Physics of the Imploding Sphere Source
An inrush of water produces an initial negative acoustic pressure.
The reduced gas volume produces an increase in gas pressure.
When the gas pressure becomes greater than the ambient pressure, water starts to decelerate.
At the moment when the shell of water gets reversed and starts flowing outward, the internal gas pressure becomes quite high.
The gas bubble oscillates with diminishing acoustic pressure on successive cycles as energy is dissipated in heating, turbulence, and radiation.