Sound absorption properties for multi-layer of composite materials using nonwoven fabrics with kapok

Kapok fiber with high degree of hollow and small fiber diameter has excellent sound absorption. To optimize the sound absorption characters at the low frequency, sound absorption coefficients and specific surface impedance of two-layer, three-layer, and four-layer materials as polyethylene film-reinforced kapok fiber nonwoven fabrics have been studied by using the impedance tube method in the frequency range of 100–2500 Hz. The layering sequence, the polyethylene films thickness, and the gradient thickness structure of nonwoven fabrics affected the sound absorption properties at the low frequency for multi-layer materials that have been analyzed. The results show that the multi-layer composite materials have better sound absorption than a single-layer nonwoven fabrics at the low frequency. The elastic vibration of polyethylene film contributes to sound absorption improvement at the low frequency. The sound absorption at the low frequency of multi-layer materials can be further improved by changing polyethylene film position or increasing polyethylene film thickness. In addition, the gradient thickness structure of nonwoven fabrics can effectively improve sound absorption at the low frequency, and the gradient change direction also exhibits remarkable effects on the sound absorption of multi-layer materials.