On the screen you see a photograph, taken by Hugh Pumphrey which shows, on the left high speed photography of a raindrop hitting water (the black horizontal line half way through the screen is the air/water interface); and on the right we see an oscilloscope trace of the hydrophone signal, which plots pressure in the horizontal direction and time running down the frame.

 

 

The droplet can be seen hitting the water in frame (a).  It causes a crater to form up to frame (e) [the black hemisphere visible at the base of frames (b) onwards is the hydrophone]. The crater begins to close, and it is not until frame (g) that  the hydrophone shows a signal. This is coincident with a bubble being pinched off from the bottom of the crater. It is the bubble pulsation which causes the exponentially decaying sinusoid characteristic of the entrainment, the sound emitted when the bubble is entrained.