What’s the difference between ultrasonic sensor and transducer?

Majority of people use ‘Transducer’ as an exchange of “sensor” and the reverse. But technically, we use ‘Transducer’ as word for collective used term form for both Sensors (which sense movements, electrical signals…) and Actuators. Actuators convert the eco into a pulse of electrical signal.

The piezoelectric element is an essential part of the transducer to generate ultrasonic waves. On both sides of the piezoelectric element electrodes are affixed and a voltage is applied. The element then oscillates by repeatedly expanding and contracting, generating a sound wave. When the element is externally applied with vibration (or an ultrasonic wave) in turn, it generates a voltage.
Among the several types of piezoelectric elements, piezoelectric ceramic (PZT: lead zirconatetitanate) is most commonly used because of its high conversion efficiency.