Cause of Annoying 'Plink Plink' Noise of Dripping Tap Finally Discovered

What causes the noise of a dripping tap? Although a seemingly simple question, until now, no one has investigated the phenomena.

The agonizing 'plink, plink' noise of a dripping tap has driven many to the point of insanity. But more have gone on to question their own sanity rather than investigate the cause of the infuriating noise of a dripping tap.

Summed up to be nothing more than the result of an impact between a droplet and a surface, most have not gone on to further investigate the cause of the infamous sound of the drip. Maybe there has been a lack of interest in questioning what is generally accepted as the cause, or perhaps the thought of studying one of the most recognizably annoying household noises has prevented scientists from wanting to further investigate and challenge the commonly accepted explanation as to exactly how it happens.

At any rate, researchers at the University of Cambridge report a new discovery claiming to definitively explain the root cause of the noise. And apparently, the way a water drop creates noise does not happen the way most have come to accept.

What Creates the 'Plink, Plink' of a Water Drip

Armed with ultra-high speed cameras and modern audio capture techniques, the scientists at the University of Cambridge captured new evidence to precisely explain the true underlying cause of the 'plink, plink' noise produced when a water droplet hits a liquid surface.

Evidently, the cause of the noise is not caused by the droplets itself, but rather, is the result of noise created when a small cavity of air becomes trapped air beneath the surface of the water. The bubble oscillates and collapses, causing the surface of the water to vibrate up and down, producing an effect much akin to how a speaker works.

What they discovered was counterintuitive of conventional explanation - according to the report, the formation of the cavity, and the jet of liquid are all effectively silent. Instead, the sound is created by a 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," said Sam Phillips, who assisted with the project and is now a PhD student in the Department of Engineering. "However, the airborne sound is not simply the underwater sound field spreading to the surface, as had been previously thought."

The Fluid Mechanics of a Water Drop - Well Understood, but Under Applied

A report published by the University of Cambridge claims despite humanity's long history of being kept awake by the sound of dripping water, there has been no research definitely explaining the mechanics of the phenomena - until now. The findings are surprising, especially for the researchers who were very likely annoyed by their experiment, further put down with the expectation of discovering the ordinary.

“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.”

The investigation was prompted by Agarwal after he visited a friend’s house with an all too familiar problem - a leaky roof.

“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,” he said. “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.”

Intrigued and unsatisfied by an unanswered question, Agarwal teamed up with Dr. Peter Jordan from the University of Poitiers to devise an experiment and definitively prove what many expected - an annoying noise produced from the impact of two objects. Together, the team used an ultra-high speed camera, a microphone, and a hydrophone, to record the process of how a water droplet falls into water to definitively explain how it creates a sound.

Dripping Water Noise: Previously Understood, but Not Explained

The fluid mechanics of how a water droplet hits a liquid surface is well known. Upon impacting the surface, a cavity is formed which quickly collapses on itself. However, it is not the immediate cavity or oscillations in the water which forms the noise.

Upon filming the phenomena, the researchers discovered the formation of a cavity as expected, but it formed a column which extended into the water. If the drop had enough momentum, a deep column forms which collapses in the middle, separating a small air bubble which remains as the rest of the cavity collapses up towards the surface (see picture below).

How Water Dripping Noise is Made
Source: University of Cambridge/YouTube

It is this small air bubble and its interactions with the surrounding water which creates the much familiar and characteristic noise of a water drop. It is a curious finding, but the researchers say it still holds relevance in modern science.

Inspiration

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Though the research was prompted by curiosity, the researchers involved in the experiment say their findings could help scientists develop equipment better suited for detecting raindrops, or perhaps could find use in the gaming industry to assist developers in creating a realistic sounding water drip - a sound which has yet to be convincingly reproduced.

As for practical applications, you can get rid of the annoying noise by adding a small amount of dish soap - a process which reduces the surface tension in the water, and prevents the formation of deep cavities which produce the sound of a dripping tap.