A frequency cut-off is almost always a part of the criteria, if not the only criterion, for selecting target modes. The cut-off is often driven by program requirements, but can also be developed on the basis of modal effective mass and transfer-function-based measures presented in the following two sections. For example, a frequency cutoff may be chosen based on a certain percentage of the cumulative modal effective mass, or based on the contribution of modes to particular transfer functions.
For the purpose of planning a modal test and choosing degrees of freedom to instrument, it is common to target all modes below a frequency cutoff, though only a subset of these modes may be correlated after the test.