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Cambridge scientists went to the bother of finding out more about the sound of a dripping tap

They have also identified a simple solution to stop it.

Image: Andrey_Popov via Shutterstock

SCIENTISTS HAVE SOLVED the riddle behind one of the most recognisable, and annoying, household sounds – the dripping tap.

And crucially, they have also identified a simple solution to stop it, which most of us already have in our kitchens.

In an unusual study, researchers from the University of Cambridge used ultra-high-speed cameras and modern audio capture techniques and found that the “plink, plink” sound produced by a water droplet hitting a liquid surface is caused not by the droplet itself, but by a small bubble of air trapped beneath the water’s surface.

In addition, the researchers found that changing the surface tension or the surface, for example by adding dish soap, can stop the sound.

Mystery through the years 

Despite the fact that humans have been kept awake by the sound dripping water from a leaky tap or roof for generations, the exact source of the sound has not been known until now.

“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.”

Agarwal first decided to investigate this problem while a friend who had a small leak in the roof of his house. Agarwal leads the Acoustics Lab, and his research investigates acoustics and aerodynamics of aerospace, domestic appliances and biomedical applications.

“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. The next day I discussed it with my friend and other visiting academic, and we were all surprised that no one had actually answered the question of what causes the sound,” he said.

Water droplets have been a source of scientific mystery for more than a century.

However, the fluid mechanics of a water droplet hitting a liquid surface are well-know – when the droplet hits the surface, it causes the formation of a cavity, which quickly recoils due to the surface tension of the liquid, resulting in a rising column of liquid.

Since the cavity recoils so fast after the droplet’s impact, it causes a small air bubble to get trapped underwater.

The answer

Previous studies have suggested that the “plink” sound is caused by the impact itself, the resonance of the cavity, or the underwater sound field pushing through the water surface, but have not been able to confirm this experimentally.

In their experiment, the Cambridge researchers found that somewhat counter-intuitively, the initial splash, the formation of the cavity, and the jet of the liquid are all effectively silent.

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The source of the sound is 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,” Phillips said.

For the “plink” sound to be significant, the trapped air bubble needs to be close to the bottom of the cavity caused by the drop impact.

The bubble then drives vibrations of the water surface at the bottom of the cavity, acting like a piston driving sound waves into the air.

According to the researchers, while the study was purely curiosity-driven, the results could be used to develop more efficient ways to measure rainfall or to develop a convincing synthesised sound for water droplets in gaming or movies, which has not yet been achieved.

The study has been published in the Scientific Reports journal.

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