## 5 Discussions  
The performance of acoustic measurements upon the 1:10 scaled models was carried out under controlled conditions, typical of an anechoic chamber, to limit the effects that can potentially affect the results. However, these circumstances are considered more optimistic than real conditions, where the variance of the external gradients is dominant and hence should be taken into account during calculations.  
This paper demonstrated that the sound attenuation is variable and dependent on the number of rows composing the barrier, the diameter of the scatterers, and the distance between source and receiver from the barrier.  
The measured results highlight that the coupled acoustic barriers create a broader gap band than a barrier composed of only one size of diameter; these outcomes are in line with the principles of mitigation applied to broadband noise, like road traffic.  
A short distance between the sound source and the edge of the barrier in all the configurations becomes favorable to increase the IL in the desired frequency bands, ranging between 100 and 1 kHz by reflecting a real scale. In summary, the greater the number of rows composing the barrier, the higher the IL value shifted to high frequencies, emphasized especially when the sound source is at a 400 mm distance from the barrier.  
A common factor between the first and second sets of measurements, independently from the specific configuration, is that the sound attenuation was found mainly at 4-5 kHz, corresponding to the spikes reaching up to 19 dB. Based on the third set of measurements, the distance of the receiver from the barrier is also a variable to be considered during the design of an acoustic barrier. By considering D as the distance between the source and the barrier, the results obtained by the third set of measurements indicate that the IL values are higher when the receiver is at D/2 from the barrier, in configurations 4 and 6, at 5 kHz and 6.3 kHz. These values are found to be reduced when the receiver is at a shorter and further distance from the barrier.  
Since each environmental site is characterized by peculiarities, it would be difficult to make a standard barrier for all the outdoor contexts. As such, the height of the barrier plays an important role. Although this research study is not focused on the height variance of the barrier, we found that a metamaterial acoustic barrier having a maximum thickness of 0.80 m and height of 3.0 m (as real dimensions and scaled from a 1:10 model) can be applied along the external edges of boulevards and wide roads, as infrastructure of suburban areas characterized by a flow rating around 70-80 km/h (defined as average-speed limit), where the side properties are no more than two-story buildings. This range can be considered restricted for big cities where the blocks are dominated by skyscrapers and tall buildings; however, it can represent an applicable solution for many villages and small towns where the buildings are most likely to be less than two stories.  
In terms of structural stability, a multiple-row acoustic barrier does not require the deep foundations of opaque assemblies, which are more sensitive to wind direction and wind speed. Regarding maintenance, the dust that can accumulate in the gaps between scatterers at grade level, while not visually pleasant, does not limit the efficiency.