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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.

This paper discusses the approach and application of controlling material and manufacturing parameters in development of lightweight acoustic interior systems. First addressed is the theoretical premise of noise control mechanisms and their relationship to material property/process sensitivity through poroelastic model simulation. The optimal balance of sound transmission loss and absorption in achieving optimally tuned acoustic performance is then presented along with material sample and in-vehicle experimental results. The ability to acoustically tune the vehicle interior to a desired sound level and frequency content through proper design & control of the elastic porous properties achieved by unique acoustic material/process flexibility & capability is demonstrated.

This paper reviews how sound transmission loss (STL) of insulators is affected by gravitational and thermal effects. A special STL test fixture was designed and fabricated to quickly and accurately obtain the STL measurement of a sample oriented at various controlled angles. The STL apparatus was designed to roll into a large reverberation chamber and act as the anechoic termination for a two-microphone approach to measuring STL. The fixture was also built with the intention of studying the temperature effects on a material's STL performance. A variety of samples, including lightweight and traditional barrier decoupled insulators, were tested in the horizontal, vertical, and inverted positions to evaluate gravitational/inertial effects. Thermal effects were investigated by bringing the STL apparatus and sample to a low temperature by moving outdoors, and then rolling the system into the reverberation chamber, at normal room temperature.

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.