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Acoustic performance of auto interiors is definitely important to control the NVH (noise, vibration, and harshness) performance inside a vehicle, and it is determined by the material parameters, such as density (ρ), thickness (d), open porosity (OP), airflow resistivity (σ), tortuosity (T), viscous characteristic length (VCL), thermal characteristic length (TCL), young's modulus, poisson's ratio, and damping coefficient. Firstly, by making different felt samples (of different surface density and thickness), the sound absorption performance and related parameters were obtained. Then the correlation between the parameters and the sound absorption coefficient (SAC) was summarized. Through this method, database of acoustic parameters and the corresponding SAC for porous materials can be established and sound package design and adjustment can be easily conducted based on the database.

PE (polyethylene) membranes are widely adopted in sound insulation pads inside vehicle. However, there are few studies on the acoustical effects of these inserted membranes. This study focuses on these effects. Frequently sound insulation is made up of two layers of felt (a pad made of cotton or synthetic fiber), separated by a PE membrane. The normal incidence sound absorption coefficient and sound transmission loss for this type of insulation construction were calculated through the micro perforated membrane theory and the analytical model (NOVA) which is based on Biot theory. Impedance tube measurement was used to derive the poroelastic properties needed to utilize these models. Comparison between the calculated and measured results showed that the absorption coefficient obtained from the micro perforated membrane theory was closer to the measured value above 3 kHz. And that calculated using NOVA was closer to the measured value below 3 kHz.

Several methods have been established to measure the normal incidence transmission loss of noise control materials using the standing wave tube. In the automotive NVH field, multi-layered systems are common-place, for example in the interaction between the traditional mass-decoupler dash insulator and the front dash sheet metal. Most of the sound transmission loss studies utilizing the standing wave tube have so far been focused on single layer systems with only a limited number of studies on multi-layered systems. Therefore there is only some degree of information on the correlation between this said method and the more widely accepted two-room methods of determining sound transmission properties in these systems.