Consider, for your listening pleasure, the next frontier in sonic miracles: so-called  "audible enclaves," where only you can hear a sound without disturbing anyone else in open space — no headphones required.

As developed by a team of researchers at Penn State, whose findings are published as study in the journal Proceedings of the National Academy of Sciences, this technology can create pockets of sound isolated from their surroundings that can be targeted at a specific location, potentially picking someone out in the middle of a crowd.

Crucially, the sound waves — specifically, ultrasound waves — used to create the audible enclaves can't be heard along the path they travel and can even be bent to avoid obstacles. Only when they reach their destination are the sound waves picked up by human ears. 

"We essentially created a virtual headset," said lead author Jiaxin Zhong, an acoustics researcher at Penn State, in a statement about the work.  "Someone within an audible enclave can hear something meant only for them — enabling sound and quiet zones."

As Zhong and coauthor Yun Jing, a Penn State professor of acoustics, explain in an essay for The Conversation, sound waves diffract or spread out as they travel, especially at lower frequencies. That makes containing them difficult.

To sidestep that issue, the researchers use two beams of ultrasound waves — the type used in medical imaging — which vibrate at a frequency way above human hearing as a "carrier for audible sound." That way as they travel, they're silent to human ears and only become audible when they reach their target.

"The person standing at that point can hear sound, while anyone standing nearby would not," Jing said in the statement.

But what if there are obstacles in the way? That's where another key innovation comes in: ultrasound beams that can bend. To achieve this, the researchers used "acoustic metasurfaces" which can precisely manipulate the paths of sound waves as they travel — "similar to how an optical lens bends light," wrote Zhong and Jing in The Conversation.

The other feat is transforming the ultrasound beams into something we can hear. This is accomplished by projecting each beam at slightly different frequencies — one at 40,000 Hz and one at 39,500 Hz, for example. When they intersect, the crossing beams generate a new sound wave equal to the difference between the beams, creating, in this scenario, a sound wave at 500 Hz, which is comfortably in the midrange of human hearing.

"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," explained Zhong in the statement. "We confirmed that sound was not audible except at the point of intersection, which creates what we call an enclave."

Right now, the researchers say they can create these audible enclaves at a distance of about three feet away at a moderate 60 decibels, roughly the volume of a conversation. Though they caution that the tech is still a long way off from hitting the market, the potential applications are thrilling. In public spaces, audio enclaves could provide sound to certain groups without disturbing others, like in a library or museum, the researchers write in their essay. It could potentially even be used for noise-cancellation in an entire area, providing peace and quiet in noise-polluted cities.

More on acoustics: Scientists Say They've Invented a Speaker That "Mutes" Annoying People


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