Ultrasonic sensors are used in many applications. One of their most common applications is in robotics. That is because they are easy to interface and comprehend. Ultrasonic sensors contain ultrasonic transducers. A transducer can convert one energy form to another. The transducer is the reason why a sensor can emit a sonic pulse. An echo
The purpose of most ultrasonic transducers is to convert electrical energy to acoustic energy, do something useful with the acoustic energy, and convert the acoustic energy back into electrical energy. The conversion of electrical energy to acoustic (and vice versa) is a function of the piezoelectric element.