A thin material was developed that can control the redirection and reflection of sound waves with almost perfect efficiency. While many theoretical approaches to engineer such a device have been proposed, they have struggled to simultaneously control both the transmission and reflection of sound in exactly the desired manner, and none have been experimentally demonstrated. The new design is the first to demonstrate complete, near-perfect control of sound waves, and is quickly and easily fabricated using 3D printers.
The design uses metamaterials — artificial materials that manipulate waves like light and sound through their structure rather than their chemistry. For example, while this particular metamaterial is made out of 3D printed plastic, it is not the properties of the plastic that are important — rather, the shapes of the device's features allow it to manipulate sound waves.
The metamaterial is made of a series of rows of four hollow columns. Each column is nearly half an inch on a side with a narrow opening cut down the middle of one side, making it look somewhat like an Ethernet port. While the device is 1.6” tall and nearly 3.5’ long, its height and width are irrelevant — it could theoretically stretch on forever in either direction. Control of how the device manipulates sound is done through the width of the channels between each row of columns, and the size of the cavity inside each individual column. Some columns are wide open while others are nearly closed off. Each column resonates at a different frequency, depending on how much of it is filled in with plastic. As a sound wave travels through the device, each cavity resonates at its prescribed frequency. This vibration not only affects the speed of the sound wave, but interacts with its neighboring cavities to tame both transmission and reflection.
The vibrating columns not only interact with the sound wave, but also with their surrounding columns. A computer optimization program was written to work through all the design permutations. The program is fed the boundary conditions needed on each side of the material to dictate how the outgoing and reflected waves should behave. After trying a random set of design solutions, the program mixes various combinations of the best solutions, introduces random “mutations,” and then runs the numbers again. After many iterations, the program eventually evolves a set of design parameters that provides the desired result.
One such set of solutions can redirect a sound wave coming straight at the metamaterial to a sharp, 60-degree outgoing angle with an efficiency of 96 percent. Previous devices could only achieve 60 percent efficiencies under such conditions. While this particular setup was designed to control a sound wave at 3,000 hertz — a very high pitch not dissimilar to ringing in the ears — the metamaterials could be scaled to affect almost any wavelength of sound. The researchers plan to transfer these ideas to the manipulation of sound waves in water for applications such as sonar.