Imagine listening to your favorite song or podcast in privacy without using headphones and without annoying people around you with TikTok blasting on the phone’s speakers (there’s a special place in hell for that). We’re not exactly there yet, but acoustic engineers at Penn State College of Engineering just showed us a glimpse into how far you can go with manipulating sound.
The researchers created localized pockets of sound, called “audible enclaves.” These enclaves are like small, invisible sound bubbles where only those inside it can hear information from the system’s sound source. They essentially allow a listener to hear audio clearly, while others nearby remain unaware of it—even in enclosed spaces like small rooms.
How It Works: Sound Beams That Bend and Intersect
The researchers use two ultrasound transducers — devices that emit high-frequency sound waves beyond the range of human hearing. These transducers are paired with acoustic metasurfaces, which are carefully designed 3D-printed lenses whose geometry and material properties bend sound in specific directions.
“We use two ultrasound transducers paired with an acoustic metasurface, which emit self-bending beams that intersect at a certain point,” explained Yun Jing, a professor of acoustics. “The person standing at that point can hear sound, while anyone standing nearby would not.”
The beams travel along crescent-shaped paths, intersecting at a precise location. Then, at this intersection point, a nonlinear interaction occurs, generating audible sound. Neither beam is audible on its own — it’s only at the point where they meet that sound becomes perceptible.
To test the system, the team used a simulated human head and torso, equipped with microphones in the ears to mimic human hearing. A third microphone scanned the surrounding area to confirm that sound was only audible at the intersection point.
“We essentially created a virtual headset,” said Jia-Xin “Jay” Zhong, the study’s first author and a postdoctoral scholar in acoustics at Penn State. “Someone within an audible enclave can hear something meant only for them — enabling sound and quiet zones.”
The system has been tested in a variety of environments, including rooms with normal reverberations, suggesting it could work in real-world settings like classrooms, vehicles, or even outdoor spaces.
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What’s Next for This Technology
For now, the technology can transmit sound about a meter away from the source at a volume of around 60 decibels — roughly the level of a normal conversation.
The researchers believe they can increase both the distance and volume by boosting the intensity of the ultrasound beams. This could pave the way for applications beyond personal listening.
Imagine visiting an art museum. When you come close to a painting, you can hear an audio guide playing in the background without having to wear headphones. Step back and the audio is gone. Something similar could be useful in private announcements in public spaces or even immersive gaming experiences.
This research builds on decades of work in acoustics and ultrasound technology. Previous attempts to create directional sound systems often relied on complex arrays of speakers. The Penn State team’s approach, using metasurfaces to bend sound, offers a more flexible and precise solution. The setup is extremely cumbersome though, so it’s not clear how something like this could be miniaturized for mobile applications. But neither scenario is out of the question.
The findings appeared in the Proceedings of the National Academy of Sciences.
