To understand what aspects can determine the sound of raindrops falling in water, consider the rain drops as spherical droplets. When a droplet falls onto an initially ‘flat’ water surface, the surface deforms. This impact of the droplet acts as a generating force. Consequently, additional restoring forces acts to return to the flat surface. This interaction between the generating and restoring forces will create waves. The frequency of the created waves is what we consider to be the ‘sound of rain’.

 

Restoring forces are due to gravity and surface tension. There are some common waves due to falling droplets:

• The impact of the droplet on the flat surface will create a shockwave. Bigger droplets will create louder shockwaves.
• Gravity-capillary waves are created at the free surface. Surface tension will cause capillary waves and gravity will cause gravity waves (not the ones space organizations try to observe). One can compare these waves to the waves created by throwing a stone into a lake.
• Shockwaves are created because of cavitation. When (depending on droplet size) droplets reach the surface with a certain velocity, a closed bubble will develop underwater which will implode resulting into a shockwave. The shockwave frequency depends on the size of the formed closed bubble. According to certain studies, the sound created by cavitation is the loudest one observed under water.

In addition, jets may arise. These jets can also propel a small droplet up into the air, which will fall back onto the surface and create a new soundwave.