Controlling sound has long been a staple of science fiction and fantasy. In Dune, the cone of silence allows characters to converse privately, even in open spaces. The eerie billboards of Blade Runner 2049 whisper advertisements into the ears of those passing by.
In the real world, quirks of architecture, intentional or not, can direct where sound goes. In the U.S. Capitol’s hall of statues, for example, a whisper can travel silently across the room from one spot to another. The sound waves interact with curved surfaces to focus the audio. Now, scientists are looking to precisely control sound, perhaps one day resulting in a world without earbuds, but directing sound waves is a challenge.
The frequencies that humans can hear, 20 to 20,000 hertz, tend to diffuse more easily than higher-frequency ultrasound, which is why conversations can be overheard.
In 2019, researchers directed sound using lasers that convert light into sound when absorbed by water vapor in the air. When the laser beam was stationary, precise localization was difficult; the sound could be heard anywhere along the beam. Adding a rotating mirror localized sound more, but the method couldn’t transmit detailed audio.
Another approach relies on ultrasonic waves. Though inaudible, they can help target audio thanks to an effect known as nonlinear interactions. When two ultrasonic waves meet, they “add” together to create a higher-frequency wave. The converging waves also “subtract” each other to create a lower-frequency wave that’s the difference between the two, which can fall within the range of human hearing.
This is what happens when water hits hot oil in a frying pan; tiny steam explosions generate ultrasonic waves, which mix in the air to create the sizzle we hear.
In the 20th century, the U.S. military took advantage of the effect to develop speakers that can direct sound along a path. Eventually, companies like Holosonics commercialized directional speakers. As with the laser, however, the audio isn’t fully private because it can be heard along the sound beam’s path.
But recently, researchers developed private “audible enclaves.”
“It is like wearing an invisible headset,” says Yun Jing, an acoustics researcher at Penn State. If you stand in the right place, you can hear a voice or music, while someone nearby hears nothing at all, Jing and colleagues reported in March in the Proceedings of the National Academy of Sciences.
They achieved this marvel using acoustic metasurfaces, materials engineered to have tiny repeating structures to manipulate sound in ways that natural materials can’t. “A metasurface is a lens that’s thinner than the wavelength of the sound waves it manipulates,” says Michael Haberman, a mechanical engineer at the University of Texas at Austin. Like lenses for light, acoustic metasurfaces can bend, shape and direct sound by changing the wave’s shape.
Jing’s team 3-D printed acoustic panels with zigzag air channels. Adjusting the path length of each channel let the team steer ultrasonic waves into curved paths. The team then covered two speakers with thin sheets of this metasurface to bend the ultrasonic beams toward each other as they propagated through the air. At the point of intersection, nonlinear interactions transformed the waves into audible sound that could be heard at only this spot.
“The sound quality isn’t great; we used a $4 transducer,” Jing says. “But this is only a proof of concept. And it works.”
This tech can’t create a Dune-like cone of silence just yet, but the researchers envision a future in which private conversations can happen in open spaces — no earbuds or wires required. Libraries, offices and other public places could host numerous audible enclaves to allow for private audio streams simultaneously.
