Plink. Plink. Plink.
It's the sound of water droplets falling one after another, maybe from a leaky faucet or through a cracked ceiling. It's the kind of sound that can keep you up all night.
University of Cambridge engineer Anurag Agarwal feels your pain. While visiting a friend in Brazil in 2016, Agarwal couldn't ignore the water that steadily dripped through the leaky roof and fell into a bucket below. “It was a rainy period, and the downfall was torrential,” he says.
Equal parts annoyed and intrigued, Agarwal began to wonder how the falling droplets made their distinctive sounds—an explanation science didn't have. It couldn't be just the impact; Agarwal notes that banging your fists on a desk will make noise, but “it doesn't have that musical quality.”
In 2017, Agarwal took the question to an undergraduate lab at Cambridge, which used high-speed cameras, an underwater microphone, and a microphone on dry land to capture precisely when and how a falling droplet of water plinks.
At the moment of impact, the drop makes no sound. But just a few milliseconds after impact, the droplet forms a cavity that recoils and creates a small column of liquid. The fast recoil then creates a small, water-trapped air bubble that's responsible for the plink. The bubble oscillates 5,000 times in a second, which makes the water vibrate and generate the iconic, annoying tone.
The recognizable plink occurs only when a water droplet lands in water; a drop landing on a dry, wooden surface leaves only a dull thud. Agarwal also found that soapy water prevents the air bubble from oscillating well enough for a droplet to plink.
Agarwal and his research team published their findings in the journal Scientific Reports. He thinks this better understanding of how droplets make noise could one day help video game and movie sound engineers better replicate the sound of water dropping into a bucket, but he notes this study was done mostly out of curiosity.
His other motivation? Finally knowing “what [we] can do to get rid of it.”