A magnetostriction generator can produce ultrasonic waves of comparatively low frequency. For generating high frequency ultrasonic waves, piezoelectric generator is used.
Piezoelectric Generator. It is found that when pressure or compression is applied on two opposite faces of a quartz crystal, then charges are produced on a set of opposite faces which are perpendicular to the faces at which pressure is applied. The magnitude of charge developed is proportional to the amount of pressure applied. Charge produced on one face is positive and on other face is negative. Further more, if instead of compression, the faces of crystal are subjected to some tension, then nature of charges developed also gets reversed. “The process of appearance of charges on transverse faces of certain crystals when subjected to external stress is called piezoelectric effect.”
But if an electric field is applied across two opposite faces of a Quartz crystal, then extension or compression is produced in the crystal in a direction transverse to the direction of electric field. This effect is called Antipiezoelectric effect. The extent of compression or extension is proportional to the strength of electric field.
If applied electric field is alternating in nature then quartz crystal starts vibrating at the frequency of electric field and hence produces acoustic waves in air. If the frequency of electric field is same as natural frequency of the crystal, then amplitude of crystal oscillations is quite large. This property of Quartz crystal is used for producing ultrasonic waves. Apart from Quartz, other materials showing this property are Rochella salt (Sodium potassium tartrate), tourmaline etc. The effect is best in Quartz cut in a particular manner.