In many vibration problems, the primary excitation force typically has a repetitive or periodic nature and in some cases, the periodic forcing function may in fact be purely sinusoidal (i.e., harmonic). Examples are excitations caused as a result of mass eccentricity and misalignments in rotational components, tooth meshing in gears, and electromagnetic devices excited by ac or periodic electrical signals. The response to a harmonic excitation is also harmonic, at least for linear systems in the steady state. In basic terms, the frequency response of a dynamic system is the response to a pure sinusoidal excitation. As the amplitude and the frequency of the excitation are changed, the response also changes. In this manner, the response of the system over a range of excitation frequencies can be determined and this represents the frequency response. In this case, frequency (v) is the independent variable and hence we are dealing with the frequency domain. In contrast, in the time domain, the independent variable is time (t).