Sound Transmission Through Multilayered Shells. Experiments and Modelling

The use of porous materials to reduce sound transmission through vibrating structures was investigated in this study. Measurements carried out at LMA showed that adding a layer of melamine foam to a steel plate increases the sound transmission of the treated plate in some frequency bands. A model was therefore developed to predict this increase of the transmitted sound level, induced by covering a vibrating structure with a porous material. The existence of compressional waves, where the quarter wavelength is equal to the thickness of the added layer, was found responsible for the increase of sound transmission at the corresponding frequencies. Biot theory and the equivalent fluid model were used to model the acoustical behavior of the porous material. With the model based on Biot theory, which involves heavy data processing, accurate predictions were obtained. To overcome the drawbacks of the Biot model, a simpler procedure was developed for predicting the frequency range in which transmission increases. Based on the surface admittance at normal incidence, a value which can be easily determined experimentally using an impedance tube, the proposed procedure efficiently predicts the frequency range in which higher sound transmission occurs.