Jia-Xin “Jay” Zhong, a postdoctoral scholar of acoustics at Penn State, used a dummy with microphones in its ears to measure the presence or absence of sound along an ultrasonic trajectory. (Credit: Poornima Tomy/Penn State)
Imagine listening to your favorite podcast or song without disturbing those around you—without the need for headphones. A groundbreaking advancement in audio engineering, led by Yun Jing, professor of acoustics at Penn State’s College of Engineering, has made this concept a reality. The research team has developed “audible enclaves,” localized pockets of sound that allow only specific listeners to hear audio, even in enclosed spaces like vehicles or crowded rooms.
How Audible Enclaves Work
Published in Proceedings of the National Academy of Sciences on March 17, the study reveals that these sound zones are created using two nonlinear ultrasonic beams. Sound is only perceived at the exact intersection of these beams, making it possible for one person to hear audio while someone standing just a few feet away remains unaffected.
This breakthrough could transform personal audio experiences in public spaces, offering new possibilities for in-car entertainment, office environments, and even public venues.
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“We use two ultrasound transducers paired with an acoustic metasurface, which emit self-bending beams that intersect at a certain point,” said corresponding author Jing. “The person standing at that point can hear sound, while anyone standing nearby would not. This creates a privacy barrier between people for private listening.”
By positioning the metasurfaces — acoustic lenses that incorporate millimeter or submillimeter-scale microstructures that bend the direction of sound — in front of the two transducers, the ultrasonic waves travel at two slightly different frequencies along a crescent-shaped trajectory until they intersect, researchers explained. The metasurfaces were 3D printed by co-author Xiaoxing Xia, staff scientist at the Lawrence Livermore Laboratory.
Neither beam is audible on its own — it is the intersection of the beams together that create a local nonlinear interaction, which generates audible sound, researchers explained. The beams can bypass obstacles, such as human heads, to reach a designated point of intersection.
“To test the system, we used a simulated head and torso dummy with microphones inside its ears to mimic what a human being hears at points along the ultrasonic beam trajectory, as well as a third microphone to scan the area of intersection,” said first author Jia-Xin “Jay” Zhong, a postdoctoral scholar in acoustics at Penn State. “We confirmed that sound was not audible except at the point of intersection, which creates what we call an enclave.”
For now, researchers can remotely transfer sound about a meter away from the intended target and the sound volume is about 60 decibels, equivalent to speaking volume. However, the researchers said that distance and volume may be able to be increased if they increased the ultrasound intensity.
