Experimental study of inhomogeneously damped plates with complex structural modes

Kurzfassung

Acoustics of vibrating plates is often characterized by sound radiation properties of structural modes. In most cases the assumption of a homogeneous damping distribution is made, which results in real valued modes. Increasing use of high performance lightweight structures and demanding acoustic requirements lead to an application of damping treatments like constraint layer damping or embedded elastomer layers in the composite layup. To save weight and costs these damping treatments may be distributed locally at some critical locations of the structure. This violates the assumption of homogeneous damping distribution and leads to complex modes of vibration. In contrast to real modes, the complex modes are no longer dominated by pure standing waves but also by a superposition of standing and travelling waves. First numerical studies revealed an important influence of eigenvector complexity on the sound radiation properties of structural modes. In this paper, the results of an experimental study of sound radiation properties of inhomogeneously damped plates are presented. To generate complex modes a clamped rectangular plate is treated locally by constraint layer damping. The re-sulting vibration patterns are measured with a laser scanning vibrometer and characterised in terms of complexity by modal collinearity index. The acoustic considerations of complex modes focus on modal radiation efficiencies and spatial intensity distribution. Finally, it is shown that experimental results confirm the structural acoustic phenomena, predicted by numerical analysis in the previous studies