Gradually perforated porous materials backed with Helmholtz resonant cavity for broadband low-frequency sound absorption

Conventional sound absorbers can hardly own the good performance of low-frequency and broadband absorption simultaneously. In order to combine these two functions into one kind of absorbers, the gradually perforated porous materials backed with Helmholtz resonant cavity are proposed. A theoretical model and a finite element (FE) model are developed to investigate the sound absorption of the proposed sound absorber. Experimental measurements are conducted to successfully verify the developed theoretical and numerical models. In the theoretical model, the gradually perforated porous material is divided into a series of thin layers and each layer is described by the double porosity theory. The backed Helmholtz resonant cavity and the connection between these thin layers are modeled by applying the transfer matrix method, so as to achieve the surface acoustic impedance of the absorber via a bottom-up approach. The FE simulations reveal the sound absorption mechanisms of the proposed absorber from the viewpoints of particle vibration velocities, sound energy flow and energy dissipation distributions. The influences of the porous material matrix and the structural parameters on the sound absorption of the proposed absorber are analyzed. Results show that the gradually perforation hole substantially improves the impedance match between the material and air to enhance broadband sound absorption. Meanwhile, the backed Helmholtz resonant cavity brings a low-frequency sound absorption peak to augment sound absorption. This proposed absorber realizes an excellent combination of low-frequency and broadband sound absorption. In addition, its easy-to-manufacture characteristics show great application potential.