that can direct sound into a tiny, private listening area, allowing someone to hear music clearly without disturbing people nearby.
The research could one day lead to headphone-free private audio in public places, offices, cars, museums, or even at home.
The study was recently published in IEEE Transactions on Ultrasonics.
Normally, sound spreads outward in all directions after leaving a speaker. This makes it difficult to keep audio private. Some existing technologies, called parametric array loudspeakers, or PALs, can focus sound into a narrow beam using ultrasonic waves. However, these systems still have important problems.
According to the researchers, once the focused sound hits a wall or another surface, it can bounce around the room and lose its privacy.
These systems also struggle to reproduce deep bass sounds, making music sound less rich and powerful.
To solve these issues, the Penn State team designed a special 3D-printed cover called an acoustic metasurface. Metasurfaces are thin materials engineered to control waves such as light or sound in unusual ways.
In this case, the metasurface acts almost like a sound lens. It changes the direction of sound waves so they meet at one very small spot in space.
The result is a tiny “sound bubble” where the audio can be heard clearly, while people just a short distance away hear almost nothing.
The focused listening area is extremely small — only slightly wider than an inch and less than a quarter-inch tall, roughly the size of a stick of gum.
One major advantage of the new design is its simplicity. Earlier systems often required complicated electronics and signal processing to control sound waves. The new metasurface works passively, meaning it does not need extra electricity or advanced computing to shape the sound.
The researchers tested the system by attaching the circular metasurface to an array of PAL speakers and playing bass-heavy electronic music. A microphone was slowly moved through the listening area to measure the sound quality.
Inside the focal point, the music sounded clear and high-quality. But moving the microphone just two inches away caused the sound volume to drop dramatically by up to 50 decibels, making the audio much harder to hear.
The system also performed surprisingly well with low-frequency sounds. It successfully projected audio as low as 38 hertz, which is close to the deepest sounds humans can hear. Normally, producing such low bass requires large subwoofers.
The researchers believe the technology could have many practical uses. In cars, different passengers could listen to separate audio streams without headphones. Retail stores or ticket machines could provide private spoken information without disturbing others nearby. Museums and public displays could also offer personalized audio experiences.
Because the metasurface can be easily produced using 3D printing or plastic molds, the team says it may also be relatively inexpensive to manufacture in the future.
The work suggests that personalized sound systems may eventually become much smaller, more private, and more practical than today’s speaker technology.
Source: Penn State.
