The sound of dripping water from a tap or a roof can easily and quickly get on your nerves. And the “plink, plink” noise had another annoying feature – we weren’t sure how it formed, until now. Thanks to high-speed cameras and state-of-the-art audio recording tech, researchers have discovered the cause of the noise and even how to stop it.
As detailed in Scientific Reports, the sound is not caused by the droplet itself but by a small bubble of air trapped beneath the water's surface as the droplet hits the water. The air bubble makes the surface vibrate, creating the distinct sound.
"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," lead researcher Dr Anurag Agarwal, of Cambridge University's Department of Engineering, said in a statement. "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."
The visual side of water droplets has been known for many years, with the first images dating back to the first decade of the 20th century. A droplet hits the surface of a liquid which causes the formation of a cavity. The reaction to that is due to surface tension. The liquid recoils quickly after the hit, closing the cavity and lifting a small column of fluid above the surface. At the same time, an air bubble is trapped below the surface.
Seeing with superhuman precision has been easier than hearing with superhuman precision. Therefore, the cause of the sound remained a mystery, until events conspired to make Dr Agarwal face the annoying "plink, plink" sound head on.
"While I was being kept awake by the sound of water falling into a bucket placed underneath the leak, I started thinking about this problem," he said. "The next day I discussed it with my friend and another visiting academic, and we were all surprised that no one had actually answered the question of what causes the sound."
The key feature they identified was that the air bubble needs to be very close to the bottom of the cavity to create the oscillation, a very efficient position for creating the sound, but also vulnerable to something easily found near every tap. Soap. A bit of soap in the water breaks the surface tension and attenuates the annoying noise.