Combining state-of-the-art acoustics, 3D printing and materials science, researchers at Penn State University and Lawrence Livermore National Laboratory (LLNL) have unveiled a novel technique that allows sound to be delivered to a precise location, circumventing physical barriers without the need for on-ear devices.
Published in Proceedings of the National Academy of Sciences, the study explores the concept of "audible enclaves" - localized audio spots created through the nonlinear interaction of self-bending ultrasonic beams. This research overcomes longstanding challenges in audio engineering by harnessing advanced wavefront manipulation and material design.
LLNL engineer Xiaoxing Xia played a key role in fabricating intricate 3D-printed acoustic metasurfaces, components that enabled precise control of ultrasonic wave trajectories. Using a high-resolution stereolithography 3D printer, Xia fabricated the metasurfaces with complex air channel networks, which shape the phase and amplitude of sound waves as they pass through. The design and printing process involved carefully constructing individual meta-atoms, then spatially arranging these meta-atoms to modify the wavefront and achieve the desired acoustic effect.
"LLNL's capability in additive manufacturing (AM), design for manufacturability and materials feedstock is very useful for pushing the metasurface community forward," Xia said. "We can use metasurfaces, optical or acoustic ones, to solve mission-related challenges, like high-throughput 3D printing or hyperspectral imaging, and also help move the research community forward by demonstrating and scaling up AM capabilities."
The challenge of directed sound
For decades, audio engineers have sought to isolate and direct sound in environments where traditional methods - such as loudspeakers or noise-canceling headphones - are impractical. While parametric speaker arrays and digital signal processing have made strides in this direction, they remain limited by the diffraction of long-wavelength audio waves and bulky device size, according to the researchers.
The team tackled this issue by leveraging self-bending ultrasonic beams that generate audible sound only at specific intersecting points, creating a personal audio experience without affecting the surrounding environment. Xia said that prior collaborations helped address many of the fabrication challenges, particularly in printing small channels with overhanging structures.
"I had to try various printing angles, supporting structures and post-processing methods to get it right, but that [prior] experience carried over, so this time, printing was easy," Xia said.
A major innovation in the study was the use of nonlinear acoustic interactions - where two self-bending waves merge to produce excitation at the difference of their frequencies - to generate sound in the audible range while keeping the ultrasonic wave components inaudible.
"Controlling longer wavelengths (audible frequencies) would require very large structures, plus longer wavelengths tend to diffract and reverberate with the room," Xia said. "The [Penn State] method circumvented the difficulties, making the device much more compact."
Reducing the devices' size could open up the application space - from private speech communications and immersive spatial audio to advanced noise control in urban environments, audible enclaves could transform how sound is transmitted and experienced, researchers said. While the enclaves' nonlinear effects provide flexibility, Xia acknowledged that further research is needed to refine the technique and explore additional applications.
"I think we still need to find the most useful application," Xia said. "It can be used in making quiet zones and audible zones, but cannot justify the difficulties in making it happen, practically speaking. Maybe further application can include encrypted information communication or sending information around some kind of blockage … This [paper] is fundamental work that will open doors to more practical applications later."
Xia envisions broader applications of 3D-printed metasurfaces beyond acoustics, such as medical imaging, concealed object detection and wireless communications using electromagnetic metamaterials in the terahertz range. Xia and LLNL scientists and engineers Wonjin Choi, Widiano Moestopo and Songyun Gu are collaborating in this area.
As Xia and his colleagues at Penn State push the boundaries of acoustic engineering through advanced manufacturing, a future where sound is as precisely engineered as light in a laser beam - targeted, controlled and tailored to individual needs - may not be far on the horizon.
Co-authors on the paper included Jia-Xin Zhong, Jun Ji, Hyeonu Heo and Yun Jing of Penn State.
