What causes the sound of a dripping tap – and how do you stop it?

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.

A dripping tap. Image credit: PumpkinSky/Wikimedia Commons (CC BY-SA 4.0)

Using ultra-high-speed cameras and modern audio capture techniques, the researchers, from the University of Cambridge, found that the ‘plink, plink’ sound produced by a water droplet hitting a liquid surface is caused not by the droplet itself, but by the oscillation of a small bubble of air trapped beneath the water’s surface. The bubble forces the water surface itself to vibrate, acting like a piston to drive the airborne sound.

In addition, the researchers found that changing the surface tension of the surface, for example by adding washing-up liquid, can stop the sound.

Water droplets have been a source of scientific curiosity for more than a century: the earliest photographs of drop impacts were published in 1908, and scientists have been trying to figure out the source of the sound ever since.

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

In their experiment, the Cambridge researchers found that somewhat counter-intuitively, the initial splash, the formation of the cavity, and the jet of liquid are all effectively silent. The source of the sound is the trapped air bubble.

In order for the ‘plink’ 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 oscillations of the water surface at the bottom of the cavity, acting like a piston driving sound waves into the air. (University of Cambridge)

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