Sound transmission loss of absorber-barrier-absorber structures with kapok fibers as absorption layers

The sound insulation of the front wall of a car is mainly used to isolate the noise of the engine from being transmitted to the passenger cabin. A front wall is generally a kind of structure of Absorber-Barrier-Absorber (ABA), where A is sound absorption layer and B is sound insulation layer. The sound absorption layer (A) usually consists of porous cotton of superfine fiber or microfiber. In this paper, kapok fiber is used to make the sound-absorbing layer A, and its sound absorption is compared with that of superfine fiber. Microstructure of the two fibers is compared, and it is found that kapok fiber has a large hollow structure. The sound absorbing coefficients of the layer A made with the two fibers are measured and it is concluded that the layer A made of kapok fibers can absorber noise greatly and thus has high sound absorption coefficient. The prototypes of ABA structural are made using two kinds of fibers as layer A, and the sound transmission loss (STL) was measured using reverberation room and anechoic room. The measurement shows that the ABA using kapok fiber as sound absorption layer has better sound insulations, and the STL of ABA using kapok fiber as sound absorption layer is increased by about 3 dB. A model for estimating STL of ABA structure is established and the calculated STL is validated. Using the developed model, the STL of ABA structures with different structural characteristics of Layers A or B is calculated and analyzed.