Modal testing has become very popular today for dynamic structural definition and problem resolution of a multitude of dynamic problems. A criterion of purity is presented which is applicable to all modal data regardless of the method of excitation. The modal acceptability and purity ratio allows a one-number linear ratio that describes a percentage deviation from zero, where zero is defined as a pure, natural mode. This ratio was used on the Space Shuttle modal program to provide instant visibility of modal data quality as well as a real-time guide to potential problems associated with certain modes.
