The irritating sound of the dripping water from a faucet or from a leaking roof during the rainy season has kept generations of people awake at night.
The noise has been extremely frustrating that scientists have been trying to decipher its source ever since the earliest photographs of water droplets were published in 1908.
There had been previous studies that proposed the "plink" sound to be the result of the impact of the droplets into the surface of the sink similar to the "bang" created when bigger things were slammed on a wall. Other studies tried to be more precise saying that annoying plinking sound was created by an underwater sound field propagating through the water surface.
None of these conclusions were proven by any experiments and the source of sound remained elusive up until this present experiment conducted by a team of researchers from the University of Cambridge.
More interestingly, they suggested one simple way to stop the annoyance.
The 'Plink Plink' Sound
Dr. Anurag Agarwal, the lead researcher for the team from Cambridge's Department of Engineering, had once visited a friend who had a small leak in the roof. Agarwal is an expert in the 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," shared Agarwal. The next day, he discussed the problem with another academic and was shocked to find out that they were both curious about the irritating sound.
They then decided to set up an experiment. They used an ultra-high-speed camera and highly advanced audio capture techniques to record the droplets of falling water.
Their experiment revealed that the "plink plink" sound was in fact produced by the oscillation or the movement back and forth at a regular speed of as little as one tiny bubble of air trapped beneath the water's surface. One bubble can force the water surface itself to vibrate and send sound waves to the ears just like how a piston trigger an airborne sound.
The best part is that the researchers found one simple way to stop the nuisance: simply adding a small amount of washing-up liquid or dishwashing soap to the container catching the dripping water.
When a droplet hits the surface, it creates an air cavity or a tiny hollow space which abruptly recoil due to liquid surface tension. The recoil happens so abruptly that a single water droplet causes a smaller air bubble to get trapped underwater. Previous studies supposed that the initial impact of that recoil produces the "plink plink" sound. As mentioned, however, the current experiment showed that those tiny air bubbles underwater are in fact what produced the airborne plinking sound.
The results of the experiment could be applied in developing more accurate ways of measuring rainfall. The insight could also be applied in creating sound effects used in movies or games that closely imitate the genuine sound of water droplets.