Science News for Students - Spring 2021

material.” He is a mechanical engineer who worked on the new project at EMPA. That Swiss research institute, based in Dübendorf, specializes in designing newmaterials. His team described its new chiral sound dampening materials October 4, 2019 in Nature Communications . How it works Snap your fingers. The sound it makes creates a disturbance in the air as some particles push against others. Those jiggled particles push against still more particles, and so on. Sound moves from one place to another—such as from your fingers to your ears—through this type of particle-shoving. Vibrations can move through solids, too. That’s why your pencil quivers if you hit a desk hard with your hand. It’s also why you can hear people talking on the other side of a thin wall. The new invention controls at least some of those sound waves as they attempt to pass through. The secret is that twist. The building blocks used in the newmaterial look like small cylinders of stacked round plates (clear marshmallows). Narrow diagonal beams (like those toothpicks) separate each disk. Both the plates and beams are made of a hard plastic. When assembled, these structures resemble a type of large crystal. (A crystal is a material with an internal structure made of repeating geometric patterns.) Entering sound waves pass through one plastic plate, then past the small plastic beams, and on through the next stacked plate. The crystal’s shape twists the waves of some sound frequencies. (Frequency describes how closely together waves travel.) Those twists serve to bump the waves off course. The material refracts —changes the direction of—some or all of the

waves passing through. The researchers unveiled two designs for their crystal-like structure. One twists the waves, but rotates them in only one direction. That means sounds pass through. By the time the waves reach the end, however, they’ll be distorted. If someone at one end had been speaking, their words could be unintelligible at the other end. Such a design might be useful for creating private rooms. Outsiders would not be able to eavesdrop on what was said inside. The other design twists first one way, then the opposite. (Imagine you build a tower of many stories of marshmallows, each story separated by and supported by toothpicks. However, the direc- tion of the marshmallow’s rotation reverses with each new floor.) The researchers demonstrated this design by sandwiching the twisting structures between two plastic windows. Their twisty material completely blocked—or cloaked—some ranges of vibra - tions. This shows that structures can be designed to have a bandgap, meaning a selected range of blocked frequencies, the researchers say. The design was challenging Finalizing this design took about five years of trial and error, says Bergamini. His team began by studying crystals. They have an internal struc- ture made of repeating patterns. People often use crystals to redirect light and other types of electromagnetic radiation. Recently, scientists realized that if they made larger structures out of repeating patterns, they could redirect larger waves. Such projects allowed them to cloak sounds and seismic waves. Ber- gamini’s group wanted to find a newway to use a crystal-like design to dampen or mute sound waves. The breakthrough, says Bergamini, came

These new anti-noise materials, which have a crystal-like appear- ance, can twist certain vibrations to block some unwanted sounds.

EMPA

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