The reason for using sinusoidal excitation is simple - any general time varying forcing function can in principle be expressed as a summation of several sinusoids ( Fourier series ). Assuming linear system behavior, we will be able to superpose the response to individual sinusoids to get the total system response. Moreover, many mechanical systems involve a fundamental operating speed and the excitation source is often predominantly at this speed or its harmonics – for example, unbalance in a shaft causes an excitation at the running speed.