I completed acoustical department independent research by building lots of underwater microphones to investigate the properties of a piezoelectric polymer film in water. My research was selected by the US Navy to be developed further into a sonar skin for submarines.

A piezoelectric sensor is a device that uses the piezoelectric effect, to measure changes in pressure, acceleration, temperature, strain, or force by converting them to an electrical charge. The prefix piezo- is Greek for 'press' or 'squeeze'.

This video is not mine but it is almost exactly what I built and did. Except that I captured sound rather than emitted sound and I did it in a pool.