Researchers in England have discovered how to stop an annoying sound that has kept many awake at night – the sound of a dripping tap. Scientists at the University of Cambridge have found that the noise is not directly caused by impact, but rather by an air bubble created when the drop hits a container filled with liquid.
Although previous studies have suggested the sound was caused by the drop’s impact, the source of the sound had not been proven until now. Driven by curiosity, the researchers decided to investigate the issue after one of them was kept awake during the night by a leaky roof.
“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 PhD student Sam Phillips, one of the study’s authors.
The cameras and audio capture technology revealed that the initial impact of the drop, the cavity formed by that impact and the jet of liquid that shoots up afterwards are all “effectively silent,” researchers said in a press release published by University of Cambridge. Surface tension of the liquid causes the cavity formed to quickly recoil, thereby creating a small air bubble that becomes trapped underwater. The bubble then forces the water to oscillate and drives sound waves into the air by “acting like a piston.”
Scientists have been curious about water drop impacts for over a century. Ever since the first photographs of these impacts were published in 1908, scientists have been attempting to identify the cause of the sound.
“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,” said Dr Anurag Agarwal, from Cambridge’s Department of Engineering. “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.”
The team also discovered a simple way to stop the noise – add dish soap to the container. Doing so changes the surface tension of the liquid the water droplets fall into, thereby getting rid of the sound.
Researchers said their findings could be used to develop a more efficient way to measure rainfall and could also help sound engineers create more realistic sound effects in movies and videogames.