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

Scientists have solved the riddle behind one of the most recognizable, and annoying, household sounds: the dripping tap.

Water tap in bathroom
Water tap in bathroom Image: Shutterstock

Researchers have tackled the riddle behind a standout amongst the most conspicuous, and irritating, household sounds: the dribbling tap. Furthermore, vitally, they have likewise recognized a basic answer to stop it, which a large portion of us as of now have in our kitchens.

Utilizing ultra-high speed cameras and modern sound capture technology, the scientists at the University of Cambridge, found that the ‘plink, plink’ sound delivered by a water droplet hitting a liquid surface is caused not by the drop itself, but rather by the oscillation of a small bubble of air caught underneath the water’s surface.

The bubble forces the water surface itself to vibrate, acting like a piston to drive the airborne sound. Scientists also discovered that hanging the surface tension of the surface, for example by adding washing-up liquid, can stop the sound.

Dr Anurag Agarwal of Cambridge’s Department of Engineering 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.”

Scientists primarily investigated the problem while visiting a friend who had a small leak in the roof of his house. The research investigates acoustics and aerodynamics of aerospace, domestic appliances, and biomedical applications.

Agarwal said, “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 another visiting academic, and we were all surprised that no one had actually answered the question of what causes the sound.”

Water droplets have been a wellspring of scientific interest for over a century: the earliest images of drop impacts were distributed in 1908, and researchers have been attempting to make sense of the wellspring of the sound from that point onward.

The fluid mechanics of a water droplet hitting a fluid surface are notable: when the droplet hits the surface, it causes the arrangement of a cavity, which rapidly recoils because of the surface pressure of the fluid, bringing about a rising column of liquid. Since the cavity recoils so quick after the droplet’s effect, it causes a little air rise to get caught submerged.

Sam Phillips, who is now a PhD student in the Department of Engineering said, “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. However, the airborne sound is not simply the underwater sound field spreading to the surface, as had been previously thought.”

“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. This is a more efficient mechanism by which the underwater bubble drives the airborne sound field than had previously been suggested.”

The study is published in the journal Scientific Reports.