Vibro-acoustic analysis of the low frequency insertion loss of finite double-panel partitions

Due to the combination of a relatively low specific weight and a high insertion loss, double-panel partitions are often used in noise control engineering applications. In applications like machine encapsulations and the double-panel fuselage structure of propeller aircrafts, the excitation of the partition contains important low frequency components. In order to investigate the physical phenomena governing the low frequency insertion loss characteristics, a detailed vibro-acoustic analysis of a finite double-panel partition has been performed. In a first part a theoretical analysis of the dynamic behaviour of the double-panel partition is developed and experimentally validated. Due to the low stiffness to mass ratio of both panels and the small volume of the enclosed cavity, the low frequency dynamic behaviour is strongly influenced by the fluid-structure coupling interaction. The theoretical model, in which this vibro-acoustic coupling is taken into account, is based on a modal superposition technique. It reveals a selective modal coupling interaction, based on the geometrical matching of the acoustical and mechanical mode shapes at their interface. The second part discusses the insertion loss characteristics of the double-panel partition in the low frequency range. Based on the modal response of the panels, the radiation has been calculated and experimentally validated. In comparison with infinite double-panel partitions, the mass-air-mass phenomena are still present, but a resonant behaviour is superimposed on the insertion loss characteristics of infinite double-panel partitions. The theoretical and experimental results indicate the frequency-dependent modal radiation efficiencies and intermodal radiation coupling phenomena to be the reasons for this resonant behaviour.