If you blow across the top of an empty bottle you'll hear a clear, steady note. Now, engineers have used that same concept to power tiny robots that move without a single motor on board.

These new devices are powered entirely by sound. When the right frequency is aimed at them, hollow chambers built into their structure resonate and push out a jet of air, generating enough thrust to steer a small boat or lift a tiny flying robot off the ground.

The scientists described their innovation in a study published Aug. 12 in the journal Science Advances.

A 19th-century tuning trick, miniaturized

The effect behind it, known as Helmholtz resonance, was first studied in 1856 by the German physicist Hermann von Helmholtz, who was trying to invent a tool for tuning musical instruments. He noticed that when air trapped inside a cavity resonates, it also pushes out a faint jet of air.

"It isn’t very powerful when you do it with a musical instrument, because pressure is low," said study co-author Selman Sakar, an associate professor of mechanical engineering at the Swiss Federal Technology Institute of Lausanne (EPFL) in Switzerland. "But if you could crank up the pressure, all of a sudden that jet could become significant... in a way that the force can be harnessed for machinery," he told Live Science.

The team realized that shrinking Helmholtz's resonators down would push the frequency needed to activate them into the ultrasonic range — meaning it's too high-pitched for humans to hear and, crucially, easier to focus with precision. Larger versions generating the same force would need audible sound loud enough to be both annoying and potentially harmful, Sakar said.

Using a specialized 3D-printing technique called two-photon printing, the researchers built hollow structures based on Helmholtz's original equations and confirmed with lab tests and computer simulations that they generated thrust as predicted.

Boats, rockets and tiny helicopters

The team built small boats measuring roughly 2 inches (5 centimeters) fitted with multiple resonators, each tuned to a different frequency and pointed in a different direction. By changing the pitch of a nearby speaker, the researchers could steer the boats left, right or straight ahead.

At a far smaller scale — some just 0.04 inches (1 millimeter) across — the team also built "microfliers" that generated lift two different ways: some pushed thrust downward like a rocket — but using air for the thrust — while others span tiny attached blades to fly like a helicopter, using sound waves to rotate the blades.

The technology has an advantage over conventional means of power generation because it can be built at extremely small scales, Sakar said. Conventional motors have "a fundamental limit" on miniaturization because of the physical components like magnets, coils and shafts that a motor needs to work, he said. Because these resonators are just precisely shaped hollow cavities, there's no comparable limit on how small they could eventually build.

The researchers said in the study that the same principle could eventually be used to precisely rotate and manipulate small objects in midair without touching them, or to build soft, flexible surfaces that bend and change shape on command when they "hear" a particular frequency which can be used for biomedical applications like heart stents.

For now, the study is a foundation for the researchers to build on, Sakar said. "This paper, I think, is important in the sense that we put the design principles out," he said, adding that follow-up work could focus on more applied designs, control systems or navigation.