Metamaterial shows control of acoustic waves.
Metamaterial shows control of acoustic waves.

Researchers at Duke University have designed a thin, engineered material that can control the redirection and reflection of sound waves with near perfect efficiency. The new design is the first to demonstrate this control of acoustic waves, and can be quickly and easily fabricated with 3D printers.

Although the theoretical means to produce this kind of metamaterial device has already been proposed, these did not control both the transmission and reflection of sound in precisely the way sought, and could not be experimentally implemented. However, this study produced and tested such a device – with the metamaterial surface being engineered to perfectly and simultaneously control the transmission and reflection of incoming sound waves.

The use of thin structures to control sound is not new, but the usual approach to controlling only the transmission and reflection phase leads to some sound energy being lost. However, as described in Nature Communications [Li et al. Nat. Commun. (2018) DOI: 10.1038/s41467-018-03778-9], by designing and controlling the wave impedance on both sides of the structure, it was possible to manipulate the transmission and reflection of sound perfectly with no energy loss.

The design uses metamaterials, which manipulate waves such as light and sound using their structure. Here, the metamaterial is made from 3D printed plastic, with the shapes of the device's properties allowing it to manipulate sound waves, while the metamaterials could be scaled to affect almost any wavelength of sound. It is comprised of rows of four hollow columns, with each column being around half an inch on a side with a narrow opening cut down the middle of one side. The researchers demonstrated how the device manipulates sound through the width of the channels between each row of columns and the size of the cavity inside each individual column. Each column resonates at a different frequency based on how much of it is filled in with plastic.

“This is an idea that can be deployed any time one needs to control or redirect the reflection or transmission of sound”Steve Cummer

When sound waves travel through it, each cavity in the device resonates at a specific frequency. The vibration has an impact not just on the speed of the sound wave but also interacts with its adjacent cavities to tame both transmission and reflection. Also, the vibrating columns not only interact with the sound wave, but also with surrounding columns.

The team are now looking to apply their ideas to more exotic forms of sound control, including the manipulation of sound waves in water for applications such as sonar. As team leader Steve Cummer told Materials Today, “this is an idea that can be deployed any time one needs to control or redirect the reflection or transmission of sound”.