# Structure and Sound Absorption Properties of Spiral Vane Electrospun PVA/PEO Nanofiber Membranes
## 3 Results and Discussion
### Sound Absorption Properties of Nanofiber Membranes  
Application of electrospun nanofiber membranes was contributed to its ease of manufacture and light weight. They can be used for indoors and automotive decoration to reduce noise. The sound absorption coefficient can be expressed as a percentage ratio of the absorbed sound wave and the incident sound wave. It is called sound absorbing material when its average sound absorption coefficient is greater than 0.2; when its average sound absorption coefficient is more than 0.56, it is known as excellent sound absorption material [18]. In order to improve the accuracy of results, we tested the sound absorption properties of the nanofiber membrane with the same thickness (2 mm) and composited with the same kind of needle-punched nonwovens (4 mm). In this paper, the materials hardly have sound absorption properties over the range of 80-500 Hz, so we mainly studied the sound absorption properties over the range of 500-6300 Hz. Figure 7 showed that the needle-punched nonwoven had poor sound absorption property, while nano-fiber membranes with a thickness of 2 mm showed good sound absorption property. The weird thing was that the sound absorption curve trends of sample a and sample b are similar, while as well as sample c and sample d. It was noticed that sample a showed maximum sound absorption coefficient 0.63 at the frequency of 4000 Hz, sample b reached up to 0.99 at the same frequency. The absorption peak sample c and d shifted to low-middle frequency, sample d showed better sound absorption performance. Figure 6: Depth of field three-dimensional microscopic images of PVA/PEO membranes. (a) 8 wt % PVA; (b) 8 wt % PVA/3 wt % PEO (90/10); (c) 8 wt % PVA/3 wt % PEO (80/20); (d) 8 wt % PVA/3 wt % PEO (70/30); (e) 3 wt % PEO. It can be seen from Figure 8 that sound absorption performance of the needle-punched nonwoven had been greatly improved after being compounded with the nanofiber membranes. Because of the increased thickness, the path of the sound waves inside the material became longer thus acoustic energy lost mostly [37; 38]. Sample a showed maximum sound absorption coefficient 0.78 at the frequency of 1580 Hz. The sound absorption curves trend of sample b-d were consistent, absorption peaks reached up to 0.78, 0.82, 0.89 and 1, 0.79, 0.91 at the frequencies of 1000 Hz and 2000 Hz, respectively. [PERSON] et al. studied that more cavity volume, wider frequency band and larger thickness were beneficial for sound absorption. Materials acquired excellent sound absorption at low frequency with a sound wavelength and thickness ratio of 169.9 [39]. Samples a, b, c, and d showed notably better sound absorption performance in the range of 1081-1945, 750-6300, 780-3077, and 750-6300 Hz, respectively. [PERSON] et al. studied that the coustic absorption band of PVA nanofiber membrane was broadened when the surface irregularity increased [40]. With the addition of PEO, the absorption peak shifted to low-middle frequency, and the sound absorption property was improved in the whole frequency range generally that probably because the blend membranes possessed cavity structure and distributed irregularly. Nanofiber membranes belong to porous structure with high porosity, the vibration of the sound waves entered material drived the vibration of the surrounding air and pores. A large amount of sound energy was consumed along with the viscous friction and heat conduction between the sound waves and the airhole walls. Figure 8: Effect of composite sound absorption properties. (**a**) composite of 8 wt % PVA and needle-punched nonwoven; (**b**) composite of 8 wt % PVA/3 wt % PEO (90/10) and needle-punched nonwoven; (**c**) composite of 8 wt % PVA/3 wt % PEO (80/20) and needle-punched nonwoven; (**d**) composite of 8 wt % PVA/3 wt % PEO (70/30) and needle-punched nonwoven. Figure 7: Effect of 2 mm PVA/PEO membranes sound absorption properties.