Tortured by sound of leaky taps? Do the washing-up!

MOST of us have known the irritation of a leaky tap dripping on and on.

But the answer to that relentless noise could be as simple as a squirt of washing-up liquid.

Engineers at the University of Cambridge have discovered the source of the deeply annoying ‘plink’ sound of a falling drop of water.

It is not simply the splash of the droplet hitting water in a bucket or sink, as might be expected.

Instead, the acutely annoying sound happens up to 15 milliseconds later, from an air bubble under the surface which sets the water vibrating and makes sound waves.

A laboratory experiment using a water tank found that washing-up liquid can silence the noise.

The researchers tested out the solution using ultra-highspeed cameras and sensitive microphones both above and below the water.

It works, simply, by toughening up the surface of the water so that the air bubble cannot form underneath.

Study co-author Samuel Phillips, of Cambridge’s Department of Engineering, said: ‘The air bubble which causes the “plink” sound forms under the water surface, vibrates and causes the water to vibrate, acting like a piston to make the sound.

‘It is like putting your foot through a trampoline, the air bubble getting under the surface. Washing-up liquid acts to make the “trampoline” more flexible so that it cannot be broken through.

The fluid mechanics of a water droplet hitting a liquid surface are well known.

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

The British researchers, whose study is published in the journal Scientific reports, believe the ‘drip’ is louder if the droplet is larger.

A smaller droplet should produce a more high-pitched ‘plink’ noise.

Mr Phillips said: ‘The main thrust of the research was to find out what is producing the sound, but the washingup liquid solution in the laboratory could work in people’s homes. It is always good when your research has an application which could work in real-life situations and gets people thinking.’

And Dr Anurag Agarwal, who led the research, said: ‘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.

‘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.’