Engineers at EPFL have built machines that run on sound. A team in the school's MicroBioRobotic Systems lab designed hollow cavities that convert sound waves into thrust, and used them to move small robots and ultralight flying vehicles that carry no motor, no battery and no electronics whatsoever. The work was published in Science Advances.
The physics is the one you demonstrate by blowing across the neck of a bottle. Air trapped in a cavity oscillates, and at particular frequencies it oscillates hard. Earlier attempts used sound to push passive objects around from outside; these cavities instead make their own propulsion. When sound sets the trapped air moving, it leaves as a narrow jet while the air coming back in is spread out, and the imbalance is a push. The chambers can be printed in ordinary plastics, cast in rubbery polymers, or made of glass.
The centimetre-scale boats carried up to three cavities, each tuned to a different audible note and each aimed to push the hull a particular way. Changing the frequency coming out of a speaker was therefore the whole of the steering: the researchers picked which cavity fired, drove the boats around obstacles, and set them to navigate on their own. A second line of work used 3D nanoprinting to build fliers with three microscopic cavities in the body, driven at frequencies too high to hear. One weighed 150 micrograms and went straight up like a rocket; another turned tiny blades at 13,000 revolutions a minute for helicopter-style lift.
The issue is HUM DRIVE, an original cartridge for the Nintendo Entertainment System, and it hands the reader the speaker rather than the boat. Tune the dial, sound the note, and whichever bell answers that pitch blows air out and shoves the hull. Low pushes it ahead, middle swings the bow to port, high swings it to starboard, and between the bands is dead air where the same note does nothing at all. The console's own sound chip is the engine: the tone steering the boat is the tone coming out of the speaker. Ring all five buoys before the tank is booked by someone else. Play it here in the browser, or download the ROM and run it on real hardware.