Piezoelectric transducers are widely used, for example, in distance sensing applications. Recently, there have been many studies on the application of piezoelectric transducers to antennas used for very low frequency radio wave communication, especially those used for underwater communications and IoT applications. These transducers are essentially narrow-band electromechanical resonators. In order to drive these transducers efficiently, accurate electrical equivalent circuits are required when designing the drive circuit. This paper introduces a method for determining the values of equivalent circuit components over a relatively wide frequency range, taking into account two series resonating circuits. We use evolutionary programming to find the circuit constants. Although the algorithm does not calculate the constant values analytically, the resulting equivalent circuit is evaluated in terms of the residual admittance over the range of frequencies, and it is shown to emulate the actual circuits reasonably well.