Piezoelectricity

Some crystals convert mechanical stress into electric charge and electrical excitation into motion.

The Curie brothers demonstrated the direct piezoelectric effect in 1880. Reciprocal electromechanical behavior later became central to ultrasonic transmitters and receivers. A piezoelectric element can therefore act as both a transmitter and a receiver: an electrical pulse makes it vibrate, while a returning vibration produces a measurable voltage. That reversible behavior remains fundamental to many ultrasound probes.

Sources

• My ultrasound used to look like what? — Lauren Rever, 2017-03-10