In my humble opinion, headphones are one of the greatest inventions of all time. They provide wearers with an immersive experience that few other products can replicate. Whether you're listening to your favorite song, learning something new from a podcast, or finding inner peace with a guided meditation, headphones block out the distractions of the day, allowing you to escape into your thoughts and emotions. However, headphones are not without drawbacks. The classic over-the-ear style can be cumbersome and difficult to use on the go. Wired models tether you to your device, while wireless earbuds are useless if you forget to charge them. So, what’s the solution? Is there a way to enjoy your favorite media without disturbing those around you, that doesn’t require headphones? The answer may be audible enclaves.

Researchers at Penn State College of Engineering have developed localized sound zones known as audible enclaves. According to the research team, which is led by Yun Jing, a professor of acoustics at Penn State College of Engineering, within these enclaves, sound can only be heard by a specific listener. Others nearby, even in an enclosed space like a vehicle or directly in front of the audio source, are unable to hear the sound. These audible enclaves are created using two nonlinear ultrasonic beams. The ultrasonic beams are inaudible on their own, but when they intersect, they generate audible sound through a nonlinear interaction. The waves can also bypass obstacles, like human heads, to reach the specific point where the sound is heard.

The researchers used metasurfaces—acoustic lenses with microstructures—to direct two ultrasonic waves at different frequencies along a crescent-shaped path until they intersect. These metasurfaces, 3D printed by Xiaoxing Xia, a staff scientist at Lawrence Livermore Laboratory, enable the precise targeting of sound.

In a recent quote, Jia-Xin “Jay” Zhong, a postdoctoral scholar in acoustics at Penn State, said, “To test the system, we used a simulated head and torso dummy with microphones inside its ears to mimic what a human being hears at points along the ultrasonic beam trajectory, as well as a third microphone to scan the area of intersection. We confirmed that sound was not audible except at the point of intersection, which creates what we call an enclave.”

Currently, researchers can transmit sound up to one meter from the target at a volume of 60 decibels, similar to normal speaking volume. However, they believe that both the distance and volume could be increased by enhancing the ultrasound intensity.

The team recently published their findings in the Proceedings of the National Academy of Sciences. The paper, titled "Audible enclaves crafted by nonlinear self-bending ultrasonic beams,” outlines the team’s efforts to overcome the physical limits in linear acoustics. In an excerpt from the abstract, the researchers write: “Here, we introduce an approach for creating remote audio spots, dubbed audible enclaves, by harnessing the local nonlinear interaction of two self-bending ultrasonic beams with distinct spectra. The self-bending ultrasonic beams created by acoustic metasurfaces, though inaudible, can bypass obstacles such as human heads. At their intersection behind obstacles, highly localized audible enclaves are formed due to the local nonlinear interactions. Additionally, we demonstrate the ultrabroadband capabilities of our metasurface-based implementation both numerically and experimentally, spanning from 125 Hz to 4 kHz (6 octave bands), covering the majority of the audible frequency range. The practicality of our proposed technique is underscored by its compact implementation size (0.16 m, equivalent to 0.06 wavelengths at 125 Hz), as well as its robust performance under wideband transient audio signal excitation and in a common room with reverberations.”