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The ability to deliver sound to a specific audience without the need for headphones (i.e., directional sound) has been a long-standing research area in audio engineering. However, achieving this typically requires large and complex sound sources, and the audio signal is often audible from anywhere along the beam path.
A new study published in the Proceedings of the National Academy of Sciences shows that a research team at Pennsylvania State University has developed a new method that breaks these limitations.
They combined a compact array of ultrasonic transmitters with a specially designed two-dimensional structure called a metasurface, which can manipulate the properties of waves to produce “self-bending” ultrasonic beams. These ultrasonic beams are not only inaudible to the human ear, but can also bypass obstacles. When two such ultrasonic beams intersect, they interact to produce sound within the range of human hearing, confined to a point only a few centimeters wide-what the researchers call the “audible zone.”
“The key innovation is that sound is generated only where the two beams intersect, which allows us to precisely transmit audio to a specific location while keeping the beams themselves from producing audible sound,” explained Jiaxin Zhong, the paper’s first author and a postdoctoral scholar in acoustics at Pennsylvania State University.
Previous studies have confirmed that audible, bendable beams can bypass obstacles, but due to the long wavelengths of audible sounds, the sound source typically needs to be on the order of meters, and the signal can be detected anywhere along the beam's path. A research team at Pennsylvania State University has devised a new technique that utilizes ultrasonic beams, inaudible to the human ear, and can generate them using much smaller hardware.
To enable the beam to bend, they 3D-printed a 16×8 cm mesh-like metasurface and placed it in front of the ultrasonic transmitter array to precisely control the phase of the output beam. "These metasurfaces act like an acoustic lens, controlling the wavefront and causing the beam to bend during propagation," Zhong Jiaxin said.
The research team demonstrated that two such beams can bypass a dummy's head and intersect in front of its face. When the two sound waves interact, they generate secondary waves with frequencies equal to the difference between the original waves. By using a pair of beams with frequencies of 40 kHz and 39.5 kHz, the researchers created a sound zone only a few centimeters wide and with a frequency of 500 Hz in front of the dummy's head, within the range of human hearing.
Researchers say they were able to generate six octaves of audio in the 125 Hz to 4 kHz frequency range using the same metasurface by changing the frequency of one of the beams. While these experiments primarily created simple single-frequency tones, the team also demonstrated that the method could be applied to a 9-second excerpt of the "Hallelujah Chorus" from Handel's Messiah, which contains a series of fluctuating frequencies.
Zhong Jiaxin pointed out that the main problem with the current method is that distortion occurs when the two beams interact, causing the audio signal to become cluttered. However, researchers believe that this problem can be solved by using new signal processing techniques, including deep learning algorithms that can learn to compensate for distortion.
Currently, the trajectory of a self-bending beam is fixed, meaning the location of the sound source must be precisely set to avoid obstacles. Zhong Jiaxin stated that the team hopes to eventually create a reconfigurable beam that can dynamically adjust to avoid different objects. "We envision using adaptive processing algorithms to dynamically adjust the beam trajectory in real time, allowing the beam to intelligently bypass obstacles based on environmental feedback," he said. "This will make the technology more versatile in practical applications, because in real-world environments, the position of obstacles may move or change."
Chung Chia-hsin also stated that this technology has a wide range of potential applications. For example, it can create personalized audio in public places, such as audio guides for museums, without the need for headphones; in cars, different passengers can listen to different audio without interfering with each other; and it can also create private voice zones for confidential information. In addition, by projecting audio that can cancel out the existing sound field, this technology can also achieve local noise cancellation.
More information: Jing, Yun, Audible enclaves crafted by nonlinear self-bending ultrasonic beams, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2408975122. www.pnas.org/cgi/doi/10.1073/pnas.2408975122
Information source: IEEE.ORG
This article is reprinted from 21dB Acoustics.