Accurate knowledge of the sound field generated by an ultrasonic transducer is important in certain applications, such as custody transfer oil and gas flow measurement. Near- and farfield diffraction effects may influence on the measurement result, demanding a need for control of magnitude and phase influences. In many situations, the simplified model of a planar and uniformly vibrating baffled piston is used to approximate the transducer’s sound field. The Rayleigh distance of the piston model is often used as a measure of the near- and farfield transition range. Real transducers typically exhibit a non-uniform vibration distribution at the front, with significant side and rear vibration. It is of interest to investigate the accuracy of the traditional piston and Rayleigh distance approaches for real transducer sound fields. A circular and baffled Pz27 piezoelectric ceramic disk with diameter 12.7 mm and thickness 2.0 mm is studied, operating in air. Finite-element modeling, supported by analytical and numerical modeling, is used to investigate the near- and farfield sound pressure field over the 0-500 kHz range, including radiation at the two lowest radial modes of the piezoelectric disk. Significant nearfield effects are observed well beyond the Rayleigh distance, both on and off the acoustic axis.