Improvement of Structural Damping Numerical Assessment

Structural damping is known as one of the most efficient design variable in order to reduce structure-borne noise. At medium frequencies (200 - 800 Hz) this damping can be obtained from various devices such as interior trim packages or viscoelastic layers. In order to improve car body design (cost, weight, performance, reliability), one must have a good understanding of ‘where’ and ‘how much’ damping must be specified. Because dissipative devices are usually localized, a single panel damping specification is more convenient than a global specification. The damping loss factor seems to be an appropriate indicator in the medium frequency range. The damping loss factor is the ratio of the dissipated power to the time derivative of the strain energy. Considering an isolated panel, the dissipated power is equal to the injected one. Knowledge of strain energy being usually (and especially in tests) quite difficult, the common assumption consists in considering that strain and kinetic energy are equal. This assumption, only valid for high modal density, can not be used in the frequency range of interest (medium). In this paper we will show how to compute the damping loss factor of a car body panel using only measurable quantities: input mobilities and kinetic energies. Numerical examples on different car panels are shown to validate the proposed method. The effect of current and improved assumptions is discussed.