Nonwoven fabric sheet with back air space serving as Helmholtz resonator

The sound absorption effect of a Helmholtz resonator was induced by creating an aperture in a nonwoven fabric sheet with a back air space, thus obtaining a sufficient sound absorption effect in a broad frequency range. In this study, transfer matrices were used to obtain the sound absorption coefficient. Transfer matrices were used to represent the following mathematically: a nonwoven fabric model based on the simplified limp-frame model, an acoustic element that considers the vibration of the nonwoven fabric sheet, the air space behind the nonwoven fabric sheet, and the aperture of the nonwoven fabric sheet. Equivalent circuits combining these transfer matrices were used to obtain sound absorption coefficient. The Helmholtz resonator sound absorption effect was more pronounced for the nonwoven fabric sheets with greater ventilation resistance. The results showed consistency between the experimental and theoretical trends. As for the sound absorption coefficient derived using the theoretical models for nonwoven fabric sheets, the results of the simplified limp-frame model were the closest to the experimental values. As the length of the back air space increased, the peak sound absorption value shifted toward low frequencies due to the sound absorption principles of both the nonwoven fabric sheet with a back air space and the Helmholtz resonator.