Apr 11, 2018 | By David

A group of researchers at Duke University have made use of 3D printing to create a device that can control the redirection of soundwaves with an unprecedented level of efficiency. It is the first device of its kind to successfully demonstrate near-perfect control of both the transmission and reflection of sound. The design makes use of a class of materials known as metamaterials, which are artificial materials that can manipulate waves like light and sound through their structure, as opposed to their chemical composition. The research could prove useful for various underwater sound applications, such as sonar technology.

The thin plastic device was fabricated using 3D printing technology. Its structure was engineered such that it is composed of a series of rows of four hollow columns. Each column is nearly one-half of an inch on a side with a narrow opening cut down the middle of one side. Alteration of the width of the channels between rows, and the size of the cavities inside each individual column, allowed the researchers to precisely manipulate sound waves. The cavities would vibrate at a prescribed frequency, and this would affect the vibrations of neighbouring cavities in order to limit transmission and reflection of the waves.

The results of the research were detailed in the paper, ''Systematic design and experimental demonstration of bianisotropic metasurfaces for scattering-free manipulation of acoustic wavefronts'', published in the journal Nature Communications. According to Junfei Li, a doctoral student at Duke and first author of the paper, ''Previous devices could shape and redirect sound waves by changing the speed of different sections of the wave front, but there was always unwanted scattering. You have to control both the phase and amplitude of both the transmission and reflection of the wave to approach perfect efficiencies.''

A special computer program was designed in order to find the optimal structure for the metamaterial. Into this program, the researchers fed the boundary conditions needed on each side of the material, in order to dictate how they want the outgoing and reflected waves to behave. The program tries out random solutions, learns from what worked and what didn’t, and eventually finds an appropriate set of design parameters after a number of different iterations.

(All images, credit: Junfei Li)

The set-up demonstrated in the paper involved a soundwave of 3,000Hz. The structure was capable of redirecting the wave, which came straight at it, to a sharp outgoing angle of 60 degrees. The efficiency was around 96 percent, which is near-perfect, and much higher than the 60 percent efficiencies to be expected from similar set-ups in the past. The same system can be modified and scaled to successfully control soundwaves at almost any wavelength.

''When talking about waves, I often fall back on the analogue of an optical lens,'' said Steve Cummer, professor of electrical and computer engineering at Duke. ''If you tried to make really thin eyeglasses using the same approaches that these sorts of devices have been using for sound, they would stink. This demonstration now allows us to manipulate sound waves extremely accurately, like a lens for sound that would be way better than previously possible.''

 

 

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