Vibrothermography, also known as Sonic IR and Thermosonics, is an NDE technique for finding cracks and flaws based on vibration-induced frictional rubbing of unbonded surfaces. Vibration is usually generated by an ultrasonic welder or a broadband piezoelectric stack transducer which transduces electrical energy into mechanical vibrations. Defect detection in vibrothermography depends on specimen vibration, which in turn is proportional to velocity spectrum (V_oc) of the transducer (with sufficient coupling). A long standing problem has been generation of repeatable specimen vibrations. The broadband piezoelectric stack transducers give much better trigger-to-trigger repeatability than the welder system. Even with the broadband piezostack, at sufficiently high excitation voltages, the transducer behavior becomes non linear and less repeatable. Also, as the transducer degrades over time, its V_oc changes significantly. We investigate the reasons for this non linear behavior and present experimental results on the effects of excitation voltage and transducer degradation on V_oc. Using these results, we suggest how to improve the repeatability of vibration generation and to enhance the longevity of the transducer.