If scientific research and development do not end up boggling the mind, then what is it even doing? A new research on sound may ‘bend’ the rules (pun intended) of how sound is heard and transmitted, forever changing the way we listen through wired or bluetooth earphones, for example. Researchers at The Pennsylvania State University and Lawrence Livermore National Laboratory, Livermore in the United States, have developed sound that can bend itself through space and travel only to the ear in a crowd. It could allow individuals to hear music or a podcast without the use of headphones or earbuds — and without annoying anyone around them. Picture holding a personal conversation in the midst of a crowded street or in a busy metro, with no one else able to hear a single word of it. It appears to be straight from a sci-fi movie scene but in reality, it is an achievement now underpinned by experimental findings.
Localised sound
The research, titled Audible enclaves crafted by nonlinear self-bending ultrasonic beams published in Proceedings of the National Academy of Sciences (PNAS) earlier this year, is based on the concept of ‘audible enclaves’ — strongly localised areas of sound created at precise points in space, invisible and silent to everyone beyond them. “Delivering audible content to a targeted listener without disturbing others is paramount in audio engineering. However, achieving this goal has long been challenging due to the diffraction of low-frequency (long-wavelength) audio waves in linear acoustics. Here, we introduce an approach for creating remote audio spots, dubbed audible enclaves, by harnessing the local nonlinear interaction of two self-bending ultrasonic beams with distinct spectra,” wrote the researchers. In other words, they have developed a technology that could create sound exactly where it needs to be — to be heard by the recipient.
The study by Jia-Xin Zhong, Jun Ji, Hyeonu Heo and Yun Jing of The Pennsylvania State University and Xiaoxing Xia of Lawrence Livermore National Laboratory, Livermore, demonstrates how sound can be engineered to travel silently through air and become audible only where desired. This could transform everything from entertainment and personal communication to military operations and smart environments.
The physics of it
To understand this development, we need to understand what indeed is sound and how does it work? Sound is a mechanical wave, created by oscillating particles moving through air. Such waves will disperse while travelling — a phenomenon called diffraction. Lower-pitched sounds such as bass notes or voices have larger wavelengths and tend to spread out more easily so it’s difficult to trap them in a confined space. In an article in The Conversation, researchers Jiaxin Zhong and Yun Jing explain that traditional technologies such as parametric loudspeakers have the potential to focus sound into directional beams, yet they continue to release noise along the line of transmission. The research, thus, stands apart in that it can produce audible sound only at a focused location and nowhere else.
It’s accomplished through ultrasound, above 20 kilohertz frequency sound waves inaudible to the human ear. Two self-folding ultrasonic beams are pointed so they will cross at a specified location. Independently, neither makes a sound. But where the beams do coincide, nonlinear acoustic interaction causes the third resultant wave, and that is one audible frequency, the difference between the first two.
“When two ultrasonic beams with slightly different frequencies, like 40kHz and 39.5 kHz, overlap, they produce a new sound wave at the difference between their frequencies – in this case 0.5 kHz or 500 Hz, ” they explain. “Sound is only audible where the beams intersect. Outside that intersection, the ultrasound waves are silent.” Even more remarkable is that these beams are not linear, they can be curved around barriers, thanks to acoustic metasurfaces. These are custom-designed materials that warp the sound wave path of sound, in much the same way a lens warps light.
What can it lead to?
The uses are mind-bending. Museums might provide individualised audio guides without a single headset. Office workers might attend conference calls in open-plan areas without disturbing neighbours and co-workers. Cars might provide individual soundtracks to each passenger. Military and security contexts might be improved by highly localised communications. And apart from providing sound, the technology can be reversed to cancel out sound. Through the creation of areas where individual noise frequencies are destructively interfered with, scientists can provide quiet areas even in noisy conditions. The researchers tried out their prototype in simulated environments, rooms with echoes and obstructions like human heads, and still obtained results. Their 16-centimetre device produced sound spots that lasted six octaves, from 125 Hz to 4 kHz, covering almost the entire human hearing range. “The pragmatism of our suggested approach is highlighted by its small implementation size…and its stable behaviour under wideband transient audio signal excitation,” they mention.
Is it consumer ready yet?
The idea works, but there are hurdles to overcome. The technique uses high-intensity ultrasound to initiate the nonlinear interaction. That takes a tremendous amount of energy, which introduces questions about efficiency and heat management. Nonlinear distortion, sound quality degrades, also has to be refined before the system can compete with conventional headphones or speakers in fidelity. “None of this is going to sit on the shelf in the short term,” the authors concede. But the potential is real. By “remapping how sound interacts with space,” they explain, the work “opens up new possibilities for immersive, efficient and personalised audio experiences.”
How does the future ‘sound’?
Whether or not earphones become extinct, the very notion of sound is changing. For centuries, sound has followed the same physical rules, radiating out, bouncing off walls, impacting everyone within earshot. Now, due to breakthroughs in nonlinear acoustics and metamaterials, sound can be personalised, private, and exactly directed. In an ever-noisier world, the ability to decide what you hear, and don’t hear, could be the next sensory technology revolution.
