Whys and Wherefores of Transmissibility

Abstract

The present chapter draws a general overview on the concept of transmissibility and on its potentialities, virtues, limitations and possible applications. The notion of transmissibility has, for a long time, been limited to the single degree-of-freedom (SDOF) system; it is only in the last ten years that the concept has evolved in a consistent manner to a generalized definition applicable to a multiple degree-of-freedom (MDOF) system. Such a generalization can be and has been not only developed in terms of a relation between two sets of harmonic responses for a given loading, but also between applied harmonic forces and corresponding reactions. Extensions to comply with random motions and random forces have also been achieved. From the establishment of the various formulations it was possible to deduce and understand several important properties, which allow for diverse applications that have been envisaged, such as evaluation of unmeasured frequency response functions (FRFs), estimation of reaction forces and detection of damage in a structure. All these aspects are reviewed and described in a logical sequence along this chapter. The notion of transmissibility is presented in every classic textbook on vibrations, associated to the single degree-of-freedom system, when its basis is moving harmonically; it is defined as the ratio between the modulus of the response amplitude and the modulus of the imposed amplitude of motion. Its study enhances some interesting aspects, namely the fact that beyond a certain imposed frequency there is an attenuation in the response amplitude, compared to the input one, i.e., one enters into an isolated region of the spectrum. This enables the design of modifications on the dynamic properties so that the system becomes “more isolated” than before, as its transmissibility has decreased. Usually, the transmissibility of forces, defined as the ratio between the modulus of the transmitted force magnitude to the ground and the modulus of the imposed force magnitude, is also deduced and the conclusion is that the mathematical formula of the transmissibility of forces is exactly the same as for the transmissibility of displacements. As it will be explained, this is not the case for multiple degree of freedom systems. The question that arises is how to extend the idea of transmissibility to a system with N degrees-of-freedom, i.e., how to relate a set of unknown responses to another set of known responses, for a given set of applied forces, or how to evaluate a set of reaction forces from a set of applied ones. Some initial attempts were given by Vakakis et al. (Paipetis & Vakakis, 1985; Vakakis, 1985; Vakakis & Paipetis, 1985; 1986), although that generalization was still