Sound-Powered Micro-Robots with No Motor Needed

That humming sound you hear when you blow over an empty glass bottle is a phenomenon in physics called Helmholtz resonance. What’s happening is that the air trapped inside the bottle bounces back and forth.

Now, researchers at EPFL’s MicroBioRobotic Systems (MICROBS) Lab are using this exact phenomenon to build tiny machines powered entirely by sound.

Building the Sound Engines

Instead of using sound waves just to push passive objects around, the team built hollow, bell-shaped cavities that turn sound into thrust. When sound hits these cavities, the air inside shoots out in a concentrated jet. This pushes the object forward. They can make these parts out of common 3D-printing plastics, rubber-like polymers, or even glass.

To test this out, they built small boats at the centimeter scale. The boats have up to three cavities, and each one reacts to a different audible sound frequency. By playing different sounds from a speaker, the team could steer the boats around obstacles and program them to navigate autonomously.

“Instead of pushing devices around with sound waves, we have created acoustic resonators that are tuned to harness sound at specific frequencies to generate directional thrust and controlled motion,” Lab head Selman Sakar explained. “Our work shows the feasibility of transforming a simple, cleverly designed mechanical piece into robotic matter.”

Helicopters and Rockets

The team didn’t stop at boats. They used a 3D nanoprinting technique to build ultralight flying vehicles powered by high ultrasonic frequencies that human ears cannot hear.

One flier weighed just 150 micrograms and used its cavities to shoot straight up like a rocket. Another combined the cavities with tiny blades. The blades spun at 13,000 revolutions per minute, creating stable lift just like a small helicopter.

Because these machines do not need heavy motors, gears, or magnets, they can be made extremely small.

“Our concept is compatible with even further miniaturization, enabling advanced designs that push the boundaries of robotics and aeronautics,” said first author and MICROBS Lab PhD student Junsun Hwang. “This would allow specific parts of the device to move, bend or vibrate, potentially leading to aerodynamic robotic devices that can change shape in response to sound.”