Music lovers may one day be able to blast their favorite artists, headphone-free, without angering the neighborhood or colleagues, thanks to researchers at Penn State.
The team designed a system that can manipulate sound waves so that they are only audible at a precise spot slightly wider than an inch. Despite this tiny focal point, their system can produce high-quality audio, potentially offering listeners a crisp, yet private, sound experience. The team detailed their work in a paper recently published in IEEE Transactions on Ultrasonics.
Sound waves traditionally spread outwards from their source, explained Jee Woo Kevin Kim, an acoustics doctoral candidate and first author on the paper. Parametric array loudspeakers (PAL) use high-intensity ultrasonic waves to focus audible sound into a narrow, laser-like beam of sound. Kim said that although these arrays are used in spaces like museums and broadcast rooms to transmit sound discretely, they face issues that make their application outside of these specialized environments difficult.
"These arrays are so directional that once the sound beam comes in contact with a surface, the sound can reflect all around the room, compromising privacy," Kim said. "Additionally, they struggle to produce low-end frequencies, which can take away from the experience of listening to bass-heavy music, for example."
Yun Jing, professor of acoustics and corresponding author on the paper, said that acoustic metasurfaces can address this key issue. Metasurfaces are a class of materials that can manipulate waves - including light, sound, heat and more - with just their thin structures. Jing explained how acoustic metasurfaces used to manipulate waves of sound can be easily 3D-printed, and have been widely used to direct specific sounds, such as in speakers.
"To develop an acoustic metasurface, we use a large surface that works like a lens focusing a beam of light," said Jing, who holds an additional affiliation in biomedical engineering. "The surface modulates sound waves in such a way that they converge at a central point after leaving the speaker, allowing us to focus the audio into a precise area."
