New Metamaterial Manipulates Sound to Improve Acoustic Imaging
For Immediate Release
来自北卡罗来纳州立大学和杜克大学的研究人员开发了一种由纸和铝制成的超材料，可以操纵声波，以使声学的分辨率，聚焦声波和控制声音经过的角度的角度的分辨率两倍以上。声学成像工具均用于医学诊断和测试从飞机到桥梁的所有事物的结构完整性。
“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 and corresponding author of a paper describing the work.
Metamaterials are simply 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. This 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 – just 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 you to expect.
The practical effect of this is that the metamaterial has some very useful applications.
For one thing, 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) – so it couldn’t achieve image resolution smaller than 1.7 mm.
“But our metamaterial improves on that,” says Chen Shen, a Ph.D. student at NC State and lead author of the paper. “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, which makes 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.”
Lastly, the metamaterial gives researchers more control over the angle at which acoustic waves can pass through it.
“For example, the metamaterial could be designed to block sound from most angles, leaving only a small opening for sound to pass through, which might be useful for microphones,” Shen says. “Or you could leave it wide open – it’s extremely flexible.”
目前，超材料的原型大约为30厘米，可有效1至2.5 kHz。
“Our next steps are to make the structure much smaller, and to make it operate at higher frequencies,” Jing says.
The paper, “宽带声音双曲线材料,” was published online Dec. 16 in the journalPhysical Review Letters。该论文由NC State的Ni Sui和Yangbo Xie，Wenqi Wang和Duke的Steven Cummer合着。
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编辑报告s:The study abstract follows.
“Broadband Acoustic Hyperbolic Metamaterial”
Authors: Chen Shen, Ni Sui, and Yun Jing, North Carolina State University; Yangbo Xie, Wenqi Wang, and Steven A. Cummer, Duke University
Published: Jan. 16,Physical Review Letters
DOI: 10.1103/PhysRevLett.115.254301
Abstract:在这封信中，我们报告了宽带声音双曲线材料的设计和实验表征。所提出的超材料由面向Y方向的多个夹紧的薄板组成，并显示出在X和Y方向的有效密度相反的迹象，低于一定的截止频率，因此产生双曲线分散体。在1.0 kHz和2.5 kHz之间的频率下，在实验上证明了部分聚焦和亚波长度成像。拟议的超材料可以为声波操纵开辟新的可能性，并可能在医学成像和非破坏性测试中使用。
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