Sep 20th 2017

The Light and Sound of Metamaterials

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Metamaterials mark a step forward in engineering. Humans have been manipulating light and soundwaves with natural materials for years. A pair of lenses, for instance, uses a natural material — glass — to control the propagation of light waves. Lens designers do so by precisely controlling the shape of the lens.

These new materials take that idea a step further by inventing new materials not found in nature to further manipulate light and sound. With a metamaterial, you could conceivably make a lens that’s completely flat and achieves the same effect because it controls the wave propagation. Such materials can even conceivably be used to create an invisibility cloak. At the moment, since these materials are still pretty new, scientists and engineers are still looking for applications. A team of engineers at the University of Colorado at Boulder, for instance, has recently developed a metamaterial that can cool objects even under direct sunlight without using any energy or water.

The idea behind metamaterials has been around for some time. Russian physicist Victor Veselago first advanced the concept in 1967. In 1999, physicist David R. Smith and a group of researchers realized the idea. Now, 18 years later, we are seeing an explosion of use cases around the concept. In this article, we’ll look at acoustic metamaterials.

Acoustics

Metamaterials can manipulate sound waves just like they manipulate light waves. For instance, a researcher at Duke University has demonstrated a device called a Sound Propagator made of plastic acoustic metamaterials that can bend sounds into shapes, such as the letter A. One possible application for this technology is wireless charging; operating at 40 kHz (outside the range of human hearing), the Propagator could send inaudible soundwaves to a device that can then be converted into electric energy.

Acoustic metamaterials recently bent an incoming sound into the shape of an A, which the researchers called an acoustic hologram. (Duke Research Blog)

Steven Cummer, a professor at the electrical and computer engineering department for Duke University, said that the primary commercial applications for acoustic metamaterials are for soundproofing. While those applications might seem limited to recording studios and the like, Cummer pointed out that cars, planes and helicopters, among other items can use soundproofing. Typically, engineers use fiberglass for soundproofing or sound absorption, which Cummer said works “reasonably well.”

But such materials will likely provide a higher level of soundproofing. “There are plenty of applications where if it worked a little bit better, it would be useful, even if it cost more,” Cummer said. “If we have full control over being able to build some complicated structure, surely we can do better than just packing some fiberglass together.”

Nicholas Xuanlai Fang, a researcher at MIT, said that for autos, acoustic applications can provide more soundproofing at a lighter weight. He said that ideally, a car could weigh up to 40% lighter with such materials and provide the same quiet ride as a heavier car.

How They’re Made

Compared to lightwaves, soundwaves are very large, which makes the process of creating acoustic materials much easier. Cummer said that the rule of thumb for such materials is that they need to be 10X smaller than the wavelength they’re manipulating. A lightwave is around 500 nanometers (for comparison’s sake, an atom is around 1 nanometer). A soundwave can be measured in centimeters, which means that such materials can be measured in millimeters or even centimeters.

Split-ring resonator array 10K sq nm
Split-ring Resonator (WikiCommons)

While optical metamaterials need to be micro-engineered with equipment similar to that used to create silicon chips, acoustic materials can be created using 3D printers. As for the material, Cummer said plastic is great for prototyping, but often in applications like nautical engineering, metals are required. Industrial use for such materials is still a few years out, Cummer said. Presently, scientists like himself are testing the materials for real-world conditions.

These all present intriguing possibilities. For those looking to be optimistic about the future, the continuing development of metamaterials represent a solid foundation.

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