Monday, May 22, 2017

Focusing Sound with Metasurfaces: A New Way to Reduce Noise and Power Devices?

Whether it’s the neighbor’s barking dogs, pounding rain, the din of traffic, or the music of your own choosing, most of us are constantly surrounded by noise. Noise is energy, so that means most of us are constantly surrounded by a relatively safe, renewable, and clean form of energy. What if we could harvest this energy?

It seems like a match made in heaven—unwanted noise is everywhere and its levels are rapidly rising, as is our demand for power. Crunch the numbers on how long you’d have to yell to heat up a cup of coffee, though, and it’s clear that sound isn’t an optimal source of energy, especially compared to the potential of sunlight and wind. As a result, sound has often been ignored as a potential energy source, but interest is growing with the rise of low-power electronic devices and other technological advances.

In research recently described in the American Physical Society’s journal Physical Review Applied, a team of scientists from the Institut Jean Lamour (CNRS University of Lorraine) in France recently proposed an innovative method for harnessing sound energy. Their method collects and efficiently focuses noise that can be converted into electrical energy, while simultaneously reducing noise in the environment. A system like this could be used in airplanes, for example, to capture low frequency noise and vibrations, providing a quieter ride and powering small electrical systems at the same time.

This image shows how sound waves combine from two adjacent metasurfaces (top and left side). The maze-like elements that make up the metasurface are shown in the dashed red boxes.
Image Credit: S. Qi, Y. Li, and B. Assouar, Physical Review Applied,
Acoustic focusing and energy confinement based on multilateral metasurfaces.

In this work, the researchers focused their attention not on the process of turning sound energy into electrical energy, but on simply and efficiently focusing sound waves. You don’t have to sit next to a jet engine to know that there is a lot of noise out there, the problem is that the energy is really spread out. Capturing sound waves that are bouncing around in all directions and focusing them in one place is not an easy task, but it is key to an efficient energy harvesting system.

About six years ago, Badreddine Assouar started thinking about how to harvest sound energy with metamaterials. Designed to have properties that aren’t typical in nature, metamaterials are artificial materials composed of tiny, often repeating structures. With well-chosen structures, metamaterials can be designed to interact with light, sound, or heat in very specific and unusual ways.

At the time, Assouar was working on a research project using metamaterials to reduce noise and vibrations. “Instead of using metamaterials to attenuate or to reflect sound waves, why not harness them, harvest them, and convert them into electrical energy?” he wondered. Inspired by the desire for a clever way to reduce noise and save energy, he began working in earnest on this idea a couple of years later.

In collaboration with his PhD student Shuibao Qi and post-doc Yong Li, Assouar came up with a method for using metasurfaces (very thin metamaterials) to focus sound waves and confine their energy. The metasurfaces are built from a series of tiny, maze-like elements that restrict the path that sound waves can travel. Physicists have shown that you can control how sound waves interact using these elements. The mazes coil up space in a way that can change the phase of a sound wave leaving the element. By combining two or more metasurfaces of this design, the team found that you can accurately focus sound waves.

The team considered different geometrical arrangements of 2-4 of these metasurfaces, including a closed box, a box with only three sides, and boxes with only two sides (in parallel and adjacent). Using theory and simulations, they analyzed and mapped how well these arrangements focused sound waves originating from within the box. This analysis included not only how sound waves were affected by the maze-like elements, but also how they were affected by the geometric arrangements of the metasurfaces, which led to reflections and interactions between waves.

The team’s results show that this method can efficiently and accurately focus sound waves in all of the geometric arrangements. The more enclosed the sound is by the metasurface geometry, the more energy you can confine. The system is highly controllable, by tuning properties of the maze-like elements and the overarching geometry, you can focus the sound waves accurately at any desired point.

Combined, the efficiency, accuracy, simplicity, and low cost of this method make harvesting energy from noise a much more appealing prospect than traditional methods, which are bulky and provide little focusing control. The team is now working on the “electrical part” of the project to determine just how well a metasurface system could perform. While noise won’t solve the world’s energy crisis, hopefully it will soon be able to save us some energy, and maybe even a little sanity.

Kendra Redmond

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