Using the same basic physics based on the familiar hollow tone produced by blowing across an empty bottle, researchers in Switzerland have developed a sound-powered propulsion system that can steer microrobots and even lift tiny machines.
In a new study published in Science Advances, engineers at the Swiss Federal Institute of Technology Lausanne (EPFL) demonstrated that 3D-printed cavities known as Helmholtz resonators can convert sound into directed jets of air and usable thrust.
The researchers built devices ranging from centimeters down to micrometers, used sound to propel and steer tiny boats, and created two microscopic flying machines—one microrobot lifted directly by acoustic thrust and another using rapidly spinning rotor blades. One version generated nearly five times its own weight in thrust, while another spun at up to 13,000 revolutions per minute to produce aerodynamic thrust.
The work points to a new way to power small robots without conventional motors, while also exposing the considerable engineering problems that remain before such machines can operate autonomously.
“While acoustic resonators have been extensively studied in liquids for the realization of untethered microrobots, their implementation in air remains largely unexplored,” researchers write. “The actuation scheme is compatible with even further miniaturization, pushing the boundaries of robotics and aeronautics.”
The idea behind the technology is surprisingly familiar. Blow across the top of an empty bottle and you hear a deep, hollow note created as the air trapped inside begins to vibrate.
Known as Helmholtz resonance, the effect happens because the air inside the bottle and its narrow opening behave somewhat like a mass attached to a spring. When sound at the right frequency excites a specially designed resonator, those vibrations become much stronger, forcing air rapidly through the opening. The resulting airflow can form a directional jet powerful enough to generate thrust.
The researchers set out to harness the same physics behind Helmholtz resonance as an unconventional propulsion method for microrobots.
The researchers designed tiny cavities that take advantage of the same principle. When sound hits one of these resonators at just the right frequency, the air in its narrow neck begins moving rapidly back and forth.
Because that airflow is slightly uneven as it enters and exits, it creates a small but focused jet of air. That jet produces thrust, giving the resonator enough force to help move or even lift a microrobot.
The team 3D-printed resonators ranging from centimeter to micrometer scales, designing them to respond to frequencies between roughly 200 hertz and 40 kilohertz. Experiments showed that larger cavities generally produced greater thrust, while the geometry and thickness of the resonator’s neck played a particularly important role in determining performance.
Importantly, researchers could tune different resonators to different frequencies. That means sound does more than supply power; frequency can effectively serve as a control signal.
The researchers demonstrated the concept using miniature boats equipped with acoustic resonators. One early prototype carried a resonator tuned to 1.2 kilohertz and moved only when exposed to sound at that frequency.
More advanced versions used separate resonators for propulsion and steering, allowing the researchers to guide a boat around obstacles remotely and along a figure-eight course with a mean tracking error of just 5.70 millimeters.
The team also showed that an external speaker is not always necessary. By attaching small vibration transducers directly to resonators, they could send sound through the robot’s structure itself.
This approach allowed the researchers to build a fully integrated boat equipped with its own battery, control electronics, and acoustic transducers. Bluetooth commands activated resonators tuned for propulsion and steering, with a total communication latency of about 30 milliseconds. The boat successfully traced the letters “EPFL,” navigated past obstacles, and could be steered back on course after researchers deliberately disturbed it by blowing air or physically pushing it.
However, shrinking the technology into flying microrobots provided the study’s most striking demonstration.
One microflier consisted of three microscopic resonators operating at 40 kilohertz and weighed only about 150 micrograms. Focused ultrasound caused each resonator to eject a tiny jet of air downward. Together, the three produced a thrust-to-weight ratio of approximately 4.9. The machine accelerated upward, climbed roughly 12 millimeters before settling around seven millimeters above its starting position.
A second design took a different approach. Instead of pointing the acoustic jets downward, the researchers used them to spin a rotor fitted with tiny aerodynamic blades. The entire aircraft weighed 184 micrograms.
Once exposed to ultrasound, the rotor accelerated to approximately 13,000 rpm. Its blades generated enough aerodynamic thrust for takeoff before settling to roughly 6,500 rpm while hovering.
Turning the device upside down prevented it from ascending, helping confirm that aerodynamic thrust, rather than acoustic radiation pressure alone, enabled flight.
Researchers demonstrated unconstrained flight with both microrobot designs. The direct-thrust version climbed rapidly but sometimes flipped in the air. The spinning version was more stable, apparently because its swift rotation provided a gyroscopic stabilizing effect similar to that found in helicopters and spinning discs.
Ultimately, the experiments remain proof-of-concept demonstrations rather than prototypes of free-ranging autonomous microrobots. Airborne propulsion requires external acoustic hardware capable of keeping a sufficiently strong sound field focused on a tiny moving object. That essentially limits the robot’s operating area. Putting the acoustic source onboard offers greater autonomy but might significantly increase the robot’s weight.
The current flying system also operates at a single 40-kilohertz frequency, restricting how independently its resonators can be controlled. The researchers suggest future multifrequency ultrasonic arrays could selectively activate different resonators, allowing small aircraft to steer and coordinate their movement through sound alone.
Further miniaturization is technically possible. Modern 3D nanoprinting can fabricate structures with resolutions around 700 nanometers, possibly allowing resonators only several micrometers across. However, at those scales, viscous forces become increasingly important and may dramatically reduce thrust, meaning simply shrinking the existing design may eventually stop working.
Still, the wider concept could go beyond flying machines. Researchers envision flexible structures containing many differently tuned resonators that can bend, vibrate, or change shape depending on the applied sound frequency, possibly resulting in shape-morphing microrobots and mechanical metamaterials.
“Our work extends the concept of acoustic manipulation toward active propulsion by harnessing cavity resonance to generate directional thrust and controlled motion,” the researchers write. “This approach broadens the scope of airborne acoustic actuation toward small-scale robots capable of autonomous operation in open environments.”
The recent study, “Acoustic Resonators as Wireless Actuators in Air for Small-Scale Robots,” was published in Science Advances.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com