Piezoelectric transducers have a number of advantages over other transducer types. They are based on solid state piezoelectric materials often specially formulated ceramics that will change shape when an electric voltage is applied. This is called the inverse piezoelectric effect. In turn, they will generate an electric charge when a mechanical pressure is applied to the piezoelectric transducer which is also known as the direct piezoelectric effect. This piezo effect was first discovered by the Curie brothers more than 100 years ago. Often these piezoelectric electroacoustic transducers are used in ultrasound sonar echolocation, as distance sensors, as flow sensors, level sensors, and so on. A pair of ultrasound transducers operates in transmitter and receiver mode, and the timing or signal strength can be evaluated and converted into the measured quantity. Some of the advantages of piezoelectric transducers are small size, robustness, wide variety of size, shape and form, zero maintenance, long service life, and ease of control.