## 4 Results and Discussion
### Effect of Fiber Diameter on the Sound Absorption Performance of Porous Materials  
Figure 7a,b illustrates the acoustic properties of porous materials made from fibers of two different diameters under varying densities as a function of frequency. Figure 8c summarizes the corresponding NRC results (the specific values of NRC can be found in Table S3). This study found that as density increases (leading to a reduction in pore size), the NRC first increases and then decreases. The sound absorption coefficient curves show that increasing the density improves the low- and mid-frequency sound absorption performance of fiber porous materials, while the high-frequency performance decreases. This trend is attributed to friction effects between the air and external pore walls (the surfaces of adjacent fibers) [14]. This phenomenon is more pronounced with fibers of smaller diameters. Due to the improvement in sound absorption performance at low frequencies and the decrease at high frequencies, the overall improvement in NRC between adjacent densities is not significant, with a maximum increase of 5.36%. The NRC reaches its maximum value at a density of 120 kg/m\({}^{3}\), corresponding to median pore sizes of 68.50 \(\upmu\)m and 58.16 \(\upmu\)m for the two fiber diameters, respectively. When the density is 140 kg/m\({}^{3}\), the median pore size further decreases, and the sound absorption performance declines. This suggests that the sound absorption performance of fiber-based porous materials in the low- and mid-frequency ranges improves as pore size decreases, but there exists an optimal pore size range. Exceeding this range can lead to reduced performance. This observation aligns well with the theoretical analysis presented in Section 2.  
Figure 6: Pore size distribution of fibrous composite porous materials with different densities: (**a**) median pore size; (**b**) pore size distribution at 80 kg/m\({}^{3}\); (**c**) pore size distribution at 100 kg/m\({}^{3}\); (**d**) pore size distribution at 120 kg/m\({}^{3}\).  
Figure 7: Sound absorption performance test results of single-fiber porous materials: (**a**) sound absorption coefficient of 7 \(\upmu\)m fiber porous material; (**b**) sound absorption coefficient of 5 \(\upmu\)m fiber porous material; (**c**) Percentage Increase.  
Comparing the sound absorption coefficients of the two materials at the same density, it is evident that a reduction in fiber diameter results in smaller pore sizes and better sound absorption performance. This improvement is due to the increase in the contact area between the internal pores and air as the fiber diameter decreases, leading to more frequent collisions of sound waves with the material's surface, which enhances energy dissipation. Fiber porous materials with a diameter of 5 \(\upmu\)m achieve a comparable sound absorption performance at a density of 60 kg/m\({}^{3}\) to that of fiber porous materials with a diameter of 7 \(\upmu\)m at a density of 100 kg/m\({}^{3}\). At this point, the median pore sizes of the two materials are also similar, further confirming that pore size significantly influences the sound absorption performance of fiber porous materials. As the pore size decreases, the sound absorption performance first improves and then diminishes, with an optimal pore size of approximately 60 \(\upmu\)m exhibiting the highest sound absorption performance.