Researchers at Penn State have developed a new way to deliver private audio without headphones. The system creates a tiny pocket of sound that only a nearby listener can hear, even in shared spaces. The technology could eventually change how people listen to music in public, watch videos in cars, or interact with digital kiosks.

The research team built the system using a 3D-printed acoustic metasurface. The device directs sound waves into a confined area slightly larger than an inch. Outside that narrow zone, the audio quickly fades.

Focused sound bubbles

Traditional speakers spread sound in every direction. Even directional systems often lose privacy because sound bounces off nearby surfaces. Existing parametric array loudspeakers also struggle with deep bass sounds.

Jee Woo Kevin Kim, an acoustics doctoral candidate and lead author of the study, said current systems work well in controlled environments but face challenges elsewhere.

“These arrays are so directional that once the sound beam comes in contact with a surface, the sound can reflect all around the room,” Kim said. He added that this can weaken privacy and reduce the listening experience for bass-heavy music.

The team turned to acoustic metasurfaces to solve the problem. These engineered materials can manipulate waves using carefully designed structures. Researchers can also produce them with standard 3D printers.

Yun Jing, a professor of acoustics at Penn State, explained that the metasurface works much like a lens.

“The surface modulates sound waves in such a way that they converge at a central point after leaving the speaker,” Jing said. “That allows us to focus the audio into a precise area.”

Smaller speakers, deeper bass

Earlier experiments from Jing’s lab used multiple ultrasonic speaker arrays to create a private listening zone. However, those systems required complex processing and consumed more energy.

The new design takes a simpler approach. The 3D-printed metasurface passively bends the sound waves into a fixed focal point. It does not require additional electronics or advanced signal controls.

The researchers attached the six-inch circular metasurface to an array of ultrasonic speakers. During tests, the setup produced a listening point about four inches away from the source.

The team played electronic music with strong bass frequencies while moving a microphone around the focal area. Inside the targeted spot, the microphone captured crisp audio. Moving it just two inches away sharply reduced the sound level by nearly 50 decibels.

The system also handled lower frequencies better than many existing directional speakers. Tests showed it could reproduce sounds as low as 38 hertz, close to the lower limit of human hearing.

Kim said that capability could help companies build smaller sound systems without sacrificing audio quality.

Commercial uses ahead

The researchers believe the technology could support a wide range of commercial applications. Public kiosks, ATMs, and ticket machines could deliver private instructions without headphones. Vehicles could also let passengers listen to different audio streams at the same time.

Kim said manufacturers could produce the metasurface cheaply using plastic molds or 3D printers. That could make the technology easier to scale for consumer devices.

Jia-Xin Zhong, a postdoctoral scholar in acoustics at Penn State, also contributed to the research. The U.S. National Science Foundation funded the work.

The study is published in the journal IEEE Transactions on Ultrasonics.