## 1 Introduction  
Composite materials with honeycomb sandwich structures can be formed by bonding honeycomb core materials and panels. They are widely used in various fields (including aerospace, railway vehicle construction, and architecture) as they possess several diverse advantages, including high specific strength, favorable shock resistance, and vibration reduction [1; 2; 3]. Moreover, by changing the honeycomb structure, thickness, and materials used in the panels, the panel's properties and performance can be tuned as required [4; 5]. In general, sandwich structures with honeycomb cores are typically more effective than homogeneous panels for sound-transmission reduction applications [6; 7; 8]. The research mentioned above has shown that composite materials with a honeycomb sandwich structure can improve noise insulation. Scholars have proposed numerous methods for improving the sound transmission loss (STL) of honeycomb sandwich panels. [PERSON] et al. [9] found a new material for their honeycomb core that increased damping, and thus improved sound insulation performance at frequencies between 50 and 100 Hz. [PERSON] et al. [10] found that using a panel made of fiber-reinforced plastic with the honeycomb core provided better vibro-acoustic and transmission loss characteristics due to the high stiffness and inherent damping associated with this material. [PERSON] [11] used a honeycomb structure of high stiffness to contain a granular filling, and subsequently found that the filled honeycomb led to lower sound pressure levels compared to empty honeycomb. [PERSON] et al. [12] used micro-perforated panels (MPPs) in combination with honeycomb materials and found that their novel structure reduced certain critical frequencies and the overall STL was improved because the MPP provided additional acoustical resistance. [PERSON] et al. [13] found that attaching gas layers with different acoustic impedance on a honeycomb sandwich panel can produce a substantial increase in STL in the mid-to-high frequency range. [PERSON] et al. [14] designed a structure in which the cross-section was split into two thinner honeycomb sandwich subpanels separated by an air gap. They found that the panel, when optimized, improved the STL at critical rotorcraft transmission tonal frequencies. Some scholars have also attempted to improve the sound absorption performance of certain composite structures by utilizing honeycombs. For example, [PERSON] et al. [15] found that the absorption of sound by MPPs backing a honeycomb cavity in a small-scale enclosure provided broadband noise control. [PERSON] et al. [16] designed a microperforated honeycomb metasurface panel (MHMP) and found that MHMP is thinner and has better sound absorption than MPP at the same peak frequency of sound absorption. Then, [PERSON] et al. [17] designed a composite structure formed by filling honeycomb with polyester fiber, and their results showed that the composite structure is better sound absorption than polyester fiber of the same thickness. By combining different concepts (additional layers, structural resonators, multi-material etc. ), the acoustic insulation of some structures can be improved. [PERSON] et al. [18] mounted air, nitrogen, argon, and helium barrier layers on honeycomb sandwich panels and found that the STL of the structure was substantially improved by the addition of a helium barrier layer. [PERSON] et al. [19] found that a glass fiber-filled honeycomb sandwich panel improved STLs at all frequencies tested and that the STL could be further improved by increasing the fiber content. [PERSON] et al. [20] found that the STL of a honeycomb sandwich plate is significantly increased by attaching a stop band. Nomex honeycomb is a nonmetallic biomimetic core material that is light in weight and has high strength. It is produced by immersing aramid fibers in phenolic resin solution. Compared with honeycomb sandwich panels made from aluminium, Nomex honeycomb sandwich panels are lighter and have a higher strength--mass ratio.