Science & Technology

Scientists Have Developed a Device That Makes Sound Go One-Way

If you ever tried to make a DIY telephone with string and a pair of cups, this article is for you. Imagine if you could hear your friend talking on the other end of the string, but they couldn't hear you. A team of researchers at Yale recently found a way to do just that: make sound and other vibrations flow in one direction. This opens up a world of possibilities for transmitting sound and heat.

Good Vibrations

All sounds are transmitted by vibrations. Your voice causes your vocal cords to vibrate, for instance, and then the sound is carried through the air by vibrating particles. In the case of the string phone, the bottom of the cup vibrates as you speak into it, causing the string to vibrate as well. The vibrations travel along the string and make the bottom of the second cup vibrate just like the first so your friend can hear what you're saying.

These vibrations occur at different amplitudes and frequencies. Amplitude, or the size of a vibration, determines how loud the sound will be. Frequency, or the speed of the vibration as measured by how many wave cycles happen per second, determines the pitch of the sound.

Some devices will use acoustic resonators to amplify or reduce sound in certain frequency ranges. For example, musical instruments are designed to amplify certain frequencies, while something like a car muffler is engineered to absorb unwanted sound.

A One-Way Street

Acoustic resonators are nothing new — they're used in everything from cell phones to gravitational wave detectors. But the Yale team broke ground by using two acoustic resonators to make sound go one way. Their findings were published in the journal Nature in April.

"This is an experiment in which we make a one-way route for sound waves," said Jack Harris, a Yale physics professor and the study's principal investigator, in a press release. "Specifically, we have two acoustic resonators. Sound stored in the first resonator can leak into the second, but not vice versa."

To achieve one-way sound, the team used a laser light that works like a "tuning knob." The force of the laser, when trapped between two mirrors, can weaken or strengthen a sound wave's vibrations depending on the wave's direction.

This discovery of one-way sound technology offers possibilities for improving a wide range of electronic devices that use acoustic resonators. But sound isn't the only thing the team figured out how to control.

Turning up the Heat

The team expanded the one-way sound technology to apply to other types of vibrations, like heat waves. The movement of heat from one object to another uses vibrations similar to sound waves, so the researchers figured their findings might be applicable to heat flow.

"By using our one-way sound trick, we can make heat flow from point A to point B, or from B to A, regardless of which one is colder or hotter," Harris said. "This would be like dropping an ice cube into a glass of hot water and having the ice cubes get colder and colder while the water around them gets warmer and warmer. Then, by changing a single setting on our laser, heat is made to flow the usual way, and the ice cubes gradually warm and melt while the liquid water cools a bit. Though in our experiments it's not ice cubes and water that are exchanging heat, but rather two acoustic resonators."

With development, this technology could be used to protect the components of delicate electronics, quieting noisy equipment, and creating clearer two-way communication between things like cell phones — you know, the modern-day version of cups on a string.

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Learn more about sound science in "The Sound Book: The Science of the Sonic Wonders of the World" by Trevor Cox. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Andrea Michelson June 13, 2019

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