Why are quartz crystals useful for producing ultrasonic waves?

Quartz crystals are highly effective for producing ultrasonic waves due to the piezoelectric effect. When an alternating electric field is applied to a quartz crystal, it undergoes the converse piezoelectric effect, causing the crystal to distort and vibrate mechanically.12

This process is particularly efficient because quartz possesses a very high quality factor ($Q$), which can reach values as high as ${10}^6$. By applying an alternating electric field at the same frequency as the crystal's natural mechanical resonance, the resulting vibrations are significantly enhanced. These high-frequency mechanical vibrations are then used to generate ultrasound—sound waves with frequencies exceeding 20 kilohertz. Because these piezoelectric crystals can be shaped to focus waves or used across a wide range of frequencies, they serve as versatile transducers for applications ranging from medical imaging to industrial sonar systems.12