As smartphone and tablet users increase, various designs of earphones and headphones are gaining popularity. Despite their convenience, wireless headphones have the drawback of needing to be charged frequently and the risk of being lost. Researchers in the United States have developed a new sound technology that allows individuals to hear sound privately without the use of headphones.
Researchers from Pennsylvania State University and Lawrence Livermore National Laboratory introduced a technology called audible enclaves, which creates localized sound zones that only emit sound at specific locations, in the Proceedings of the National Academy of Sciences (PNAS) on the 17th.
Localized sound zones precisely narrow down the area where sound can be detected. In designated spaces, sound can be heard, but it becomes inaudible just outside the area. This technology can be applied not only in enclosed spaces like vehicles but also around audio equipment like large speakers.
The research team developed a technology that emits two nonlinear ultrasound beams to create a small area where sound can be heard. By placing a metasurface, an acoustic lens made of microstructures that bend sound waves, in front of the transducer, the two ultrasound beams fly along crescent-shaped curved trajectories at different frequencies until they intersect.
The two ultrasound beams are inaudible on their own. However, sound can be heard precisely at the point where the two beams intersect. This interaction locally creates audible sound. The ultrasound beams can also be directed around obstacles like a person's head to the desired location. Professor Yoon Jin of Pennsylvania State University, who led the research, noted, "When shooting refracted ultrasound beams using two ultrasound transducers paired with an acoustic metasurface, they intersect at a specific point," adding, "People standing at this point can hear the sound, but those nearby cannot, creating a privacy barrier."
The researchers tested the new sound system using a mannequin with a head and torso equipped with a microphone. The microphone detected sound at the intersection of the ultrasound beams, simulating a person listening to music. Jia Xin Zhong, postdoctoral researcher at Pennsylvania State University and first author of the paper, confirmed, "It was verified that sound is inaudible outside the intersection point," stating, "This forms an audible enclave in this way."
The research team also tested this system in a typical reverberant room to examine whether it works in various environments such as classrooms, vehicles, or outdoors. Research Institute Zhong said, "This technology is essentially used to create a virtual headset," stating, "Someone within the audible zone can hear sound that is exclusive to them."
The research team confirmed that sound can be heard at a volume of about 60 decibels (dB) from a distance of approximately 1 meter. They noted that increasing the intensity of the ultrasound could extend the distance and volume.
The sound personalization technology developed by the research team is garnering significant interest from both scientists and corporations. Traditionally, listening to sound without external noise was primarily accomplished through earphones or headphones. However, concerns about the discomfort of wearing devices, fatigue or hearing loss from prolonged use, and difficulty in perceiving surrounding situations or dangers have led to growing attention.
Harman, an American automotive electronics company, has developed personal sound zone technology that creates an audio zone in vehicles, allowing passengers to enjoy music and make phone calls without interference from ambient sounds. This method combines signal processing software with standard automotive speakers and micro speakers to ensure sound is only heard in specific locations within the vehicle. The Japan NTT Data Science Research Institute is developing an augmented reality (XR) technology that combines ambient and specific sounds to deliver only desired sounds without the need for earphones, as well as vehicle space technology that analyzes the characteristics and propagation direction of external sounds using artificial intelligence (AI) to block noise while delivering audio and ambulance sounds.
References
PNAS (2025), DOI : https://doi.org/10.1073/pnas.2408975122