A new model for piezoelectric ultrasonic transducers is proposed. Using a systems engineering approach, the concept of feedback is used to explain secondary piezoelectric effects and to clearly describe electromechanical interaction. The model is derived from the fundamental piezoelectric equations and it embraces the relevant practical situations where the transducer is subject to arbitrary electrical and mechanical loading. Block diagram representations of the piezoelectric transducer acting as a generator and receiver of ultrasound are developed. The physical significance of each element in the block diagrams are explained to provide a clear and distinct relationship between electrical and mechanical quantities. This permits the factors which control secondary piezoelectric action to be readily defined, along with the importance of this effect on the overall transfer function. It is considered that no other transducer model illustrates these concepts with the same degree of clarity and the technique offers significant advantages over existing transducer analogies. A number of computer simulations and experimental water tank measurements of acoustic wave profiles are presented. Close substantiation of the theoretical analysis is shown.