Modal testing is the practice of gathering empirical test data that can be used to define dynamic response of a structure. The name “modal” implies that the data gathered will be adequate to define one or more dynamic mode shapes of response to some stimulus. The values often calculated from measured data include modal stiffness, damping, mass, natural frequency, and the deformed shape of the structure corresponding with each natural frequency.
In addition to definition of the dynamic response of the structure in question at or near resonance, the same test data can be used to predict off-resonant response based on the characteristics of the modal data.
The tools that are used for modal testing may also be used to generate an operating deflection shape. This field test technique provide visual information about how a machine and/or structure is responding do dynamic forces. By measuring vibration data at several locations with a common phase reference, it is possible to create computer models that visually show the machine's movement at various frequecies. This technique makes diagnosing some machinery faults
These analysis techniques allow a vast amount of information to be presented in an easy to understand visual form.