The simple physics that makes a bottle hum when you blow across its opening has inspired a new way to power tiny robots.
Researchers have used sound to propel miniature boats and even microscopic flying machines without conventional motors.
Scientists at EPFL in Switzerland developed hollow structures that capture sound energy and turn it into directional thrust.
Their research, published in Science Advances, could lead to extremely small and lightweight robots controlled simply by changing sound frequencies.
The technology is based on Helmholtz resonance. When air moves across the opening of a bottle, the air trapped inside begins to oscillate. At certain frequencies, those vibrations become especially strong, producing the familiar humming sound.
The EPFL researchers realized the same effect could be used to create movement.
They designed tiny hollow cavities with round or bell-like shapes. When sound waves at the correct frequency reach a cavity, the trapped air begins vibrating strongly. Air leaving the opening forms a concentrated jet, while incoming air is more spread out. This difference produces thrust that pushes the structure in a particular direction.
Rather than using sound waves to simply push an object from the outside, the new devices effectively capture sound and convert it into their own propulsion.
To demonstrate the idea, researchers built centimeter-sized miniature boats containing as many as three cavities. Each cavity was designed to respond to a different audible sound frequency and provide thrust in a particular direction.
By changing the frequency produced by a nearby speaker, the scientists could choose which cavity became active. This allowed them to move and steer the boats, guide them around obstacles and even demonstrate autonomous navigation.
The team then took the idea to a much smaller scale.
Using advanced 3D nanoprinting, the researchers created ultralight “microfliers” with microscopic cavities built directly into their structures. Instead of audible sound, these tiny flying machines responded to ultrasound, which has frequencies too high for humans to hear.
One microflier weighed only 150 micrograms. Its cavities generated upward thrust directly, allowing it to operate somewhat like a miniature rocket.
Another version added tiny blades. Sound-driven cavities caused the blades to rotate at speeds of up to 13,000 revolutions per minute, producing stable aerodynamic lift similar to a tiny helicopter.
An important advantage of the technology is its simplicity. The robots do not need traditional motors, gears or magnetic components to produce motion. Their propulsion comes mainly from carefully designed hollow structures, which can be manufactured from materials including ordinary 3D-printing plastics, flexible polymers and glass.
Researchers believe the technology could be miniaturized even further.
Future robots might contain many cavities, with each tuned to a different sound frequency. Changing the sound could then activate specific parts of a machine independently, causing them to move, bend or vibrate.
Eventually, this could lead to tiny robots or flying devices that change their shape and behavior in response to sound—turning an everyday acoustic phenomenon into a new form of wireless robotic control.
