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How do piezoelectric materials help sonar generate sound?

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Figure 1: Ferroelectric properties of barium titanate (BaTiO3). (Left) Above 120° C the structure of the BaTiO3 crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal. — capacitor dielectric and piezoelectric ceramics Figure 1: Ferroelectric properties of barium titanate (BaTiO₃). (Left) Above 120° C the structure of the BaTiO₃ crystal is cubic, and there is no net polarization of charge; (right) below 120° C the structure changes to tetragonal, shifting the relative positions of the ions and causing a concentration of positive and negative charges toward opposite ends of the crystal.

Piezoelectric materials, such as specific ceramics and crystals, are essential to sonar technology because they function as acoustic transducers that convert electrical energy into acoustic energy.1

This process relies on the converse piezoelectric effect. When these materials are subjected to an electromagnetic field, they undergo a physical change in dimension, effectively vibrating to produce mechanical motion. By applying an alternating electric field that matches the material's natural mechanical resonance frequency, these vibrations are significantly amplified.234

When these materials are suitably mounted—often in an oil-filled housing—these induced mechanical vibrations generate sound waves that propagate through the water. In active sonar systems, this generated sound travels outward, reflects off target objects, and is subsequently detected by the same or a different transducer, which uses the direct piezoelectric effect to convert the returning acoustic pressure back into an electrical signal for analysis.14