Numerical Method for Predicting the Sound Reduction Index of a Double Panel Including an Active Control System and Porous Material

Abstract

In this paper, a numerical method for predicting the sound reduction index of a double wall including an active control system and a porous layer inside the double wall cavity is presented. The double wall is placed in a laboratory situation between an emission and a reception room. Active control sources and error microphones are distributed in two vertical planes located inside the double wall cavity. The secondary sources role is to improve the sound reduction index at low frequencies by reducing the acoustical pressure at microphone positions created by the primary source placed inside the emission room. The simulation principle is based on the decoupled Green method where the radiated pressure inside the reception room is deduced from the double wall velocity and from the reception room Green's functions. The double wall and the active control system are modelled by finite element method using Nastran software. The two rooms Green's functions are analytically modelled using a modal method. The model allows investigating effects of various parameters on active control efficiency, such as the effect of evanescent modes inside the double wall cavity, the double wall cavity thickness, the number of control channels as well as the effect of a porous layer inserted inside the double wall cavity placed between secondary sources plane and error microphones plane.