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The Physics Behind a Leaky Faucet’s Maddening ‘Plink’

Microphones and high-speed cameras show that what happens when a water droplet hits water is surprisingly complicated

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smithsonian.com

The sound of dripping water is dependent on context—if droplets are falling from a cliff into a cool, shady pool, we might think the sound is beautiful. If they are dripping all night from the faucet into the sink, we might think the plink sound was designed by nature as a form of torture. In either case, one thing is certain: The sound of dripping water is distinctive, and now, reports Sarah Gibbens at National Geographic, curious scientists have figured out why.

In 2016, University of Cambridge engineer Anurag Agarwal was visiting a friend in Brazil during the rainy season. Water was dripping from a crack in the roof to a bucket below, plinking all night long. That annoying, sleepless night led Agarwal to wonder just how the droplets produce that particular sound. If it was just created by a water droplet smacking the surface of the water in the bucket, he reasoned, it would be much harsher. Something else, he thought, was going on.

So when he returned home, he decided to investigate. George Dvorsky at Gizmodo reports that he wasn’t the first to be intrigued by the plink. As far back as 1908, a scientist named Arthur Worthington took photos of droplet impacts. Over the last century, scientists have continued to analyze the physics behind fluid droplets, since understanding the fluid dynamics of droplets has uses in things like printing and in improving combustion engines. Researchers have also spent the last century or so trying to figure out how the plink sound is produced. While there are many hypotheses, no one had been able to figure it out.

Agarwal decided to take a shot, using an ultra-high-speed camera and microphones above and below the surface of the water. That allowed the team to watch and hear exactly what was happening and trace the sound to its origin.

“A lot of work has been done on the physical mechanics of a dripping tap, but not very much has been done on the sound,” Agarwal says in a press release. “But thanks to modern video and audio technology, we can finally find out exactly where the sound is coming from, which may help us to stop it.”

So what’s going on? It’s a little complicated, but Gibbens explains that when the water droplet hits the surface of the water, it makes no sound. Instead, it creates a small cavity in the surface of the water with a little column of water spurting up in the middle, the classic image of a water droplet. The creation of that water column also produces a small bubble under the water column which oscillates 5,000 times a second. That bouncing bubble then causes the water below the cavity to also oscillate. That's what produces the audible plink. It all happens within about 35 milliseconds. “Using high-speed cameras and high-sensitivity microphones, we were able to directly observe the oscillation of the air bubble for the first time, showing that the air bubble is the key driver for both the underwater sound, and the distinctive airborne ‘plink’ sound,” co-author Sam Phillips, also from Cambridge, says in the release.

While the study solves one of life’s little mysteries, according to the release, it could have some practical applications. The info could be used to find new ways to measure rainfall or make plink sounds for movies and video games, which, it turns out, is surprisingly hard. Dvorsky reports that it also offers a solution if a dripping faucet or leaky ceiling is driving you batty—add a little soap to the container catching the water. It will disrupt the surface tension, changing the physics and eliminating the plink.

The research appears in the journal Scientific Reports.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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