## 1 Introduction  
The noises generated by large mechanical and electrical apparatuses are major component of industrial noise, which not only impair health of the operators, but also are detrimental to the residents living around the workshops [1]. In addition to traditional control strategies [2; 3], such as the application of equipment with less noise, reasonable layout of the workshops, appropriate planting of vegetation, and wearing of personal protective appliance, installation of a proper acoustic absorber around the noise source or on the wall of the workshop is an effective method to reduce reflecting superposition of the noise [4]. This is favorable to effectively decrease the noise inside and outside the workshop, therefore the development of a novel acoustic absorber is one of the research focuses in the field of industrial noise reduction. Many sound absorbing materials and structures have been developed to reduce industrial noise. [PERSON] [5; 6] proposed the classical microperforated panel and proved it an effective and simple method to reduce the industrial noise within the low-and-medium frequency range. [PERSON] et al. [7] further developed the novel microperforated panel with arbitrary cross-sectional perforations, and the research products indicated that changing cross-sectional roughness of the perforations was an efficient method to improve its sound absorption performance. [PERSON] et al. [8] pointed out that porous metal could achieve a high sound absorption coefficient in the medium-and-high frequency range, and [PERSON] et al. [9] further improved sound absorption efficiency of the porous metal through the compression process and quantificationally investigated the effect of the compression ratio. In addition to these traditional sound absorbing materials and structures, some novel acoustic meta-structures and metamaterials are also proposed [10; 11; 12; 13; 14]. [PERSON] et al. [10] has developed a 0.15\(\lambda\)-thick, 15%-filling ratio ultra-sparse meta-surface based on artificial Mie resonance. With careful design of the coupled resonators by [PERSON] et al. [11; 12], over 99% energy absorption at the central frequency of 511 Hz with a 50% absorption bandwidth of 140 Hz was achieved. An acoustic super-lens using single-phase metamaterials with star-shaped structure were proposed by [PERSON] et al. [13; 14], which could obtain band gap and double-negative properties. However, fabrication costs of the acoustic metamaterials are high in general and the structures of them are usually complicated at present, which limit their practical applications. Meanwhile, each single traditional sound absorber has its weakness, such as finite absorption band, excessive occupied space, and so on. Thus, the composite structure consisting of a single sound absorber is considered as the development direction of novel acoustic absorbers. Sound absorption performances of flax fiber and its reinforced composite, as well as balsa wood, were evaluated by [PERSON] et al. [15] and [PERSON] et al. [16] respectively, and it showed that the natural-materials-based sandwich structure has a high sound absorption coefficient within the high frequency range. [PERSON] et al. [17; 18] proposed a composite sound absorption structure, and the composite helical-shaped sound absorber coated with the carbon fiber exhibits excellent sound absorption property, especially within the low frequency range. Barium titanate/nitrile butadiene rubber (BT/NBR) and polyurethane (PU) foam were combined to prepare sound absorbing material with an alternating multilayered structure by [PERSON] et al. [19; 20; 21], and it has excellent sound absorption performance within the low frequency range due to the organic combinations of airflow resistivity, resonance absorption, and interface dissipation. [PERSON] et al.