## 1 Introduction
Nomex honeycomb sandwich panels also possess other advantages including resistance to aging, flame retardancy, and heat insulation; they can also improve the acoustic performance of structures at and below resonance [21]. Therefore, numerous investigations have been made to improve the acoustic performance of Nomex honeycomb sandwich panels. [PERSON] et al. [21] found that the noise transmitted through a light-weight panel can be reduced by attaching a Nomex honeycomb, and that a honeycomb sandwich panel used as a double-layered core gives a much greater STL than that based on a single-layered core. [PERSON] et al. [22; 23] analyzed the influence of different panel design parameters on the sound transmission loss of honeycomb sandwich panels using a reverberant source room and an anechoic receiver room. They found that core density influences STL above the coincidence frequency, but other parameters like cell size and skin type show negligible effects. Moreover, Kevlar core has much higher stiffness than Nomex core, and hence it can be expected to result in poorer acoustical performance than Nomex core. Additionally, a few scholars have explored the effect of varying the structural parameters of the honeycomb sandwich panel on STL. [PERSON] et al. [10] found that varying the thickness of the face sheet has a significant effect on the panel's vibration and sound radiation characteristics, but the vibration and acoustic response are not sensitive to cell size. [PERSON] et al. [24] found that the cell angle of the honeycomb core had no effect on STL, but increasing the panel thickness lead to a significant improvement in STL in the mid-frequency range. [PERSON] et al. [25] found that increasing the face sheet thickness can lead to an improvement in STL, both below and above the coincidence frequency. [PERSON] et al. [26] found that honeycomb sandwich panels have an increased number of resonances in the 1-1000 Hz range, but there is greater sound pressure transmission loss between these resonant frequencies. It is thus apparent that changing various structural parameters of the honeycomb sandwich panels can, on the whole, affect the sound insulation performance and local frequency bands of the panel to different degrees. The abovementioned research indicates that the honeycomb sandwich structure can achieve better acoustic insulation by combining different concepts [18; 19; 20; 21] and changing the various dimension parameters of the structures can have a large influence on their sound insulation performance. Nevertheless, reverberation room testing is a more expensive and time-consuming experiment. For the impedance tube test, the small sample size makes it difficult to make general conclusions about the sound insulation of that structure [27]. But impedance tube measurement samples are small, with low cost. They are suitable for the initial stage of material design, and the test can study the effects of the dimension parameters on STL in the structure. However, whether the influence rule studied by the impedance tube test is consistent with the rule of the actual panel is yet to be investigated. In this study, Nomex honeycomb sandwich structures were tested using an impedance tube to determine their STL values. The results were used to explore the effect of changing the size of the honeycomb cells, panel thickness, and thickness of the honeycomb core on the acoustic performance of the Nomex honeycomb sandwich structures. Both single- and double-layered structures were employed. In addition, the acoustic boundary element method (BEM) and finite element method (FEM) with the scattering matrix method were employed to simulate the STL performance of the materials in the impedance tube tests. The simulation methods and ideas are hoped to help others to improve simulation accuracy of the STL of the honeycomb sandwich core material tested by impedance tube.