Audio breakthrough: Silently listen to music, podcasts without headphones

A new breakthrough in audio engineering could make it possible to listen to music, podcasts, or private conversations without disturbing those around you—without the need for headphones.

A team of researchers has developed a method to create isolated pockets of sound, called audible enclaves, where only specific individuals can hear audio while others nearby remain unaware.

Delivering targeted sound without affecting surrounding listeners has long been a challenge due to the way sound waves naturally spread. Low-frequency sounds, like human speech, travel in long wavelengths that diffract around objects, making it nearly impossible to contain them within a specific area.

Traditional approaches to directional audio, such as highly focused speaker arrays, struggle to provide a truly isolated listening experience, especially in spaces with reverberation like classrooms, offices, or vehicles.

Researchers at Penn State, led by acoustics professor Yun Jing, have developed a new approach that bypasses these limitations by using self-bending ultrasonic beams. By emitting two ultrasonic waves that meet at a precise point, they create a localized zone where sound is heard only at the intersection of the beams. Outside this narrow zone, the sound remains inaudible.

“We use two ultrasound transducers paired with an acoustic metasurface, which emit self-bending beams that intersect at a certain point,” said 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.”

The key to this breakthrough lies in acoustic metasurfaces—thin, engineered materials with microscopic structures designed to control the direction of sound waves. These metasurfaces, positioned in front of ultrasound transducers, shape the beams into crescent-like paths. As the beams travel through space, they remain inaudible to the human ear until they converge at a precise point, creating a highly localized sound zone.

Co-author Xiaoxing Xia, a staff scientist at Lawrence Livermore National Laboratory, used 3D printing to create the metasurfaces, enabling fine-tuned control over the wave paths.

The technique takes advantage of a nonlinear interaction between the ultrasonic waves, meaning the sound only emerges at the pre-determined intersection point. Even if obstacles, such as people or furniture, are in the way, the beams bend around them, ensuring the sound remains targeted.

To test their system, the researchers used a simulated human head and torso equipped with microphones inside its ears. They measured the sound along the ultrasonic beam path and at the point of intersection.

“We confirmed that sound was not audible except at the point of intersection, which creates what we call an enclave,” said Jia-Xin “Jay” Zhong, a postdoctoral scholar in acoustics at Penn State and the study’s first author. “We essentially created a virtual headset. Someone within an audible enclave can hear something meant only for them—enabling sound and quiet zones.”

The ability to create highly targeted audio zones has broad implications. In environments like classrooms, lecture halls, and offices, multiple people could receive personalized audio streams without interfering with one another.

Vehicles could use the technology to allow passengers to listen to different media without overlapping sounds. Even public spaces, such as museums or libraries, could offer personalized audio tours without requiring headphones.

The researchers tested their system in a standard room with typical reverberations, demonstrating that the technology works in real-world conditions. The sound transferred from the ultrasonic beams was measured at approximately 60 decibels—equivalent to a normal speaking voice—and could be delivered up to a meter away from the intended target.

While the current system is limited in range, the researchers believe it can be improved by increasing the intensity of the ultrasound signals, potentially allowing for greater distances and higher sound volumes.

The team’s work, published in the Proceedings of the National Academy of Sciences, marks a significant step forward in audio engineering.

Supported by the U.S. National Science Foundation and Lawrence Livermore National Laboratory, the research demonstrates that the future of sound may be more customizable and private than ever before.

As advancements continue, the prospect of seamless, immersive, and private audio experiences without the need for headphones is becoming more realistic.

Whether for entertainment, communication, or accessibility, the ability to control where sound is heard could transform the way we interact with audio in daily life.

Note: Materials provided above by The Brighter Side of News. Content may be edited for style and length.

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