## 6 Conclusions  
This paper deals with the study of sound attenuation expressed in terms of insertion loss (IL) related to different configurations of metamaterial acoustic barriers. A brief introduction is given in relation to the use of modular elements (composed of solid circled scatterers and lattice constants) employed in the field of applied acoustics and noise control. The combination of two diameters chosen for the wooden scatterers (installed for staggered diameter size) and the placement of the sound source in two positions (at 400 mm and 800 mm from the edge of the barrier) allow for a robust data analysis. The sound attenuation gap band was found between 4 kHz and 12.5 kHz, dependent on the number of rows installed to compose the acoustic barrier [32]. A common factor among the results is that the greater the number of rows composing the barrier, the higher the IL value.  
The benefit of having an acoustic barrier made of modular elements instead of an opaque surface area could represent an alternative solution to mitigate vehicular road traffic noise, without the necessity of building deep foundations. This was assessed to be applicable, especially along the sides of boulevards and wide roads, where the average speed limit is around 70-80 km/h, in the context of small towns and villages with the prevalence of two-story buildings [33]. The efficiency of a coupled acoustic barrier was found in a broader bandwidth of sound gap attenuation, reflecting the characteristics of road traffic noise.  
Further research studies will be focused on the combination of different sizes of lattice constants, as well as on different sizes of scatterer diameters. This methodology aims to enlarge the band gap, especially at low frequencies. However, it was demonstrated that the thinner diameter assigned to scatterers shifts the sound attenuation towards higher frequency bands, in accordance with [PERSON]'s theory. A larger size of scatterer diameter should be considered as more effective at low frequencies in real dimensions, which can be represented by trees.  
One of the potential applications of this study could be extended also to the filters installed for attenuating the steady noise of heating, ventilation and air conditioning (HVAC) systems [34], where the noise generated by the fan is centered upon a specific octave band (pure tone). For this scope, the model size tested would be suitable for the cabins and enclosures of the mechanical units. The application of the multiple-row acoustic barriers composed of modular elements, could find development in the field of mechanical ventilation.  
Conceptualization, G.I. and A.T.; methodology, G.I.; software, I.L.; validation, A.B.; formal analysis, A.B.; investigation, G.I.; resources, A.T.; data curation, A.B. and LL; writing--original draft preparation, G.I. and I.L.; writing--review and editing, A.B and A.T.; visualization, A.B.; supervision, G.I.; project administration, A.T.; funding acquisition, A.T. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by Ministero dell'Istruzione, dell' Universita e della Ricerca (MIUR)-PRIN 2017 Progetto Settore PE8 Codice 2017T8 SBH9_001; \"Theoretical modeling and experimental characterization of sustainable porous materials and acoustic metamaterials for noise control\".  
The authors declare no conflict of interest.