Acoustical prediction of the effect of porous materials on the noise level inside a coupled structural and acoustic system

The prediction and optimization of the acoustic effects of interior trim acoustic products (porous sound absorbing materials) on cabin interior noise level are complex and challenging. This research aims to develop an experimentally validated acoustic model to effectively predict/optimize the cabin interior noise level and its interaction with porous sound absorbing materials. Firstly, the sound absorption coefficients of porous sound absorbing materials are experimentally measured by using the two microphone transfer function method and the reverberation chamber method. Then, an inverse method for acoustic material characterization based on normal incidence impedance tube measurements is used. In order to measure/predict the cabin interior noise level due to structure borne noise (force excitation) and airborne noise (acoustic excitation), a scaled down coupled structural acoustic system imitating a car cabin is set up. The car cabin is made of six rigid walls, and one flexible plate is mounted on the front firewall position. The porous sound absorbing material is applied to the inner surface of the plate and modifies the coupling between the plate and the cabin air cavity. The plate is mechanically excited by using an electromagnetic shaker, which is imitating the structure borne radiated noise, and the airborne noise is acoustically excited by using a loudspeaker. The radiated noise is recorded by using pressure microphones at different locations inside the car cabin. The prediction of the acoustic effects of the porous sound absorbing material on a car cabin structural acoustic coupling system is conducted by using FEM (Finite Element Method) and SEA (Statistical Energy Analysis) for structure borne noise and airborne noise analysis, respectively. Based on the model proposed, the effect of the interior trim porous materials on the acoustic properties is investigated by using the Sound Pressure Level (SPL) at the microphone locations. Finally, the experimentally acquired acoustic properties of the porous sound absorbing material are compared with the numerical simulation results. The simulation results show that the proposed model agrees well with the experimental data. The noise propagating inside the car cabin is predicted to be at a similar level in both the experimental method and in the numerical analysis. Moreover, the effects of the acoustic pressure response of porous material on the structural acoustic system are analysed and discussed. The experimental data and results presented in this thesis, are useful for enhancing vehicle interior trim design and refining vehicle cabin acoustic quality in future vehicles.