A ground-breaking method to create “audible enclaves” – localized zones where sound is perceptible while remaining completely unheard outside – has been unveiled by researchers at Pennsylvania State University and Lawrence Livermore National Laboratory. Their innovation could transform personal audio experiences in public spaces and improve secure communications.
“One of the biggest challenges in sound engineering is delivering audio to specific listeners without disturbing others,” explains Penn State’s Jiaxin Zhong. “Traditional speakers broadcast sound in all directions, and even directional sound technologies still generate audible sound along their entire path. We aimed to develop a method that allows sound to be generated only at a specific location, without any leakage along the way. This would enable applications such as private speech zones, immersive audio experiences, and spatially controlled sound environments.”
To achieve precise audio targeting, the researchers used a phenomenon known as difference-frequency wave generation. This process involves emitting two ultrasonic beams – sound waves with frequencies beyond the range of human hearing – that intersect at a chosen point. At their intersection, these beams interact to produce a lower-frequency sound wave within the audible range. In their experiments, the team used ultrasonic waves at frequencies of 40 kHz and 39.5 kHz. When these waves converge, they generated an audible sound at 500 Hz, which falls within the typical human hearing range of approximately 20 Hz–20 kHz.
To prevent obstacles like human bodies from blocking the sound beams, the researchers used self-bending beams that follow curved paths instead of travelling in straight lines. They did this by passing ultrasound waves through specially designed metasurfaces, which redirected the waves along controlled trajectories, allowing them to meet at a specific point where the sound is generated.
Manipulative metasurfaces
“Metasurfaces are engineered materials that manipulate wave behaviour in ways that natural materials cannot,” said Zhong. “In our study, we use metasurfaces to precisely control the phase of ultrasonic waves, shaping them into self-bending beams. This is similar to how an optical lens bends light.”
The researchers began with computer simulations to model how ultrasonic waves would travel around obstacles, such as a human head, to determine the optimal design for the sound sources and metasurfaces. These simulations confirmed the feasibility of creating an audible enclave at the intersection of the curved beams. Subsequently, the team constructed a physical setup in a room-sized environment to validate their findings experimentally. The results closely matched their simulations, demonstrating the practical viability of their approach.
“Our method allows sound to be produced only in an intended area while remaining completely silent everywhere else,” says Zhong. “By using acoustic metasurfaces, we direct ultrasound along curved paths, making it possible to ‘place’ sound behind objects without a direct line of sight. A person standing inside the enclave can hear the sound, but someone just a few centimetres away will hear almost nothing.”
Initially, the team produced a steady 500 Hz sound within the enclave. By allowing the frequencies of the two ultrasonic sources to vary, they generated a broader range of audible sounds, covering the frequencies from 125 Hz–4 kHz. This expanded range includes much of the human auditory spectrum, increasing the potential applications of the technique.
The ability to generate sound in a confined space without any audible leakage opens up many possible applications. Museums and exhibitions could provide visitors with personalized audio experiences without the need for headphones, allowing individuals to hear different information depending on their location. In cars, drivers could receive navigation instructions without disturbing passengers, who could simultaneously listen to music or other content. Virtual and augmented reality applications could benefit from more immersive soundscapes that do not require bulky headsets.
The technology could also enhance secure communications, creating localized zones where sensitive conversations remain private even in shared spaces. In noisy environments, future adaptations of this method might allow for targeted noise cancellation, reducing unwanted sound in specific areas while preserving important auditory information elsewhere.
Future challenges
While their results are promising, the researchers acknowledge several challenges that must be addressed before the technology can be widely implemented. One concern is the intensity of the ultrasonic beams required to generate audible sound at a practical volume. Currently, achieving sufficient sound levels necessitates ultrasonic intensities that may have unknown effects on human health.
Another challenge is ensuring high-quality sound reproduction. The relationship between the ultrasonic beam parameters and the resulting audible sound is complex, making it difficult to produce clear audio across a wide range of frequencies and volumes.
“We are currently working on improving sound quality and efficiency,” Zhong said. “We are exploring deep learning and advanced nonlinear signal processing methods to optimize sound clarity. Another area of development is power efficiency — ensuring that the ultrasound-to-audio conversion is both effective and safe for practical use. In the long run, we hope to collaborate with industry partners to bring this technology to consumer electronics, automotive audio, and immersive media applications.”
The research is reported in Proceedings of the National Academy of Sciences.
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