# A Novel Acoustic Ventilation Barrier Based on the Block-Shaped Space-Coiling Metamaterial
## 1 Introduction  
With the rapid development of human living standards, noise reduction schemes are now of crucial significance in creating a more comfortable environment for building residents [1, 2, 3]. One typical solution for noise reduction is the use of conventional acoustic barriers, which can be classified into acoustic composite panels and sonic crystals [4, 5, 6]. Acoustic composite panels are usually cheap, soft, and opaque and lack ventilation properties [5], while sonic crystals are usually partially optically transparent, helping them provide ventilation [7]. Nevertheless, conventional acoustic barriers are often heavy and bulky, and their use is usually limited to low frequency ranges [8]. Existing research shows that, to achieve satisfactory noise reduction performances (e.g., transmission loss (TL) over at least 10 dB for the frequency range of 500-1000 Hz), the thickness of an acoustic barrier should reach at least 600 mm [9]. Evidently, such schemes are too cumbersome to be practically applicable. The reason for these limitations lies in the fact that conventional acoustic barriers rely on traditional materials, such as solid, perforated, or porous rods, which lack fine manipulations of sound wave propagations. With the aim of breaking the limitations of conventional acoustic barriers, a variety of acoustic barriers made of acoustic metamaterials have come into being in the past three decades [10; 11; 12; 13]. Among these new acoustic metamaterials, space-coiling metamaterials can provide a desirable noise reduction effect with an extremely small thickness (e.g., less than 100 mm) [14]. Space-coiling metamaterials are constructed by spatially coiling up multiple acoustic channels, forming sub-wavelength cross sections [15]. Furthermore, space-coiling metamaterials have been reported to exhibit Fano-like resonance [16], achieving amazing sound reduction effects by triggering extreme acoustic behaviors such as an extreme refractive index [17], a double-negative media phenomenon [18], phase/amplitude modulation [18], etc. Though Fano-like acoustic resonances have been numerically and experimentally demonstrated through profiles of TL, more comprehensive analyses specifying the mechanism of sound interference are yet to be established [19]. Using Fano-like resonances, a high TL is usually achieved within a narrow frequency band [20; 21; 22]. Exploiting multiple Fano resonances can achieve a wider band (e.g., as high as 1250 Hz), which is usually obtained at the sacrifice of a much larger thickness (about 460 mm) [23]. Recently, a cylindrical space-coiling meta-unit was proposed with a cylindrical meta-unit that contains helix acoustic channels and a circular hole at the center, enabling both desirable noise reduction properties and sufficient ventilation (shown in Figure 1a) [20]. By modifying the above-mentioned cylindrical space-coiling meta-unit, a modified cylindrical space-coiling meta-unit with ultra-broadband (500-2000 Hz) TL performance has been proposed, which cooperates not only with fundamental but also with higher-order Fano resonances (shown in Figure 1b) [24]. On the other hand, a customizable cylindrical space-coiling meta-unit with intelligent sound insulation was proposed, which is able to adjust its working frequency band according to needs (shown in Figure 1c) [25]. However, the above-mentioned cylindrical space-coiling meta-units are all curve-shaped, whose practicalities are weakened by providing difficulties to form a wall-like acoustic barrier. Recently, a rectangular column-shaped acoustic barrier with a maze-like cross-section has been proposed, which provides good TL over a broad frequency range (600-1100 Hz) as well as desirable ventilation (shown in Figure 1d) [26]. Nevertheless, it is hard work to build such a slim column with intricate details inside, and mechanical stability can be a problem. In addition, a block-shaped meta-unit has been proposed, whose the cross section consists of four planar rectangular maze-like acoustic channels (shown in Figure 1e) [27].