Other descriptors of the dynamics o

Other descriptors of the dynamics of constant-parameter linear systems are defined in terms of their response to “standard” excitations. When the excita¬tion is represented by a unit impulse input, the dynamics of the system can be described by its impulse response function (IRF). When the excitation is a unit¬amplitude sinusoidal force applied at every frequency in a given range, another descriptor is obtained: it is the so-called frequency response function (FRF) defined over the considered range of frequency. As with the modal parameters, FRFs and IRFs are univocally determined from the mass, damping, and stiffness properties of the structure. Moreover, they can also be expressed in terms of modal parameters (Chap. 4). Even if each conversion from one model to the other is theoretically reversible (for instance, it is possible to compute the modal properties from the mass, stiffness, and damping matrices, and vice versa), in the case of experimental investigations only a limited subset of points of the structure are measured, and not all the modes are identified. The incomplete information has serious consequences on the possibility to reconstruct the mass, damping, and stiffness matrices from the results of modal testing.