The vibration is expressed in a statistical sense by a cross-spectral density matrix. A random vibration step is a linear perturbation analysis that gives the linearized dynamic response to a random excitation (vibration). The response is calculated based on the model's eigenfrequencies and mode shapes, which must first be extracted by using the eigenfrequency extraction procedure in a preceding frequency step. The modes are used to calculate the power spectral densities of response output variables (stresses, strains, displacements, etc.) and the corresponding root mean square (RMS) values of these variables. | ||||||