How science can silence the sound of your dripping tap

THE irritating “plink, plink” sounds of a dripping tap have caused many sleepless nights.

But now University of Cambridge researchers have discovered what causes the noise and how to stop it.

Using ultra-high-speed cameras and audio-capture techniques, they found the sound was caused by the movement of a small bubble of air trapped beneath the water’s surface.

The bubble forces the water surface to vibrate, creating the sound as it hits a bucket or sink. Changing the surface tension of the water that the drops fall on to, by adding washing-up liquid, causes the sound to vanish.

Dr Anurag Agarwal of Cambridge’s Department of Engineering, decided

‘While I was kept awake by the sound of water falling into a bucket, I started thinking about this problem’

to investigate while visiting a friend’s house which had a leaking roof.

“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 fluid mechanics of a water droplet hitting a liquid surface are wellknown, but nobody had got to the bottom of the mystery “plink”.

The researchers found that the initial splash, the formation of the cavity, and the jet of liquid were all effectively silent. The source of the sound was only down to the trapped air bubble.

“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,” said Sam Phillips, a doctoral student. “However, the airborne sound is not simply the underwater sound field spreading to the surface, as had been previously thought.”

According to the team, the research – published in the journal Scientific Reports – could be used to develop more efficient ways to measure rainfall.