Dripping tap

Cambridge engineers study - and stifle - sound of a dripping tap

Image credit: Dreamstime

Researchers at University of Cambridge’s Department of Engineering have used the latest video and audio recording technology to understand one of the most frustrating sounds – the dripping of a tap – allowing them to suggest a simple way to stop it.

Dr Anurag Agarwal, head of Cambridge’s Acoustics Lab, decided to investigate the phenomenon after visiting a friend who had a leak in the roof of his house, which kept Agarwal awake at night due to the sound of water dripping into a bucket beneath. Although countless people have been kept awake listening to the sound, its source was not understood.

“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 Agarwal. “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 researchers used a setup of an ultra-high speed camera, a hydrophone and a microphone to study the sound of droplets falling into a tank of water.

As a water droplet hits the surface of the water, it causes a cavity to form, which recoils due to the surface tension of the liquid and resulting in a rising column of liquid. A small air bubble is trapped under the surface due to the cavity recoiling quickly after impact.

Agarwal and his colleagues found that the ‘plink, plink’ sound was not caused by the droplets of water impacting on the surface, the resonance of the cavity or the jet of liquid, all of which are effectively silent. Instead, they found that the noise was caused by the vibrations of the tiny air bubble trapped beneath the surface of the water.

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

When the trapped air bubble is near the bottom of the cavity caused by the droplet’s impact, the bubble drives vibrations of the water surface at the bottom of the cavity. This causes the airborne sound field to be driven more effectively, resulting in a louder and more annoying sound.

The Scientific Reports study also revealed that changing the surface tension – such as by adding washing-up liquid – can dampen down the sound.

According to the Cambridge researchers, while the study was largely conducted to satisfy their curiosity, the results could help develop more efficient methods for measuring rainfall, or to create a convincing synthesised sound for water droplets – none of which yet exist – for use in games and films.

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