Researchers Create Sound that Bends Through Space to Reach Just One Person
Updated
Thanks to rapid advances in technology, the world we live in is extensively changing before our eyes. Soon, thanks to researchers at Penn State, you may be able to enjoy your favorite podcast or music without headphones and without disrupting those nearby. Using what they call “audible enclaves,” acoustics professor Yun Jing and his team have developed a way to confine sound—a vibration that travels through the air as a wave—to specific small zones where only the intended listener can hear it, even in close quarters like a car or right in front of the speaker, or picking someone out in a crowd, while others nearby hear nothing.
Published on March 17, 2025, in the journal Proceedings of the National Academy of Sciences (PNAS), the researchers note that recent advancements in digital signal processing and loudspeaker array design have allowed individuals to experience immersive spatial audio in virtual, augmented, and extended reality environment in their daily routines. Still, audio engineering development is restricted by “physical constraints stemming from the diffraction of long-wavelength audio waves.” Jing’s study presents a strategy to overcome this challenge by demonstrating the evolution of ultrabroadband (125 Hz to 4 kHz) and tightly focused remote audio zones, called audio enclaves. Jing shared:
“We’re using two ultrasound transducers combined with an acoustic metasurface to produce self-bending beams that cross at a specific point. At that intersection, the listener hears the sound clearly, but anyone else nearby doesn’t—offering a personal audio zone for private listening.”
This astonishing feat sounds like something out of a science fiction movie. For the study, metasurfaces (special acoustic lenses that incorporate millimeter or submillimeter-scale microstructures that bend the direction of sound) were placed in front of two ultrasonic transducers. They emit dual ultrasonic waves at slightly different frequencies, traveling in a curved, crescent-like path until they overlap. The sound becomes audible at this point—where the ultrasonic waves overlap—only to remain silent elsewhere along the trajectory. In other words, individually, the ultrasonic beams are inaudible, but their convergence triggers a nonlinear effect that generates hearable sound. The metasurfaces were 3-D printed by co-author Xiaoxing Xia, a scientist at Lawrence Livermore National Laboratory.
Remarkably, the beams can bypass obstacles, such as a person’s head, to reach a target point of intersection. “The person standing at that point can hear sound, while anyone standing nearby would not,” Jing explained, adding, “This creates a privacy barrier between people for private listening.” The researchers tested the method in an ordinary room with common reverberations. According to the research team, their system demonstrates that it could work in multiple environments, including school classrooms, vehicles, and even outdoors. First author Jia-Xin “Jay” Zhong, a postdoctoral scholar in acoustics at Penn State, shared:
“We essentially created a virtual headset. Someone within an audible enclave can hear something meant only for them—enabling sound and quiet zones.”
It is easy to understand that sound waves are created when an object moves back and forth, compressing and decompressing air molecules. The pitch of a sound is determined by the frequency of its vibrations. Lower frequencies produce deep tones, like the rumble of a bass drum, while higher frequencies create sharp, piercing notes, such as a whistle. Directing sound to an exact location is challenging due to diffraction, a process where sound waves spread out as they move. This spreading is especially pronounced for low-frequency sounds, which have longer wavelengths, making it difficult to restrict them to a specific spot. Some audio tools, like parametric array loudspeakers, can generate targeted sound beams pointed in a chosen direction. However, these systems still produce a sound that remains audible as it travels through air space.
Unless something blocks or reflects them, sound waves usually travel in straight lines. Using acoustic metasurfaces—remember, they are specialized materials that manipulate sound waves—the researchers designed ultrasonic beams that can bend on their own as they travel, similar to how an optical lens bends light. By finely tuning the phase of ultrasound waves, they were able to shape curved sound trajectories capable of bypassing obstacles and converging at a precise target point. Zhong and Jing note that the pivotal phenomenon at play is called “difference frequency generation.” They explained:
“When two ultrasonic beams of slightly different frequencies, such as 40 kHz and 39.5 kHz, overlap, they create a new sound wave at the difference between their frequencies—in this case, 0.5 kHz, or 500 Hz, which is well within the human hearing range. Sound can be heard only where the beams cross. Outside of that intersection, the ultrasound waves remain silent.”
The research team notes they can generate these audible enclaves about three feet away at a comfortable 60 decibels, which is similar to the level of a typical conversation. As they fine-tune their project, which is their hope, they believe its possibilities are exciting. In public settings, such as libraries or museums, audio enclaves could deliver sound to specific groups without bothering others, the team explains in their paper. They suggest it might even enable area-wide noise cancellation, offering calm amid the clamor of noisy urban environments. Nonetheless, converting ultrasound to audible sound requires high-intensity fields that need considerable energy to produce, so it may be a while before this technology is available. Considering that ultrasonic sound control can also be employed as a weapon, that, my friends, is fine with me.