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How can a piezoelectric transducer detect returning ultrasonic echoes?

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A piezoelectric transducer detects returning ultrasonic echoes by utilizing the direct piezoelectric effect.1

When an ultrasonic wave—such as an echo reflected from an object or body tissue—strikes the piezoelectric crystal within the transducer, the mechanical pressure of the vibration causes the crystal to deform. This stress induces a displacement of ions within the crystal lattice, which creates an internal electric polarization.21

As a result of this polarization, a potential difference (voltage) is generated across the crystal. The transducer then converts this mechanical vibration into a corresponding electrical oscillation, which can be processed and analyzed by the system. This process is the inverse of the converse piezoelectric effect, where an applied electric field is used to generate ultrasonic waves.12