto the minifig during the experiments to measure what was happening to it. The targeted Lego minifig would bounce just a little before and after the focusing of the acoustic wave, such that it was often in the air, rather than in contact with the metal plate, when the vibration hit.
"We were making the Lego vibrate on the plate before we wanted it to," Anderson told Ars. "Thus, the Lego would miss the main focus of energy that was intended to launch it into the air." Turning down the amplitude to the optimal level—enough to knock over the minifig but not enough to cause a premature bounce—resolved the issue and produced much more consistent launches.
"We also found that a different version of time-reversal signal processing, called time-reversal inverse filtering, was helpful to give us a cleaner focus of energy rather than the technique we were using, called clipping time reversal, whose main purpose is to maximize the amplitude of the focusing," said Anderson
Anderson's lab experiments relied on a $250,000 laser in order to sense the vibrations. "That's a little overkill if you're trying to turn this into a practical museum demonstration," he said. "We wanted to come up with a cheaper sensor." The solution: an eddy current sensor, which proved quite efficient at detecting vibrations in the metal plates. He and his team also optimized the pitch and frequencies used in the demonstration and determined the best thickness for the metal plate, ultimately achieving 100 percent reproducibility. A paper describing this latest work is currently under review for publication.
However, for the museum exhibit, the researchers intentionally decreased the reproducibility a bit to turn the demo into something more akin to a game of chance, allowing two children to compete to see who can knock down the other's minifig first. They just need to click on an interactive screen to choose where they each want to focus the acoustic energy—"ideally below a minifig"—and this moves their eddy current sensor below that spot.
The tightly focused vibration might be sufficient to knock over a minifig or break up a kidney stone in situ, but don't expect to see powerful sonic blasters any time soon. We're also not likely to see large, targeted seismic weapons capable of, say, destabilizing the spin at the Earth's core—the (admittedly ludicrous) premise of the 2003 film The Core. (In 2011, scientists at NASA's Jet Propulsion Lab rated The Core as being among the most notable examples of bad science in film.) But the potential to harness the power of sound on a smaller scale so precisely will no doubt enable plenty of other applications.
