Models for Prediction of Sound Transmission through the Inner Dash Insulator

Airborne sound transmission through vehicle panels is the main contributor to interior noise in high frequencies. This transmission can be reduced by the application of sound insulation materials. An insulator typically used in the dash panel treatment comprises a porous material layer bonded to a limp dense material. This porous layer dissipates sound energy mainly by viscous effects and reduces sound transmission. An accurate prediction of the insulator performance depends on the porous material model adopted and input material properties. Some simplified models take into account only a single longitudinal wave propagating in the porous medium since it is represented by an equivalent fluid with effective properties. More complex models, based on Biot's theory, also consider waves whose properties are more connected to the porous material frame. In this paper one-wave, two-wave and three-wave models are presented. The relevance of each case in the prediction of sound transmission loss of a solid panel-foam-rubber system is discussed. Models include oblique sound incidence and a diffuse sound field incidence is represented by integration of the transmission coefficient up to a field angle. It is demonstrated that elastic properties of the frame play an important role in this three-layered system.