A simple bottle can teach us an important lesson about sound. Blow across the opening of a bottle and it produces a familiar humming tone. This everyday experiment demonstrates a scientific phenomenon known as Helmholtz resonance, in which air trapped inside a cavity vibrates at particular frequencies.
Now, researchers at the MicroBioRobotic Systems (MICROBS) Lab at EPFL's School of Engineering have taken this basic principle much further. They have developed tiny hollow structures that can turn sound into motion, creating miniature machines that can move through water or even fly.
The research, published in Science Advances, could open a new path toward extremely small robots that do not need conventional motors, gears or magnetic components.
Turning Sound Into Thrust
Helmholtz resonance happens when air inside a hollow space oscillates in response to sound or airflow. At certain frequencies, the movement becomes particularly strong, producing the characteristic sound we hear from a bottle.
The EPFL researchers have designed special hollow structures, known as acoustic resonators, that use this effect in a different way.
When a sound wave reaches one of these cavities, the air inside begins to oscillate. The design of the cavity causes air to leave through its opening as a concentrated, high-speed jet. At the same time, air entering the cavity is more widely distributed.
This difference creates an imbalance in airflow. The result is a small but useful force known as acoustic thrust.
Instead of using a motor to push a machine forward, the machine can therefore use sound to create its own propulsion.
"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," explains Selman Sakar, head of the MICROBS Lab.
According to Sakar, the work demonstrates that a simple mechanical structure, if carefully designed, can effectively become a form of robotic matter.
Miniature Boats Controlled by Sound
To demonstrate the technology, the researchers first built miniature boats equipped with acoustic cavities.
The boats could contain as many as three cavities. Each cavity was designed to respond to a different audible frequency and was positioned so that its thrust would push the boat in a specific direction.
This gave the researchers a surprisingly simple way to control the boats.
By changing the frequency produced by a nearby speaker, they could activate a particular cavity without physically touching the vehicle. One frequency could move the boat forward, while another could activate a different cavity and change its direction.
The system could even be used to steer the boats around obstacles and support programmed autonomous navigation.
The approach is also versatile because the resonators can be manufactured from different materials. The researchers demonstrated structures made using materials such as common 3D-printing plastics, flexible rubber-like polymers and glass.
This flexibility could be important for future applications because different materials and manufacturing methods can be selected depending on the size and function of the robot.
Sound-Powered Flying Machines
The researchers also explored whether the same principle could work for flying vehicles.
Using an advanced 3D nanoprinting technique, they created extremely lightweight flying structures called "microfliers." These tiny vehicles incorporated three microscopic acoustic cavities directly into their polymer bodies.
Unlike the miniature boats, the microfliers were powered using ultrasonic frequencies—sound frequencies too high for humans to hear.
One particularly lightweight design weighed only about 150 micrograms. Its acoustic cavities generated upward thrust, allowing the structure to rise in a way similar to a tiny rocket.
Another design combined the acoustic cavities with microscopic blades. When activated, the blades could rotate at speeds of up to 13,000 revolutions per minute. Their rapid rotation generated aerodynamic lift, producing a flight mechanism similar in principle to that of a helicopter.
These experiments show that sound can do more than simply move an object from a distance. Carefully designed structures can actually convert acoustic energy into mechanical motion.
A New Approach to Tiny Robotics
One of the most promising aspects of the technology is its simplicity.
Traditional miniature robots often require motors, gears, batteries, magnets or other components. As a robot becomes smaller, however, manufacturing these parts becomes increasingly difficult.
Acoustic resonators could offer an alternative.
Because they are essentially hollow mechanical structures, they can be manufactured using modern 3D-printing and microfabrication techniques. This means that propulsion and control mechanisms can potentially be integrated directly into the body of a tiny robot.
Junsun Hwang, a Ph.D. student at the MICROBS Lab and the study's first author, says the concept is compatible with further miniaturization. This could allow researchers to develop increasingly sophisticated designs for robotics and aeronautics.
The technology may eventually be useful in situations where conventional motors are too large, heavy or complicated.
Toward Robots That Respond to Sound
The researchers envision an even more advanced possibility: robots containing many acoustic structures, with each structure responding to a different sound frequency.
Imagine a flexible robotic device containing dozens of tiny resonators. One frequency could make one section bend, another could make a different section vibrate, while another could activate a small propulsion system.
Such a system could allow a robot to change its shape or movement without requiring a complex collection of mechanical parts.
Sakar suggests that this approach could eventually lead to aerodynamic robotic devices capable of changing shape in response to sound.
The idea is still at the research stage, but its potential is significant. By combining acoustic physics, advanced manufacturing and robotic design, scientists may be able to create machines that are dramatically smaller and simpler than today's conventional robots.
A Tiny Technology With Big Potential
What begins with something as ordinary as blowing across a bottle could point toward a new generation of machines.
The EPFL research shows that resonance can be more than a way to create sound. With the right geometry, a hollow cavity can transform acoustic energy into directional force, allowing tiny vehicles to move, steer and even fly.
As manufacturing technologies continue to shrink, sound-powered structures could become increasingly sophisticated. Future robots may not always need motors and gears to move. Instead, their own carefully designed bodies could interact with sound to generate motion.
From a humming bottle to a flying machine weighing just a fraction of a gram, the journey illustrates a powerful idea in engineering: sometimes, the simplest physical principles can inspire the most advanced technologies.
Reference:
- Junsun Hwang et al.
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