When a voltage is applied across the piezoelectric ceramics, it stretches in its plane. When a voltage is applied to piezoelectric diaphragm, as the metal plate does not stretch, it bends as shown in Fig (a). When the polarity of the applied voltage is reversed, the piezoelectric ceramics shrinks and the metal plate bends toward the opposite side as shown in Fig (b).
When the direction of applied voltage changes alternately, the state of Fig (a) and Fig (b) is repeated as shown in Fig. (c), producing sound waves in the air.