Focusing sound waves is child’s play

In the 2003 sci-fi movie The Core, the Earth faced catastrophe in the aftermath of tests of a weapon designed to create focused seismic waves that could destroy enemy cities from half the planet away.

The movie, for the most part, is scientific bunk. But the basic idea of focused vibrational waves isn’t, says Brian Anderson, a researcher at Brigham Young University, Salt Lake City, Utah.

At the 181st meeting of the Acoustical Society of America, held earlier this month in Seattle, Washington, Young unveiled a demo of the power of focused sound waves, in which he put Lego figures on a plate, then sent ‘time reversed’ sound waves though the plate in order to knock one figure into the air (and flip it onto its head) while leaving the others undisturbed.

The details are complex, but the concept isn’t. “Time reversal is a method to focus wave energy to a point in space,” he says.

Nor is the use of Lego all that important to the overall picture. “I stole my kids’ Legos one night when nobody was looking, and set them up and did this experiment,” says Anderson.

It took a while to get it right, he admits, but once perfected, he could pick any given Lego figure out of a phalanx, focus his sound waves on the plate beneath it, and flip it end-over-end without disturbing any of the other figures.

He compares it to ventriloquism, in which a puppet master appears to ‘throw’ his or her voice to the puppet. But in this case, he says, “it’s real”.

The primary focus of Anderson’s work has been to develop a museum exhibit with which to demonstrate to children the power of focused sound waves. “Everyone loves Legos,” he says.

But there are lots of real-world applications… and no, science-fictional sonic blasters aren’t among them. “I don’t think it’s going to get to that point,” he says.

Not that there aren’t military applications. One, Armstrong says, is communicating between submarines without allowing others to eavesdrop. And outside of the military field, he says, “I have used time reversal to locate cracks or defects with ultrasound in metal structures, such as storage canisters for spent nuclear fuel.”

There are also medical applications, including the use of focused sound waves to ‘melt’ kidney stones or brain tumours without the need for surgery.

At the moment, however, Anderson’s work is focused on producing a two-player game at a children’s museum hosted by ETH Zurich, Switzerland, in which kids take turns trying to use focused sound waves to knock down each other’s Lego minifigures.

Sometimes, learning science is all about play… and what, for a child, better spells play than Lego?