Alternative method to the Oberst technique to measure the complex elastic modulus of viscoelastic materials

Viscoelastic materials have a wide range applicability in noise control. They are used to reduce vibration transmission and sound radiation. However, they are effective when the damping system is properly designed. To do that, it is necessary to characterize the elastic and damping properties of the viscoelastic material. Among the several methods that can be found in the literature, the Oberst beam technique is undoubtedly one of the most popular. However, several drawbacks of such method have been pinpointed by different authors and alternatives to overcome these issues have been presented. In this article, an alternative methodology to the Oberst technique is proposed in order to determine the elastic modulus and the loss factor of viscoelastic elements. This method allows the evaluation of the material's elastic and damping characteristics in narrow bands within a wide frequency range from the structural complex wavenumbers measured over a line of points equally spaced length-wise on the tested beam surface. The experimental test rig is easily implementable and offers the possibility to investigate the viscoelastic material as a self-supporting beam with no need to be coupled to a base beam. An electrodynamic shaker is used as excitation source. Moreover, unlike in the Oberst method, there are no assumptions regarding the boundary conditions to be fulfilled. The method is validated by comparing the narrow band values of the elastic modulus and loss factor, measured for three different materials, with the results obtained by using the classical Oberst beam method.