## 5 Conclusions  
In this study, experiments were carried out to measure the sound transmission loss of sandwich structures made of aluminium (face sheets) and Nomex (honeycomb core). Some test results were simulated using modal-based vibro-acoustic response analysis. The influence of various factors (size of the honeycomb cells, thickness of the face sheets and honeycomb core, and single-vs. double-layer structure) on the sound transmission losses investigated. Our main conclusions are:  
1. The size of the honeycomb cells had an insignificant effect on the sound transmission loss of the honeycomb sandwich structure. As the thickness of the face sheet increased, the sound transmission loss valley point of the structure decreased, and the sound transmission loss in the high-frequency band increased significantly. Because the increase in the face sheet thickness resulted in an increase in the mass per unit area, the overall sound transmission loss increased as the face sheet thickness increased after the resonance region. 2. Increasing the thickness of the honeycomb core can also improve the sound transmission loss of the structure, on the whole. However, after the thickness reached 30 mm, any further increase in the thickness increased the sound transmission loss of materials to an insignificant degree. The benefit gained by increasing the thickness of the honeycomb core further began to weaken due to the stiffness of structure approaching the maximum. This could be attributed to the fact that excessive thickness will reduce the stiffness of the honeycomb sandwich panel and optimum middle thickness values maximized the stiffness. 3. On average, the sound transmission loss value of the double-layered structure was 4 dB greater in the 1.6-2.4 and 3.7-5.2 kHz frequency bands compared to the single-layered structure. The stiffness of the composite multilayer structures increased with intermediate layers. So, the performance of the double-layered honeycomb sandwich structure was superior to that of the single-layered structure in the stiffness region. 4. The trends in the sound transmission loss curves were affected by the structural modes of the panels and were related to the resonance of the materials. In the research discussed above it can be seen that the sound transmission loss results of impedance tube tests are consistent with the influence law of the structure dimension parameters on the sound transmission loss of the large panel, indicating that the influence of the dimension parameters on the structure's sound transmission loss can be studied using an impedance tube. Conceptualization, S.X. ; Data curation, S.X. ; Formal analysis, D.W.; Funding acquisition, S.X. ; Investigation, D.W.; Resources, S.X. ; Writing--original draft, D.W.; Writing--review & editing, S.X., Z.F., X.L. and Y.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (grant number 51775558), Innovation-Driven Program of the Central South University (grant number 2018 CX023), the Nature Science Foundation for Excellent Youth Scholars of Hunan Province (grant number 2019 JJ30034), the Shenghua Yu-ying Talents Program of the Central South University (Principle Investigator: Pro. [PERSON]) and the National Key R&D Program of China (grant number 2017 YFB1201103-04). And The APC was funded by the Nature Science Foundation for Excellent Youth Scholars of Hunan Province. Figure 17: Simulated mode shapes for the D\({}_{2}\)-type panel: (**a**) mode at 5787 Hz; (**b**) mode at 7338 Hz; (**c**) mode at 7339 Hz. **Acknowledgments:** This research was undertaken at Key Laboratory of Traffic Safety on Track (Central South University), Ministry of Education, China. The authors gratefully acknowledge the support from the National Natural Science Foundation of China (Grant no. 51775558). This paper also supported by Innovation-Driven Program of the Central South University (No.