New Tech Improves Ability to Reflect Sound Back to Source

Yun Jing

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(a) Schematic illustration of a cat’s eye retroreflector composed of a focusing lens and a concave mirror. (b) Illustration of the proposed acoustic APR composed of two planar surfaces. Planar surface I directs waves with different incident angles to different spots on planar surface II. Planar surface II is a gradient surface and adds a spatially varying wave vector equal to twice that of the wave, but with the opposite sign.
Researchers have developed a device that reflects sound in the direction it came from, rather than deflecting it at an angle. The “retroreflector” can reflect sound across an operating range of 70 degrees in either direction – more than doubling the effective range of previous technologies.

“The technology makes use of two engineered materials,” says Yun Jing, an associate professor of mechanical and aerospace engineering at North Carolina State University and co-corresponding author of a paper on the work.

“The first layer focuses the incoming sound waves onto a second layer, which then sends the sound waves back to their source. We were inspired by a similar approach used in optics research, but we think we are the first to use this technique in the acoustics field.”

Previous techniques for creating retroreflective surfaces relied on rectangular pits arrayed across a material. Sound waves would ricochet from the side of the rectangle to the bottom, before bouncing back in the direction they came from.

“However, designs using that approach can be bulky, and have a fairly narrow range of angles that they can reflect properly,” Jing says. “Our technology is both slimmer and effective across a wider range of angles.”

Jing, Yun
Dr. Yun Jing
Experiments using a prototype of the new technology find that it is also fairly efficient. At 0 degrees – when the sound source is perpendicular to the surface – 60 percent of the sound is sent back to the source. At 70 degrees – the extreme end of the effective range – 40 percent of the sound is directed back to the source.

“We have a fully functional prototype now, and our next steps include fine-tuning the technology for use in specific applications, such as medical ultrasound,” Jing says. “Frankly, we think there are likely applications that we haven’t thought of yet.”

The paper, “Acoustic planar surface retroreflector,” was published June 25 in the journal Physical Review Materials. First author of the paper is Gang Yong Song of Southeast University, in Nanjing, China. Co-corresponding authors of the paper are Qiang Cheng and Tie Jun Cui of Southeast University.


Note to Editors: The study abstract follows.

“Acoustic planar surface retroreflector”

Authors: Gang Yong Song, Qiang Cheng and Tie Jun Cui, Southeast University, Nanjing; and Yun Jing, North Carolina State University

Published: June 25, Physical Review Materials

DOI: 10.1103/PhysRevMaterials.2.065201

Abstract: This article reports on the design, numerical simulation, fabrication, and experimental test of an acoustic planar retroreflector capable of effectively reflecting sound along its incident direction for a wide operating angle range (0-70°). The proposed acoustic planar retroreflector is a compound of two cascaded metasurfaces; a transmissive surface that converges the incident beam onto a second planar surface placed behind it, which serves as a reflective surface that bounces the beam back along the incident direction. Both the simulated and measured results provide evidence of the sound retroreflection effect. The structure proposed here provides a possible strategy for improving medical ultrasound, underwater communication, and illusion device design.