FREQUENCY RESPONSE
Is a method to compute structural responses to steady-state oscillatory excitations: rotating machinery, unbalanced tyres, etc.
Loading is explicitly defined in the frequency domain.
In the simplest case of a sinusoidal excitation at a single frequency, the response occurs at the same frequency.
Response may be shifted in time due to damping in the system, the peak loading and peak response no longer occur at the same time.
This phase shift is represented using complex numbers, magnitude and phase (respect to applied load) or real and imaginary components.