Noise transmission analysis through a mechanically coupled finite double wall panel

Abstract

Double wall panels with two structural leaves and air in between them form an integral part of aircraft fuselage, automobile bodies, glazed doors and windows in buildings etc. Although they are very efficient in reducing the acoustic radiation in general, they have a problem of mass-air-mass coupling in the vicinity of which the noise transmission is highly magnified, Furthermore, the two panels forming the double wall need to be mechanically connected which forms a path of acoustic transfer too. In the literatures available so far most of the noise transmission studies through double wall do not include the mechanical connectors. The studies are also limited to simplified structural forms and boundary conditions. In the present study an attempt is made to model the sound transmission behaviour through a finite double wall panel using a modal approach wherein the in-vacuo structural modes and rigid wall acoustic modes are coupled using Green’s Theorem. The two structural panels considered for this study are of size 0.5 m x 0.35 m x 0.002 m and 0.5 m x 0.35 m x 0.003 m, respectively. Two different separation thicknesses/gaps of 0.1 m and 0.05 m are taken into consideration to model the acoustic domain. The analysis domain in terms of frequency range was limited to 0 – 500 Hz. Modal analysis is performed in an ANSYS environment and the modal parameters are exported to a MATLAB environment where the analysis is carried out to obtain the transmission characteristics of the double wall. Results are compared with a full transient analysis of the complete system and it is observed that they are in very good agreement. However the major challenge in the present study was to incorporate the mechanical link of finite stiffness in the modal coupling of the two plates. It was observed that to obtain a good agreement with the full transient analysis with mechanical connections alone, mode shapes up to 2500 Hz have to be incorpo-rated in the modal analysis. Presently studies are being conducted to condense the higher