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

June 22, 2018, University of Cambridge
Credit: Sarah Collins (Cambridge University)

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.

Using ultra-high-speed cameras and modern audio capture techniques, the researchers, from the University of Cambridge, found that the 'plink, plink' 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 itself to vibrate, acting like a piston to drive the .

In addition, the researchers found that changing the surface tension of the surface, for example by adding dish soap, can stop the sound. The results are published in the journal Scientific Reports.

Despite the fact that humans have been kept awake by the sound of dripping water from a leaky tap or roof for generations, the exact source of the sound has not been known until now.

"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."

Water droplet hitting a liquid surface. The oscillation of the trapped air bubble, visible in this video, causes the water surface to oscillate and drives the airborne sound. Credit: University of Cambridge

Agarwal, who leads the Acoustics Lab and is a Fellow of Emmanuel College, first decided to investigate this problem while visiting a friend who had a small leak in the roof of his house. Agarwal's research investigates acoustics and aerodynamics of aerospace, domestic appliances and biomedical applications. "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."

Working with Dr. Peter Jordan from the University of Poitiers, who spent a term in Cambridge through a Fellowship from Emmanuel College, and final-year undergraduate Sam Phillips, Agarwal set up an experiment to investigate the problem. Their setup used an ultra-high-speed camera, a microphone and a hydrophone to record droplets falling into a tank of water.

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 are well-known: when the droplet hits the surface, it causes the formation of a cavity, which quickly recoils due to the 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.

Credit: Sarah Collins (Cambridge University)

Previous studies have posited that the 'plink' sound is caused by the impact itself, the resonance of the cavity, or the underwater sound field propagating through the water surface, but have not been able to confirm this experimentally.

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.

"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 Phillips, who is now a Ph.D. 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."

Credit: Sarah Collins (Cambridge University)

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 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.

According to the researchers, while the study was purely curiosity-driven, the results could be used to develop more efficient ways to measure rainfall or to develop a convincing synthesised sound for in gaming or movies, which has not yet been achieved.

Explore further: Marine animals can hear us swim, kayak and scuba dive

More information: Samuel Phillips et al, The Sound Produced by a Dripping Tap is Driven by Resonant Oscillations of an Entrapped Air Bubble, Scientific Reports (2018). DOI: 10.1038/s41598-018-27913-0

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11 comments

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Doug_Nightmare
4.5 / 5 (8) Jun 22, 2018
A better solution to the dripping tap noise is to REPAIR THE VALVE. That is the teleological reason for the irritating noise.
donjoseph
1 / 5 (2) Jun 22, 2018
Can you believe the time spent on this and an article too?
Besides as Doug says repair it and if you can't get to it until morning attach a short hose to it and put it down the drain
ZoeBell
Jun 22, 2018
This comment has been removed by a moderator.
dan42day
3 / 5 (2) Jun 22, 2018
I have to agree with the previous commenters, repair the leak. Never surrender to entropy!
Doug_Nightmare
3.7 / 5 (3) Jun 23, 2018
The valving interface is between the valve seat and disc, both of the hardest material in the assembly. The 'washer' is between - properly - the low pressure downstream of the seat and atmosphere, and should only see pressure while the valve is open. Commonly naive installers do not understand the bridge-wall diagram and put the stem on the pressure supply side and that washer/packing can leak.

A properly designed valve can be opened onto its backseat to allow packing changes under normal pressure.

In the extreme, an eroded valve seat can be re-ground to remove the eroded channel, but that is extreme wear and extreme repair in appropriate for consumer grade valves.

I serviced reactor plant valves among other things.
IronhorseA
1 / 5 (1) Jun 23, 2018
I have to agree with the previous commenters, repair the leak. Never surrender to entropy!


You already surrendered the day you were born, it just takes 6-8 decades for you to realize it. :D
Da Schneib
3 / 5 (2) Jun 23, 2018
Pro tip: maintain your faucets. Duh.

Given the water crisis caused by global warming this is Intuitively Obvious To The Most Casual Observer (IOTTMCO).

Teh stupid it burnz.
Whys
not rated yet Jun 23, 2018
A flat surface at a 45 degree angle also works.
FreddieOne
5 / 5 (2) Jun 23, 2018
That sound of dripping water is also the sound of money going down the drain. You would be AMAZED at how much water is lost due to dripping faucets and leaking toilets. Forget the soap to stop the sound. FIX THE LEAK. The few dollars spent on fixing the leak has the potential to save you a lot more money in the long run. Forget the plumber. Look up your faucet on YouTube and do this minor repair yourself. I ran municipal water supplies and distribution systems for over 34 years. I have seen this thousands of times over the years.
HocusLocus
not rated yet Jun 23, 2018
This revelation -- but especially the comments with practical suggestions -- puts me in mind of a delightful old story by Lucretia P. Hale (1820-1900) entitled, "The :Lady Who Put Salt In Her Coffee".

I was glad to discover The Peterkin Papers in my grade school library, but you can read this public domain story here,
http://digital.li...alt.html

Zka
not rated yet Jun 24, 2018
This has to be nominated for ignobel.

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