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. The vibration is usually generated by a piezoelectric stack actuator which transduces electrical energy into large amplitude mechanical vibrations. The amplitude and impedance transfer characteristics of the transducer system control the vibration of the sample. Within a linear contact (no tip chatter) model, the interaction between the transducer system and the specimen can be characterized using the theory of linear time‐invariant (LTI) systems and electro‐mechanical Norton equivalence. We present quantitative measurements of the performance of piezoelectric stack actuators in a vibrothermography excitation system and investigate the effect of actuator performance and specimen characteristics on the induced vibration in the specimen. We show that the system resonances generated because of metal‐metal contact of specimen and actuator are broken by adding a couplant between specimen and actuator. Finally, we give criteria for actuator and couplant selection for vibrothermography.