## 1 Introduction  
Noise is considered an environmental and social problem arising from consumption patterns and lifestyles in the accelerated urbanization process, which is generated by machines, railways, airplanes, loudspeakers, and so on [1]. Increasing noise has been treated as one of the four major environmental pollutants [2]. In particular, it has been reported that low frequency noise could result in hearing loss, headaches, sleep disturbance, inattention, and so on, and is therefore seen as one of the reasons for deterioration in people's quality of life [3]. Thus, methods and practices to reduce low frequency environmental noise are considered as one of the research focuses in the field of acoustical environment protection [4; 5; 6]. Values of sound absorption coefficients in certain frequency ranges are the major index to evaluate the performance of a sound absorber with a limited total thickness [7]. Meanwhile, considering the huge application prospects, a sound absorber reducing low frequency noise must not only have the advantage of a low manufacturing cost, good machinability, excellent fire resistance, and environmental friendliness, but also be easy to fabricate and install. Many sound absorbers have been developed for low frequency noise reduction [8; 9; 10; 11; 12]. [PERSON] et al. [8] modified a microperforated panel by using a mechanical impedance plate, and the low frequency absorption was effectively enhanced without the need to increase the total thickness of the absorber. [PERSON] et al. [9] proposed ultrathin low frequency sound absorbing panels based on coplanar spiral tubes or coplanar Helmholtz resonators, and the efficacy of sound absorption by these panels was validated by the strong agreement between the theoretical analysis and experimental measurement. Perfect absorption of low frequency sound by the critically coupled subwavelength resonant system was proposed and developed by [PERSON] et al. [10], and a highly efficient (\(>80\%\)) low frequency broadband absorption was achieved in the frequency range of 99.1-294.8 Hz. A review of sound absorption structures ranging from conventional materials with a porous microstructure to acoustic metamaterials was conducted by [PERSON] and [PERSON] [11], who found that with a sound absorber with a single sound absorbing structure, it was difficult to obtain a satisfactory sound absorption performance in a wide frequency range. Meanwhile, although the acoustic metasurface or metamaterial could achieve a high sound absorption coefficient in the low frequency range [4; 6; 12], its complex structure and exquisite design limited its practical application in large-scale products. Porous metal and microperforated metal panels are two common materials for sound absorption [13], and both have the advantage of a low fabrication cost, good loading capacity, high mechanical strength, and so on [14; 15; 16], which make the large-scale manufacture and application of sound absorbers made with these two materials feasible. [PERSON] and [PERSON] [17] studied the physical mechanisms involved in the sound attenuation and absorption of a microperforated panel backed by anisotropic fibrous material, which provided guidelines for further parametric or impedance optimization research. [PERSON] [18] investigated the sound absorption performance of porous material with perforated facing at high sound pressure levels, and a semi-empirical model was proposed. [PERSON] et al. [19] attempted to enhance low- to mid-frequency sound absorption using parallel-arranged perforated plates with extended tubes and porous material, which had been proven effective in the 100-1600 Hz range. The sound absorption coefficient of a microperforated panel backed by porous sound-absorbing material was investigated by [PERSON] et al. [20], and a novel approach named 3D printing technology was proposed to fabricate the sound absorbers for acoustic application.