Sound absorption characteristics of multilayer porous metal materials backed with an air gap

A sound absorbing structure of multilayer porous metal materials backed with an air gap is proposed to enhance the normal incidence sound absorption performance. The new sound absorbing structures could improve the sound absorption coefficient in a low frequency range and decrease the fluctuation of the sound absorption coefficient in a high frequency range. The Johnson-Allard model is employed to study the sound absorption characteristics of the porous metal materials. And acoustic wave transmitting in the porous metal materials obeys the law of acoustic wave propagation. The sound absorption coefficient and normalized acoustic impedance of the multilayer porous metal materials with or without an air gap are analyzed theoretically, and then compared with existing experimental values. Furthermore, the influence of several physical parameters on sound absorption characteristics, such as the thickness of the air gap, the number of layers, thickness, porosity and flow resistivity of the porous metal materials, are evaluated. The results indicate that the sound absorbing structures of the multilayer porous metal materials backed with an air gap present a satisfying sound absorption characteristic over a wide frequency range.