Sound engineers exploring ways to listen to podcasts and phone calls ‘privately’ in public have designed customized “audible enclaves” where only the listener can hear the projected sound.
“In an enclave, a listener can hear sound, while others standing nearby cannot, even if the people are in an enclosed space, like a vehicle, or standing directly in front of the audio source,” explains a statement from the Penn State engineers developing the novel concept.
The team hopes their technology can mature to the point of commercial applications that offer unprecedented listening privacy in public settings.
Private Audible Enclaves at the Forefront of the Science of Sound
When lead researcher Yun Jing, a professor of acoustics at the Penn State College of Engineering, began exploring the concept of private audible enclaves, several other scientists were exploring applications of technologies tapping into the unique properties of sound. Some noteworthy efforts, including bouncing lasers off of thin air using sound waves, mapping the “uncharted expanse” of the ocean floor, employing laser-acoustic technology to detect land mines, using ultrasonic acoustics to hack into smart devices, and creating a “magical” sensor that can be activated with a single word.
In the case of science imitating science fiction, researchers are exploring several ways to tap into an acoustic radiation force phenomenon to create a Star Trek-style tractor beam using sound. One project by engineers from the University of Minnesota Twin Cities coated objects with metasurfaces so an acoustic tractor beam could lift much larger objects than other methods.
More speculative research into acoustics, such as efforts to explain several mysterious sounds heard worldwide, include studying “the voices in your head” and even the possibility that the people who built Stonehenge may have factored acoustic properties into its design.
In this latest effort, Penn State College acoustic engineers say that their goal to create audible enclaves where only one person can hear a projected sound may be possible.
The Technological Magic Behind Listening Privately in Public
The researchers proposed using a pair of targeted nonlinear ultrasonic beams in the published article explaining the pursuit of audible enclaves to produce virtual headphones. According to the authors, they found that positioning a pair of metasurfaces that act as acoustic lenses to bend the direction of the sound in front of the two audio transducers generating the beams caused the waves to travel at slightly different frequencies. Depending on the tuning of the metasurfaces, the current-shaped trajectories of the other two beams can be programmed to intersect at a distant point where the sound can be heard.
“We use two ultrasound transducers paired with an acoustic metasurface, which emit self-bending beams that intersect at a certain point,” Jing explained. “The person standing at that point can hear sound, while anyone standing nearby would not. This creates a privacy barrier between people for private listening.”
audible enclaves
By positioning metasurfaces in front of two ultrasonic transducers, dual ultrasonic waves travel at two slightly different frequencies along a crescent-shaped trajectory until they intersect, forming an audible enclave where sound cab be heard. At other points along the trajectory, sound is not heard — meaning private listening is possible. Credit: Provided by Heyonu Heo/Penn State.
The team tested their concept by equipping a simulated human head and torso with ear microphones. They also tested the equipment in a typical room rather than a sound-proof lab to simulate the varying environments in which such a technology might ultimately be applied.
The test area also had a third microphone to scan where the two beams intersect. According to Jia-Xin “Jay” Zhong, a postdoctoral scholar in acoustics at Penn State and a research team member, the result was the equivalent of a virtual, completely private headset.
audible enclaves
Jia-Xin “Jay” Zhong, a postdoctoral scholar of acoustics at Penn State, used a dummy with microphones in its ears to measure the presence or absence of sound along an ultrasonic trajectory. Credit: Poornima Tomy/Penn State University.
“We confirmed that sound was not audible except at the point of intersection, which creates what we call an enclave,” Zhong said.
warp drives
Perhaps just as significant, the individual sound beams are not audible independently. Combined with the ability to bypass obstacles like furniture or humans, the system can generate sound one person can hear, but the person right next to them cannot.
Future Efforts Will Try to Increase the Distance and Volume
The engineers can remotely transfer sound about a meter from the intended target in its current configuration. At that range, the sound volume is about 60 decibels, equivalent to the speaking volume. They hope to explore increasing the system’s distance and volume by amplifying the ultrasound intensity.
While more work remains to evolve the concept of audio enclaves from the lab to a commercially viable product, the initial prototype has the research team celebrating the first-ever headphones that function without a physical device.
“We essentially created a virtual headset,” Zhong said. “Someone within an audible enclave can hear something meant only for them.”
The article “Audible enclaves crafted by nonlinear self-bending ultrasonic beams” was published in Proceedings of the National Academy of Sciences.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him onX,learn about his books atplainfiction.com, or email him directly atchristopher@thedebrief.org.