‘Audible enclaves’ could enable private listening without headphones

It may someday be possible to listen to a favorite podcast or song without disturbing the people around you, even without wearing headphones. In a new advancement in audio engineering, a team of researchers led by Yun Jing, professor of acoustics in the Penn State College of Engineering, has precisely narrowed where sound is perceived by creating localized pockets of sound zones, called audible enclaves.

In an enclave, a listener can hear sound, while others standing nearby cannot, even if the people are in an enclosed space, like a vehicle, or standing directly in front of the audio source.

In a study published in the Proceedings of the National Academy of Sciences, the researchers explain how emitting two nonlinear ultrasonic beams creates audible enclaves, where sound can only be perceived at the precise intersection point of two ultrasonic beams.

“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 creates a local nonlinear interaction which generates audible sound, the 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.”

Researchers tested the system in a common room with normal reverberations, meaning the system could work in a variety of environments, such as classrooms, vehicles or even outdoors.

“We essentially created a virtual headset,” Zhong said. “Someone within an audible enclave can hear something meant only for them—enabling sound and quiet zones.”

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.

In addition to Jing, Xia and Zhong, the co-authors include Jun Ji and Hyeonu Heo, alumni of the Penn State graduate program in acoustics.