UNIVERSITY PARK, Pa. — Music lovers may one day be able to blast their favorite artists, headphone-free, without angering the neighborhood or colleagues, thanks to researchers at Penn State.
The team designed a system that can manipulate sound waves so that they are only audible at a precise spot slightly wider than an inch. Despite this tiny focal point, their system can produce high-quality audio, potentially offering listeners a crisp, yet private, sound experience. The team detailed their work in a paper recently published in IEEE Transactions on Ultrasonics .
Sound waves traditionally spread outwards from their source, explained Jee Woo Kevin Kim, an acoustics doctoral candidate and first author on the paper. Parametric array loudspeakers (PAL) use high-intensity ultrasonic waves to focus audible sound into a narrow, laser-like beam of sound. Kim said that although these arrays are used in spaces like museums and broadcast rooms to transmit sound discretely, they face issues that make their application outside of these specialized environments difficult.
“These arrays are so directional that once the sound beam comes in contact with a surface, the sound can reflect all around the room, compromising privacy,” Kim said. “Additionally, they struggle to produce low-end frequencies, which can take away from the experience of listening to bass-heavy music, for example.”
Yun Jing , professor of acoustics and corresponding author on the paper, said that acoustic metasurfaces can address this key issue. Metasurfaces are a class of materials that can manipulate waves — including light, sound, heat and more — with just their thin structures. Jing explained how acoustic metasurfaces used to manipulate waves of sound can be easily 3D-printed, and have been widely used to direct specific sounds, such as in speakers.
“To develop an acoustic metasurface, we use a large surface that works like a lens focusing a beam of light,” said Jing, who holds an additional affiliation in biomedical engineering. “The surface modulates sound waves in such a way that they converge at a central point after leaving the speaker, allowing us to focus the audio into a precise area.”
Previous work from Jing’s lab bent ultrasonic-sound waves produced by two speakers, creating a private “audible enclave” that is about 60 decibels, or as loud as a conversation. However, he explained that this approach requires multiple arrays of PALs, and the generation of audible sound from ultrasound is relatively inefficient.
The team’s new metasurface can passively focus sound waves, without the need for complex signal processing or electricity. The 3D-printed covering can direct sound produced by the speakers into a tight “bubble” that is only audible in a space slightly larger than an inch wide and less than a quarter of an inch tall — about the size of a stick of gum.
“Using this design, our focal point is fixed in space, but the components passive nature allows us to substantially reduce the cost of manufacturing and implementing the acoustic metasurface,” Jing explained.
To test their design, the team simulated and verified the performance of the metasurface lens before 3D-printing the component. They then applied the circular metasurface to a series of PALs assembled into an array, which generated a focal point about four inches away from the speaker. The team played bass-heavy electronic music, slowly moving a microphone in and out of the focal point to test the audio quality and confinement of the sound.
While in the focal point, the microphone recorded clear, high-quality audio. However, moving the microphone just two inches — less than the width of a credit card — out from the focal point drastically reduced the volume by as much as 50 decibels.
Additionally, audio produced from PALs using the team’s acoustic metasurfaces demonstrated impressive sound quality across both high and low-end frequencies, which has traditionally been a struggle to focus into a focal point. Tests showed that the PALs could effectively project frequencies as low as 38 Hz, around the deepest, lowest-pitched range of human hearing, something that traditionally requires bulky subwoofers to accomplish. According to Kim, this could be a major step in producing smaller speaker systems that maintain sound quality.
“The acoustic metasurface is about six inches in diameter — around the size of a small plate — and can be applied directly onto the surface of any PAL,” Kim said. “We believe this holds great commercial potential, as companies would just need a 3D-printer or a plastic mold to mass produce these components.”
The team said they believe the technology could be useful anywhere where personalized, private listening would be beneficial like ATMs, ticketing terminals, retail displays and in vehicles, to allow for multiple people to listen to different streams of private audio.
Jia-Xin Zhong, a postdoctoral scholar in acoustics at Penn State, also contributed to this work.
This work was supported by the U.S. National Science Foundation.
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Experimental study
Not applicable
Audible Focal Spot Generation via a Metasurface-enabled Parametric Array Loudspeaker
1-May-2026