## 2 The Sound Absorption Model of the Composite Structure  
The schematic diagrams of the composite structure are shown in Figure 1. The composite structure includes a PP layer, an air gap, and an OFC sound-absorbing material. The sound absorption coefficients of a PP layer backed by air gap can be calculated using [PERSON]'s theory. The characteristic impedance and complex wave number of the OFC are measured by the B and K impedance tube, which is adopted to measure the characteristic impedance and complex wave number of the OFC using the four-microphone transfer function method. According to the measured results, the exponential function is used for curve fitting the characteristic impedance and the complex wave number of the OFC. The characteristic impedance of the composite sound-absorber structure is given, and the normal sound absorption coefficient is calculated. There are many physical phenomena involved in the process of the acoustic energy dissipation of composite structures. The physical phenomena involved in the structure of the composite sound-absorber are shown in Figure 2. The impact of the PP on the sound wave in the incoming hole primarily includes surface viscous, distorted flow and inner viscous, as can be seen in Figure 2a. In this paper, the normal surface impedance of the PP supported by the rear cavity is considered. First, the surface impedance of a single perforation is predicted. Air has a surface viscous effect on the surface of the PP around the holes. Distorted flow is caused by part of the viscous boundary layer, including the viscous effect around the edge of the viscous boundary layer on the panel surface caused by the distortion of the acoustic flow, and the influence caused by the non-uniform flow on the panel surface. As the surface roughness is different due to different PP materials, processing, etc., it generally needs to be corrected. Inner viscous refers to the viscous effect of the inner wall surface of the perforation. The sound mass load, the sound radiation from the perforation and the sound flow on the panel surface will affect each other. It is generally believed that the thickness of the air in the perforation is greater than the perforation depth, which increases the mass of the vibrating air. In order to better describe the movement of the air mass block, the end correction lengths can be used to describe the inertial effect caused by the increased air mass. As shown in Figure 2b, the impact of the reflected sound flow when the sound flow passes through the perforation and enters the porous materials is also considered as a part of the acoustic impedance of the PP. The linear radiation impedance at the perforated end of the OFC side is also related to the characteristic impedance, complex wave number and porosity of the porous material. According to the physical phenomenon of acoustic energy dissipation in the composite structure, the total impedance of the composite structure is divided into five parts. Where \(R_{s}\) is the acoustic impedance generated by the friction between air and PP surface, which is caused by surface viscous (viscous interactions (or effects) at the surface of the perforated plate); \(Z_{p}\) is the impedance generated by the air in the hole, which is caused by the inner viscous; \(Z_{ae}\) is the radiation impedance of the front end (left side of PP), which is mainly  
Figure 1: Schematic diagrams of composite sound-absorber structure. caused by the distorted flow when the sound flow flows into the hole; \(Z_{pe}\) is the radiation impedance at the tail end (on the right side of PP), and this side is the OFC, which is mainly caused by the distorted flow of acoustic flow reflected from the OFC in the inflow hole; \(Z_{ps}\) is the characteristic impedance of the OFC. The acoustic absorption of PP layer backed by an air gap can be calculated using [PERSON]'s theory [5].