Piezoelectric transducers are based on the use of the piezoelectric effect, which some crystals have. There are direct and reverse piezoelectric effects.
Direct piezoelectric effect - when under the action of mechanical stresses on the faces of the crystal, electric charges arise. Force, pressure and acceleration transducers are based on the use of the direct piezoelectric effect.
Inverse piezoelectric effect - when under the action of an electric field, the geometric dimensions of the crystal change. The reverse piezoelectric effect is used in ultrasonic generators, vibrators, etc.
The piezoelectric effect is most studied in quartz, tourmaline and ferroelectrics (Rochelle salt, barium titanium, etc.). The figure schematically shows the device of a piezoelectric pressure transducer. The converter consists of two quartz plates / connected in parallel. The outer edges of the plates are grounded, and between the middle edges there is a brass foil 2. The pressure on the quartz plates is transmitted through the membrane 5. The output voltage from the quartz plates is removed by a shielded cable 4. There is a hole to connect the cable to the brass foil, plugged by plug 3.
