## 1 Introduction
Research studies on this topic highlight how the gap band can be shifted across the spectrum by undertaking tests on the same structure but changing the  
Figure 1: Scattering phenomenon of plane sound waves across a two-row array of cylindrical scatterers. variables as per the above discussion [10]. This outcome is mainly due to the constructive or destructive interference that varies the amplitude of the emitted soundwaves [11; 12; 13]. Among the different categories and their applicability, this paper deals with 2D sonic crystals only, composed of regular arrays (or lattice design) of solid wooden cylinders, coupled for two different sizes of diameters in multiple rows, and tested with a scaled model with the purpose of implementing the research studies on barriers for road traffic noise mitigation. Previous research studies [14] have been carried out on real, scaled models, realized by using wooden bars of a 300 mm height. The contribution of this paper is the use of different sizes of diameters, equal to 15 mm and 9 mm. The overall dimensions have been scaled by a proportion of 1:10 compared to the effective size that should be assumed. Acoustic tests have been carried out to investigate the attenuation efficiency of different configurations of multiple-row barriers by spacing the scatterers at a constant distance. The outcomes highlight that the sound attenuation is directly proportional to the increased number of rows and is more effective by coupling two sizes of diameter. Results are reported in terms of an insertion loss (_IL_) that mirrors the attenuation obtained by testing multiple-row barriers installed between the source and the receiver. Traditionally, acoustic barriers have been considered solid screens by having the function of shielding any noise source from sensitive receptors. Although the aesthetic design of the barriers has been improved in the latest decades, also given the availability of innovative and flexible materials that enhanced the sectional shape [15], the visual impact always represents a concern for urban design, aggravated further by a social aspect that considers these construction elements as the ingredients for exclusion. Nowadays, the research is aligning with different applications of acoustic barriers to be aesthetically more pleasant, even along the edges of roads. In this direction, studies focused on the absorption of low-frequency noise from outdoor sources (e.g., road traffic) have been undertaken by designing trees to be planted on a regular-grid base [16]. Investigations have been carried out for the purpose of having solid and empty entities composing the metamaterial barrier, highlighting no difference between soundwaves when interacting with a solid and opaque surface [17; 18]. As such, acoustic tests undertaken on green barriers, e.g., natural bars such as bamboo, have shown reliability on a certain frequency band of sound attenuation, given its nature to be tubular and empty in the core [19]. In this paper, the choice of undertaking experiments on plain wood bars is due to simulating tree trunks in real dimensions rather  
Figure 2: Scheme of sound control through the structure of a sonic crystal barrier. than dictated by practical reasons, including the shear cutting of tubular materials that often results in cracking and makes materials not suitable to be subject to experiments. Nonetheless, both bamboo and plain wood composed of discarded chunks are eco-friendly. This paper aims to contribute to understanding the behavior of multiple-row and coupled acoustic barriers using different applications. In this specific case, the wooden bars are organized on a surface area of 9 m\({}^{2}\) (a sample of 3 \(\times\) 3 m), which highlights the strengths and weaknesses of the selected lattice design. Along with other research studies mainly focused on the road traffic abatement (having a broad band noise centered on 1 kHz) [20; 21; 22], the proposed acoustic barrier has a high absorption, in the range between 800 Hz and 1250 Hz.