Science & Technology

Scientists Created a Material That Blocks Sound While Letting in Light and Air

If you've ever fallen asleep to the din of highway noise in a cheap hotel or gritted your teeth through a neighbor's loud parties, you know how hard it can be to block sound from your surroundings. A new breakthrough by researchers at Boston University could make it a little easier to keep things quiet — and all without blocking air and sunlight. The future is coming, and it's just the right volume.

Turn Down for What

If you wanted to block sound from, say, your bedroom, what would you use? If the walls weren't already built with thick, heavy, sound-absorbing materials, you might cover them with big foam panels, or maybe a heavy blanket in a pinch. But what if instead of a bedroom, you wanted to soundproof a big glass-lined building lobby? Heavy materials wouldn't cut it since that would also block out the sunlight. Same goes for quieting the cacophony of a giant jet engine — thick walls would keep that plane firmly on the ground.

The mathematically designed, 3D-printed acoustic metamaterial is shaped in such a way that it sends incoming sounds back to where they came from, Ghaffarivardavagh and Zhang say. Inside the outer ring, a helical pattern interferes with sounds, blocking them from transmitting through the open center while preserving air's ability to flow through.

The answer, these Boston University researchers decided, lies in metamaterials. Those are materials that use specially engineered quirks in their shape or composition to alter light or sound waves. For an example of how metamaterials might work, think about a magnifying glass: the curved shape of the glass focuses the light waves into a point by bending them, or slowing them at different rates — those that hit the edges bend the most (or slow the least), and those that hit the center bend the least (or slow the most). As Duke University professor Steve Cummer explains in a YouTube video, you can do the same thing with the composition of an object. Instead of creating a lens with a particular shape, you can create an object that's made of different materials, each one chosen for how much they slow light waves. Materials that slow them the most could be placed at the center, and materials that slow them the least could be placed at the edges. Depending on the purpose of the metamaterial, engineers can use a variety of different compositions and shapes to bend light to their will.

The same goes for sound. Acoustic metamaterials, as they're called, can use their composition or their shape to block, slow, or reflect sound waves, depending on their purpose. But to let light and air through, a material has to almost entirely rely on its shape — no thick, heavy substances here.

"Sound is made by very tiny disturbances in the air. So, our goal is to silence those tiny vibrations," coauthors Reza Ghaffarivardavagh and Xin Zhang say in a press release. "If we want the inside of a structure to be open air, then we have to keep in mind that this will be the pathway through which sound travels."

"A Mute Button Incarnate"

To make their open-air sound-blocker (described in a paper published in January), the researchers started by combining two materials that each blocked and bent sound waves at slightly different rates so that the sound-blocking power of the two together would be greater than the sum of their parts. Next, they determined the exact shape they'd need for the greatest sound reduction. What they settled on was a ring constructed of six hollow tunnels that twisted along the length in a helix shape, which would lead the sound waves in a twisting path that passed through these sound-blocking materials.

To test their design, they 3D printed a small ring about the size of a roll of masking tape and placed it at one end of a PVC pipe. At the other end, they placed a loudspeaker. It blocked a whopping 94 percent of the sound.

"Standing in the room, based on your sense of hearing alone, you'd never know that the loudspeaker was blasting an irritatingly high-pitched note," wrote Boston University's Kat J. McAlpine. "If, however, you peered into the PVC pipe, you would see the loudspeaker's subwoofers thrumming away. The metamaterial, ringing around the internal perimeter of the pipe's mouth, worked like a mute button incarnate until the moment when Ghaffarivardavagh reached down and pulled it free. The lab suddenly echoed with the screeching of the loudspeaker's tune."

The researchers point out that their design is flexible — you can build it at any size and shape it to block virtually any frequency, all while still letting air and light through. Zhang says that opens up a lot of possibilities for the future of sound reduction. "The idea is that we can now mathematically design an object that can block the sounds of anything."

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Written by Ashley Hamer April 3, 2019

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