Abstract: An improved apparatus and method for determining the existence and severity of a crack in a shaft system is provided. The apparatus includes means for mounting multiple vibration responsive output transducers around an outer circumference of the shaft system, allowing simultaneous measurements of vibrational response of the shaft system when multiple vibration responsive output transducers are mounted to the mounting means. In a first approach, the mounting means comprises a collar having two connected arcuate members, with drilled milled and tapped apertures for mounting radial input and output transducers. Torsional input and output transducers may also be mounted tangentially on the collar in drilled milled and tapped heads of bolts used to connect the two arcuate members.