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Cambridge University discovers how to stop irritating ‘plink plink’ of dripping tap

Putting washing up liquid in the bucket or sink beneath a dripping tap will stop the noise 
Putting washing up liquid in the bucket or sink beneath a dripping tap will stop the noise

The irritating "plink, plink" sounds of a dripping tap has caused many a sleepless night.

But now Cambridge University has discovered what causes it, and how to stop it.

Using ultra-high-speed cameras and audio capture techniques, researchers found the noise is produced by the movement of a small bubble of air trapped beneath the water’s surface.

The bubble forces the water surface itself to vibrate, creating the sound as it hits an empty, or water-filled sink below.

However changing the surface tension of the water that the drops are falling onto by adding washing up liquid causes the sound to vanish.

“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,” said Dr Anurag Agarwal of Cambridge’s Department of Engineering, who led the research.

“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.”

Ultra slow speed cameras and specialist audio recorders were able to pick out the exact moment the sound is produce as a drop hits the surface  - Credit: University of Cambridge 
Ultra slow speed cameras and specialist audio recorders were able to pick out the exact moment the sound is produce as a drop hits the surface Credit: University of Cambridge

Dr Agarwal decided to investigate the problem after while visiting a friend’s house who had a leak in his 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 earliest photographs of drop impacts were published in 1908, and the fluid mechanics of a water droplet hitting a liquid surface are well-known, but nobody had got to the bottom of the mystery "plink".

In their experiment, the Cambridge researchers found that, the initial splash, the formation of the cavity, and the jet of liquid are all effectively silent. The source of the sound is 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 doctoral student Sam Phillips.

“However, the airborne sound is not simply the underwater sound field spreading to the surface, as had been previously thought.”

According to the researchers, the results could also used to develop more efficient ways to measure rainfall.

The research was published in the journal Scientific Reports.