Sound transmission through double wall aircraft structures/mounting effect.

Abstract

In the context of noise control treatment identification for turbulent boundary layer excitation, the main objective of this work is to assess the impact of mechanical links on the sound transmission loss through a double wall aircraft structure. An assembly of a stiffened panel made of composite fiber reinforced plastic (with a critical frequency around 6000 Hz) and a trim panel with a honeycomb core (with a critical frequency around 3000 Hz) has been considered in the whole study. The transmission loss of this double panel filled with glass wool blankets has been measured under diffuse sound field and point force excitation in the 100–10000 Hz frequency range for a fully decoupled configuration and a totally coupled one (rigid mounting). Extensive simulation using transfer matrix methodology has been conducted on the double panel for different glass wool filling rates, density, sound barriers, and add-on damping patches, for fully decoupled and rigid configuration. The main result is that the rigid mounting decreases significantly (up to 10 dB at 1000 Hz) the double wall transmission loss compared to a fully decoupled mounting and the acoustic efficiency of the passive treatments as well, whatever the excitation field.