The water pushes against the bubble, compressing it. As the bubble shrinks, the air trapped inside increases in pressure. This occurs so rapidly that the pressure inside the bubble becomes higher than that of the water, so it expands to equalize, again overshooting. The bubble oscillates between high and low pressure at a high frequency, creating a distinctive and well-quantified sound. The sound radiates energy, so the bubble eventually reaches equilibrium with its surroundings.
Larger bubbles ring at lower frequencies and smaller bubbles ring at higher frequencies.
As the drop size changes, the shape of the splash changes and so does the subsequent sound production. These bubbles are relatively uniform in size, and therefore frequency, and are very loud underwater. The raindrop sizes are identified by different physical mechanisms associated with the drop splashes. The only acoustic signal from these drops is a weak impact sound spread over a wide frequency band. Thus, each drop size produces sound underwater with unique spectral features that can be used to acoustically identify the presence of drops of a given size within the rain.