ABSTRACT
In simple terms, modal analysis is the process (analytical or experimental or both) that determines the properties of the normal modes that describe the dynamic response of a structure. Each normal mode of vibration is characterised in terms of a resonance frequency, mode shape and damping, much like the simple single-degree-of-freedom oscillator. The overall response of a structure to a specified excitation force over a specified frequency range can determined by summing the contributions of each mode at each specific frequency of interest in the frequency range under consideration. When summing the contributions at specific frequencies, relative phases must be taken into account and it should be remembered that a vibration mode can contribute to the structural response at frequencies other than its resonance frequency. It also should be noted that in order to obtain accurate results, it is necessary to include in the sum, modes with resonance frequencies up to twice the upper frequency of interest in the analysis and down to 50% of the lowest frequency of interest in the analysis.
