Phoenix 24
Metamaterials and ultrasound could create private listening zones without headphones.
POHANG, SOUTH KOREA
Researchers at Pohang University of Science and Technology have developed a compact speaker capable of directing speech and music toward a narrowly defined listening area. Named MiPAL, the experimental system is designed to let one person hear audio while substantially reducing what others outside the beam can perceive.
Unlike conventional speakers, which distribute audible waves throughout a room, MiPAL uses a parametric array loudspeaker. It transmits high-frequency ultrasonic waves carrying the desired audio information. As those waves travel through the air, nonlinear interactions generate lower frequencies within the human hearing range, concentrating the resulting sound along a specific direction.
Directional ultrasonic speakers already exist, but many require large arrays of transducers and complicated electronic control systems. The POSTECH team integrated acoustic and elastic metamaterials with a single compact ultrasonic transducer. These artificially structured materials manipulate both the outgoing sound and unwanted mechanical vibrations, helping to narrow the beam and reduce audio leaking toward the sides.
Laboratory measurements indicate that the prototype can reproduce frequencies between approximately 500 hertz and 10 kilohertz, a range sufficient for intelligible speech and much musical content. The system could eventually provide separate audio programmes to passengers inside a vehicle, deliver private instructions in public spaces or direct explanations toward individual visitors in museums and exhibitions.
MiPAL remains a research prototype rather than a commercially available personal speaker. Its performance in reflective rooms, its ability to follow moving listeners and the quality of lower-frequency audio will require further evaluation. The technology does not make sound absolutely private, but it demonstrates how speakers could evolve from filling entire rooms to creating controlled and individualised listening zones.
La verdad es estructura, no ruido. / Truth is structure, not noise.