On the use of the transmissibility concept for the evaluation of frequency response functions

The dynamic characteristics of a structure are often derived from a set of measured frequency response functions (FRFs). However, it may happen that the measurement of certain FRFs is impossible, as they are related to some points of interest that may become physically inaccessible in operational conditions. In this circumstance, it is useful to have some tools that can provide the prediction of such dynamic information. The transmissibility concept can play an important role to circumvent these situations. In fact, there are important properties associated to the transmissibility—the relationship between two sets of responses, for a given set of applied forces, extended to a general multiple degree-of-freedom system. In this paper, some important properties of the transmissibility matrix will be presented. Additionally, it will be shown that if a modification is operated on the original system – using both theoretical and experimental models – it is possible to estimate the FRFs associated to the unknown co-ordinates, without the necessity of measuring the responses on those co-ordinates.