Following the development of the piezoelectric transducer, ultrasound waves were also used by the navy to detect the presence of submerged submarines. Also, during War World II, the military developed ultrasound radar applications. Later, as science progressed, ultrasound waves found new applications in industry, such as, detection of defects in metals, industrial and dental drills, ultrasonic cleaning, cardiology and fundamental sciences such as the study of the molecular properties of materials. The piezoelectric effect is the physical phenomenon that made the whole field of ultrasound possible. Let us first introduce a simple description of this effect.
Longitudinal waves travel the medium in the direction they propagate.
An example of longitudinal waves is sound waves in air. Transverse waves travel the medium in a direction perpendicular to the direction they propagate. The vibration of a string is a transverse wave. A shear wave is a transverse wave that is accompanied by a shear of the bulk of a medium. The stress pattern has the form of a time-varying pattern.