Sound might seem to merely carry information and create music. But engineers from the EPFL Micro-BioRobotic Systems Laboratory have shown that a sound wave can become a direct physical force of motion.
The researchers have developed miniature devices that take to the air under the influence of ultrasound. They carry neither an engine nor a battery: the energy required for movement comes from the outside, carried by a sound wave of a specific frequency.
The results of the study were published in the journal Science Advances.
How resonance turns into thrust
At the heart of the development lies Helmholtz resonance — a physical effect that can be observed when blowing across the neck of an empty bottle. The air inside the cavity begins to oscillate, amplifying the sound at a specific frequency.
The engineers applied the same principle to create motion. They 3D-printed hollow resonators with precisely calculated geometry. When the structure is subjected to sound of the required frequency, the air oscillations inside the cavity are amplified many times over.
Air exits through the opening in a narrow, directed jet, while it returns inside in a more diffuse manner. The resulting asymmetry of the airflow creates thrust.
Thus, an invisible sound wave is transformed into mechanical motion.
For the flight prototypes, the team used ultrasound with a frequency of 40 kilohertz — above the range of human hearing. We cannot hear the control wave, yet for the microscopic device, it becomes a source of energy.
From a speck to flight
The researchers created two versions of flight devices with different lift principles.
The first micro-flyer weighs just 150 micrograms. Three downward-facing acoustic resonators create thrust, allowing the structure to lift off vertically from a surface.
The second prototype, weighing 184 micrograms , resembles a three-bladed helicopter rotor. Resonators located at the base of the blades create directed thrust and spin the structure at approximately 13 000 revolutions per minute. The rotation of the blades generates aerodynamic lift.
Thus, the same physical principle was successfully applied in two different architectures: for direct vertical lift and for rotational motion.
A boat controlled by frequencies
The experiment was also conducted on the water's surface.
The researchers installed several resonators on a miniature boat, each tuned to its own sound frequency. Activating different cavities allowed the boat to move forward and change direction.
This is one of the most important properties of the development: individual elements of the structure can be controlled selectively. A single acoustic wave passes through space, but only the resonator tuned to the corresponding frequency responds to it.
In effect, the frequency becomes a wireless movement command.
For now, this is a proof of concept
The technology is at an early experimental stage. The micro-flyers rise only a few millimeters and are not yet capable of carrying a payload.
Energy is also not generated inside the device — it is transmitted by an external loudspeaker or ultrasonic emitter. Therefore, this is not about ready-made autonomous drones, but rather a proof of a new principle of movement.
However, the absence of an onboard engine, wires, and a battery can be an advantage where size and mass are critical.
In the future, acoustic resonators could be used in microrobotics, for exploring hard-to-reach cavities, in precision manufacturing operations, and for non-contact manipulation of small or fragile objects.
Matter that has heard its frequency
The most amazing thing here is not just the microscopic size of the created devices. It is the ability of a form to adopt a specific frequency and transform vibration into directed motion.
The same acoustic wave passes through the air of the laboratory. But only the structures tuned to its resonant frequency respond to it. The rest remain motionless.
In this discovery, technology touches upon one of the fundamental principles of nature: what matters is not just the force of the impact, but the correspondence between the wave and the form that receives it.
Form meets sound.
Sound awakens the potential contained within it.
And matter begins to move.
Thus, our understanding of future technologies is gradually changing. We are learning not only to place an energy source inside every mechanism, but also to create forms capable of receiving energy from the surrounding space.
Sometimes, to gain wings, matter needs no more force. It needs to encounter its own frequency.
