## 1 Introduction  
Helicopter cabin noise is highly complex, comprising noise generated from a variety of sources, including aerodynamic noise caused by the rotor system, transmission system, engine, and other components [1]. The main gearbox is a key component of the helicopter transmission system, and the impact excitation generated by gear meshing causes structural vibrations and induces cabin noise, whose frequency is usually in the range of 200-2000 Hz. This noise has a high sound pressure level covering the sensitive range of human ear hearing, resulting in cabin noise levels exceeding 95 dB and even reaching 110 dB in many helicopter models [2]. Therefore, the noise generated by the main gear is one of the key factors limiting the development of new helicopters [3]. Figure 1 depicts the cabin noise spectrum of an S-76 helicopter cabin [4, 5]. Clearly, the helicopter cabin noise is mostly medium-high-frequency line-spectrum noise with multiharmonic, multifrequency, and high-level characteristics. Currently, helicopter cabin noise control is mainly based on the addition of porous sound-absorbing materials, honeycomb structures, and damping materials to the cabin wall sandwich to reduce sound energy propagation through vibration isolation and sound absorption techniques [6, 7]. Two helicopter cabin wall panel structures are illustrated in Figure 2. As can be seen, the  
helicopter nacelles typically have a semi-thin-walled structure consisting of an external skin and longitudinal and transverse skeletons. Some civil helicopter models are equipped with interior panels, and their air-back cavities are often filled with sound-absorbing cotton and foam for acoustic treatment. This type of material has limited ability to control low- and medium-frequency noises and has a large additional mass, which affects the overall performance of the helicopter to some extent [5]. According to the Helmholtz resonance principle, the sound absorption characteristics of a microperforated plate acoustic structure exhibit a single resonance peak. If used for noise reduction in helicopter cabins, the sound absorption band of the MPP structure will need to be broadened. The sound absorption band is mostly concentrated in the zones of middle and high frequency; the low-frequency noise suppression effect of the structure is not significant and cannot meet the requirement for noise reduction in helicopter cabins. At present, most of the domestic and international research on sound absorption and insulation materials still remains in the research stage of a single material, and it is difficult to achieve a good noise reduction effect in the wide band. Therefore, efficient new composite materials [15; 16; 17; 18] are one of the trends of future development. Hence, this study proposes three composite MPP acoustic structures for noise reduction in helicopter cabins, namely, a combined MPP acoustic structure (hereinafter referred to as the Combined MPP), an MPP-porous material composite structure (hereinafter referred to as MPP+Porous), and an MPP-acoustic supermaterial composite structure (hereinafter referred to as MPP+PAM), whose basic configurations are shown in Figure 3. The combined MPP consists of parallel MPPs with different properties, aiming to absorb multilinear-spectrum, high-level noise and broaden the absorption band. The MPP+Porous structure modifies the original interior panel for acoustic treatment through an installed MPP, filling the back cavity with porous acoustic materials to broaden the absorption band. The MPP+PAM combines an MPP and acoustic supermaterial of the thin plate type placed in the back cavity to suppress the noise in the middle- and low-frequency bands. This study takes helicopter cabin noise control as its research problem, focusing on noise in the middle- and high-frequency bands of the multilinear spectrum caused by gear meshing of the main gear reducer, and investigates the feasibility and effectiveness of using MPP composite sound absorption structures for noise suppression in helicopter cabins.