Duke engineers have designed a device that redirects sound waves almost without scattering. 

Duke researchers at the Center for Metamaterials and Integrated Plasmonics recently constructed a device with metamaterials, modified materials that redirect and reflect light and sound waves with their physical properties. This is similar to how lenses with varied thickness redirect light to different levels of precision. The researchers published a paper in Nature on this device early this month. 

Previous devices to control acoustic waves have not been able to achieve a thickness comparable to that of lenses for light without becoming too large to manipulate sound at its natural wavelengths, said graduate student Junfei Li, co-lead author on the paper. 

But the use of metamaterials—tiny and plain structures—can manipulate these relatively small waves effectively, he added.

“Typically, when you want to control the properties of a material, you use chemical reactions or molecular-level manipulations,” Li said. "[By studying] structures larger than that scale but smaller than the wavelengths, [we hope] to control the inner structure of the device instead of the molecular properties of the material."

Other developers have also designed their devices by modifying the internal structure to alter the speed of waves at various critical points, packing material along the surface to control time delay at each point and form a wave front. However, these devices have always produced unwanted scattering of energy.

Li and his co-authors aimed to design a metasurface—a sheet material with a thickness smaller than its wavelength—that could perfectly shape a wave front so all of the energy would go in the desired direction. 

By considering the infinite wave comprising the input of sound and the code of the wave energy of their desired output, the researchers later discovered a device with an asymmetric surface would serve the purpose. 

The design—composed of channels and cavities of varying depth organized into periods—allows developers to separately control the properties of both the input and output sides of the structure. Developers can thus control the amplitude and phase of both transmitted and reflected sound waves going through the device. 

“The overall performance is to direct a wave to the desired direction with 100 percent efficiency,” Li said.

The research team 3D printed their experimental devices themselves, at a cost of approximately 50 dollars per period. The cost of making one of these devices varies depending on the desired size, since they can be made for any desired length or height by adding periods or extending the height of cavities proportionately. 

This device and its design strategy can be implemented to control waves in any context. Li mentioned that a nonprofit organization looking to control ocean waves to prevent corrosion of coral reefs has contacted the team about creating a device for this purpose by using the technology Li's team developed.

“One of our next steps is to design something in the water so when a wave interacts with it, it can be redirected, or can be used to harvest energy for electricity,” Li said.