Manipulating Sound to Enhance Acoustic Imaging

North Carolina State University Raleigh, NC

A team of engineers from North Carolina State University and Duke University has developed a metamaterial made of paper and aluminum that, they say, can manipulate acoustic waves to more than double the resolution of acoustic imaging, focus acoustic waves, and control the angles at which sound passes through the metamaterial. Acoustic imaging tools are used in medical diagnostics and can also be used to test the structural integrity of everything from airplanes to bridges.

“This metamaterial is something that we’ve known is theoretically possible, but no one had actually made it before,” says Yun Jing, an Assistant Professor of Mechanical and Aerospace Engineering at NC State.

How It Works

Metamaterials are materials that have been engineered to exhibit properties that are not found in nature. In this case, the structural design of the metamaterial gives it qualities that make it a “hyperbolic” metamaterial, which means that it interacts with acoustic waves in two different ways. From one direction, the metamaterial exhibits a positive density and interacts with acoustic waves normally, like air. But from a perpendicular direction, the metamaterial exhibits a negative density in terms of how it interacts with sound. This effectively makes acoustic waves bend at angles that are the exact opposite of what basic physics would tell researchers to expect. The practical effect of this is that the metamaterial may have some very useful applications. (See Figure 1)

The researchers explained how the metamaterial can be used to improve acoustic imaging. Traditionally, acoustic imaging could not achieve image resolution that was smaller than half of a sound’s wavelength. For example, an acoustic wave of 100 kilohertz (kHz), traveling through air, has a wavelength of 3.4 millimeters (mm). Therefore, it could not achieve image resolution smaller than 1.7 mm.

“But our metamaterial improves on that,” says Chen Shen, a PhD student at NC State and a lead researcher. “By placing the metamaterial between the imaging device and the object being imaged, we were able to more than double the resolution of the acoustic imaging—from one-half the sound’s wavelength to greater than one-fifth.”

The metamaterial can also focus acoustic waves, making it a flexible tool.

“Medical personnel and structural engineers sometimes need to focus sound for imaging or therapeutic purposes,” Jing says. “Our metamaterial can do that, or it can be used to improve resolution. There are few tools out there that can do both.”

Right now, the prototype metamaterial is approximately 30 centimeters square, and is effective for sounds between 1 and 2.5 kHz. The researchers’ next steps are to make the structure much smaller, and to make it operate at higher frequencies.

For more information, visit https://news.ncsu.edu .

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