Do piezoelectric ultrasonic transducers distinguish positive and negative poles? How do you tell?

Piezoelectric transducers are generally partial positive and negative poles. Because piezoelectric transducers are ac driven. However, like cleaning and welding transducers, for convenience, the electrodes connected with the front and rear covers are generally considered to be negative. Detection of the transducer, if it is a metal shell, generally the metal shell and one of the piezoelectric transducer connected together, when the shield, this when the negative pole.