Inserting transducer model into a circuit
Assuming you are talking about a piezo transducer, there is a strong inductive component (like a tank circuit) because of the mechanical resonance. 

This site has a simple model of a piezo transducer: 



There are actually going to be multiple resonances in many cases- the above model is for a thickness resonance. Audible-frequency to ultrasonic transducers use a bending mode, so the calculator on that page will not yield useful results for such a transducer.