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Humans have been kept awake by the sound of dripping water from a leaky tap or a
roof for generations. It's one of the most recognizable and annoying household
sounds yet - the exact source of the sound has not been known. Until now.
And crucially researchers have 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 found that the plink plink sound
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 fluid mechanics of a water droplet
hitting a liquid surface are well known. When the droplet hits the surface it
causes the formation of a cavity which quickly recoils due to the surface
tension of the liquid resulting in a rising column of liquid. Since the cavity
recoils so fast after the droplets impact, it causes a small air bubble to
get trapped under water. The bubble forces the water surface itself to
vibrate acting like a piston to drive the airborne sound.
The result
In addition the researchers found that changing the surface tension of the
surface for example by adding washing up liquid can stop the sound.
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 synthesized
sound for water droplets in gaming or movies which has not yet been achieved.