Ultrasonic beams fired through a 3D-printed metasurface can create localized pockets of sound that are inaudible to passers-by. The technique could be used to create private speech zones for secure communications or enable personalized audio spaces in public spaces and vehicles.
The ability to deliver sounds to a specific listener without the need for headphones, known as directional sound, has been a long standing area of research in audio engineering. But achieving this typically requires large and complicated sound sources and it is often possible to hear the audio signal along the path of the beam.
A new approach from researchers at The Pennsylvania State University gets round these limitations by combining a compact array of ultrasonic emitters with a specially patterned 2D structure, which is designed to manipulate the properties of waves. This structure, known as a metasurface, creates “self-bending” ultrasound beams that are inaudible to humans and can steer round obstacles. When two of these beams cross paths they interact in a way that generates sound in a human’s audible range but confined to a spot just a few centimeters across, which the researchers call an “audible enclave.”
“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,” says Jia-Xin Zhong, a postdoctoral research at Penn State and lead author of a paper in the Proceedings of the National Academy of Sciences that describes the new approach.
Previous research has demonstrated audible self-bending beams that can curve around obstacles. But the long wavelengths of audible sound mean the sources typically have to be on the scale of meters and it is possible to hear the signal anywhere along the path of the beam.
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