“The key innovation is that sound is only generated where two beams intersect, making it possible to deliver audio to a precise spot while keeping the beams themselves silent,” describes Penn State postdoctoral researcher Jia-Xin Zhong. This new method of delivering sound will dramatically change the manner in which we hear in public areas, vehicles, and workplaces.
Sound, as we are used to it, travels via air as waves produced by oscillating bodies. Even though it is possible to contain these waves to a certain extent, their tendency to diverge called diffraction has long been a source of concern for engineers who needed to confine sound into specific areas. This is particularly the case with low-frequency sound due to their longer wavelength. Earlier technologies, like parametric array loudspeakers, have been able to concentrate sound beams in one direction, but the sound can still be heard along the beam path.
Researchers at Penn State have significantly pushed the idea of “audible enclaves” a notch further by using self-bending ultrasound beams and nonlinear acoustics. Ultrasound having frequencies over 20 kHz, too high for human beings to hear, is used as the carrier of the sound of audio in this method. Intersecting two slightly different frequency beams of ultrasound, e.g., 40 kHz and 39.5 kHz, researchers produce a new difference frequency wave of 500 Hz, which human beings are able to hear. Most importantly, such a noise is limited to the very place where the beams cross each other, and the rest of the distance is quiet.
What is even more incredible about the method is the self-bending capability of the ultrasound beams. With 3D-printed metasurfaces specialty materials that are meant to control sound waves, the scientists can control the phase of the beams so that they can bend around objects and meet at a point of interest. Zhong calls these metasurfaces “like an acoustic lens,” bending the wavefronts in a manner to have self-bending trajectories.
The uses of this technology are boundless. Picture strolling through a museum and listening to an audio guide directed at your exact location without the need for headphones, or car passengers listening to varying audio streams without annoying each other. Libraries would be able to provide audio lessons to students without inconveniencing others, and offices can create local zones for sensitive conversation. Even military uses, where secrecy is topmost, can tap into this technology.
The Penn State researchers tried out their method with a dummy, bending the beams around the top of its head so that they intersected just in front of its face. The outcome was a sound bubble of a few centimeters’ diameter, radiating a single-frequency tone at 500 Hz. They then showed off the system’s flexibility by making sound above six octaves, from 125 Hz to 4 kHz, and even singing a 9-second clip of Handel’s “Hallelujah Chorus.”
Encouraging as the technology is, it is not without challenges. Nonlinear distortion, which can muddy the audio signal, is still a problem. Zhong says that sophisticated signal processing schemes, such as deep learning algorithms, would be able to reduce the distortion. Besides, the existing system demands fixed beam paths, meaning sound sources must be accurately positioned so as not to hit obstacles. The team is working on adaptive processing algorithms to allow dynamic real-time adjustments to make beams more flexible in environments with dynamic obstacles and shifting positions.
Efficiency of energy is another issue. Ultrasound-to-sound translation is energy consuming because it is based on high-intensity fields. However, the scientists remain hopeful of overcoming the challenges, citing the transformative potential of sound enclaves. As per Zhong, “We envision using adaptive processing algorithms to dynamically adjust beam trajectories in real time, allowing the beams to intelligently navigate around obstacles based on environmental feedback.”
The social consequence of the innovation is significant. In addition to augmenting personal audio experiences, the technology might be used to cancel noise, generating quiet spaces in city life or offices. In altering the physics of the way sound responds to space, researchers are making new possibilities for immersive and effective sound control. With the advancing science, the dream of sending sound exactly where it’s needed without headphones, cables, or interrupting disruptions becomes closer to becoming a reality.
To technology buffs and engineers, the invention of auto-curving sound beams is an engineering achievement in the field of audio engineering. It’s a demonstration that computational algorithms, acoustics, and materials science can be put together to address centuries-old problems and create a new era where sound can be manipulated as easily as the equipment we use to produce it.
