A new technique sends sound to a specific person without the surrounding people hearing it

The institution itself acknowledges that “it sounds like science fiction.” A team of scientists from Pennsylvania State University has created a technique for sending sounds remotely to a specific person, without anyone hearing it along the way. In the experiment announced on March 17, the researchers fired two independent beams of inaudible ultrasound, each surrounding the recipient’s head on one side. When they cross in front of the face, they interact, producing the sound of the famous chorus from Handel’s Messiah: “Hallelujah! Hallelujah!” The scientists call these remote sound bubbles “audible enclaves” or “whispering beams.”

Mechanical engineer Jiaxin Zhong explains the possible applications to this newspaper, such as receiving personalized audio messages in public spaces. “Museums, libraries, or exhibitions could offer personalized sound without the need for headphones,” he notes. “Car drivers could receive navigation instructions while passengers enjoy music without distractions,” adds Zhong, one of the lead authors of the method. These personalized sound bubbles, he says, could also facilitate confidential military communications, improve the virtual reality experience, and even create silent zones in noisy environments, if the option is to selectively cancel out unwanted sound. The results of the experiment were published Monday in the PNAS journal of the U.S. National Academy of Sciences.

The experiment is promising, but it has significant limitations, according to Yun Jing, professor of acoustics and biomedical engineering at the Penn State College of Engineering. First, the chorus of Messiah sounds like a poorly tuned old radio. “To achieve better sound quality, we will need better ultrasound emitters, because the ones we are using now are very cheap ones purchased for the proof of concept,” explains Jing. His team is also exploring artificial intelligence tools to offer a clearer result.

Distance is another relevant factor. The source of the ultrasound is an acoustic metasurface, an ultrathin material capable of modifying the waves that strike it. In the experiment, the scientists placed a dummy a few centimeters from the emitter, but Jing claims they could create a sound bubble around a person standing “a few meters” away. “The challenge is that ultrasonic waves attenuate very quickly in air, so reaching 100 meters is going to be difficult unless we have very powerful ultrasonic emitters,” he explains. The advantage of their ultrasound source is that it measures just 16 centimeters in length, a compact format that would facilitate its applications.

Telecommunications engineer Juan Miguel Navarro, from the Catholic University of Murcia in Spain, collaborated with Yun Ling over a decade ago on acoustic simulations for large venues. Navarro notes that sound focusing has been used for over 20 years in security applications, such as long-range acoustic devices, considered non-lethal weapons because they emit painful sounds that incapacitate the enemy. “What’s novel about this new study is that it allows us to reproduce a sound bandwidth suitable for transmitting low-fidelity vocal and musical signals,” the engineer notes.

In 2017, Spanish sound engineer Marcos Simón founded Audioscenic in Southampton, England. He develops devices capable of detecting the listener’s ears and transmitting focused sound. Simón applauds the new work, in which he was not involved. “The methodology presented is truly innovative, and as far as I know, nothing similar has been implemented before.”

The Spanish engineer, however, emphasizes that, in his opinion, there are “significant technological limitations” to implementing this idea. “First, ultrasonic loudspeakers require very high energy levels, which implies high power consumption. Furthermore, for ultrasonic loudspeakers to emit audible sound, second-degree intermodulation products [the signals generated by the combination of ultrasonic waves] are required. This requires generating very high acoustic pressure, which raises questions about possible effects on the human auditory system, especially considering that safe exposure levels to ultrasound are not yet fully defined,” warns Simón, a visiting researcher at the University of Southampton. “As a research contribution, the work is very interesting and innovative. However, from a technological perspective, it still requires significant development before it can be applied in practical contexts,” he concludes.

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