What must limit frequency equations be used for to be considered useful?

Frequency equations are particularly useful for creating impedance diagrams because these diagrams visually represent how a material reacts at different frequencies during electromagnetic testing. In the context of electromagnetic testing, impedance is a critical parameter that can indicate material defects or properties.

When using frequency equations in this manner, the impedance can vary with frequency, and the diagram helps illustrate these variations. This insight is crucial for interpreting test results and understanding material behavior under specific conditions.

While the other options may reference important aspects related to testing and material properties, they do not directly relate to the primary application of frequency equations in the context of impedance diagrams, which serve as a crucial analytical tool in evaluating electromagnetic characteristics.