How are ultrasonic waves produced using the piezoelectric effect?

Ultrasonic waves are produced using the converse piezoelectric effect, which occurs when an external electric field is applied to a piezoelectric crystal or ceramic, such as quartz or lead zirconate titanate.123

When an alternating electric field is applied to the material, it induces a mechanical distortion or strain, causing the crystal to vibrate. To enhance this effect, the frequency of the alternating electric field is matched to the natural mechanical vibration frequency of the crystal. This resonance significantly amplifies the mechanical vibrations, which are then emitted as ultrasound—sound waves with frequencies exceeding 20 kilohertz.12

These piezoelectric transducers are highly versatile; by selecting specific shapes, such as a concave or bowl-like surface, the resulting ultrasonic waves can be focused at a precise point for applications ranging from medical imaging to industrial cleaning.123