# Resonant Acoustic Metamaterials
## 4 Measured Results  
The measured _IL_ results for all test samples are reported in Figures 7 and 8. Configurations with type A scatterers represent the effect of sound attenuation obtained with plain solid scatterers, where the distance between rows is equal to 15 mm. The scatterers with a cavity, employed in all type B configurations, were analyzed in comparison with type C configurations, whereas the first row is composed of perforated scatterers (type C) and all the other rows are composed of scatterers with cavity (type B). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Configuration** & **No. of Rows** & **Distance between Row Layers (mm)** & **Scatterer Diameter (mm)** & **Scatterer Distance (mm)** \\ \hline A1 & 1 & - & 15 & 15 \\ A2 & 2 & 15 & 15 & 15 \\ A3 & 3 & 15 & 15 & 15 \\ A4 & 4 & 15 & 15 & 15 \\ B1 & 1 & - & 20 & 7 \\ B2 & 2 & 20 & 20 & 7 \\ B3 & 3 & 20 & 20 & 7 \\ B4 & 4 & 20 & 20 & 7 \\ C1 & 1 & - & 20 & 7 \\ C2 & 2 & 20 & 20 & 7 \\ C3 & 3 & 20 & 20 & 7 \\ C4 & 4 & 20 & 20 & 7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of all the configurations assumed during the acoustic measurements. Figure 6: Configuration scheme of the test samples. Figure 8: Measured results of the comparison of type B and C scatterers with a 20 mm diameter and a distance of 20 mm between rows of staggered scatterers. The continuous black line refers to type B configurations and the dotted red line refers to type C configurations. Figure 7: Measured results of type A scatterers, composed of 15 mm diameter and a distance of 15 mm between rows of staggered scatterers. The analysis of the results was conducted in the bandwidth between 1 kHz and 10 kHz. The selection of the spectrum analysis is based on the dimensions and size of scatterers; no effects of sound attenuation can be found below 1 kHz because the obstacles/scatterers are seen as acoustically transparent; consistent _IL_ results are recorded for octaves above 1 kHz. Figure 7 shows the _IL_ results from type A scatterers. Significant effects of attenuation occur when the system is composed of two or more rows of scatterers, especially at 5 kHz. This peak at 5 kHz increases by incrementing the number of rows, although above this octave band, the attenuation remains constant, approximately around 6 dB. It has been recorded that the attenuation with only one row of scatterers is negligible, giving a maximum _IL_ of 3.5 dB only at 5 kHz, while the _IL_ is equal to 1 dB on all octaves. Figure 8 shows the _IL_ results from comparisons of the configurations B and C on the same number of rows, where the scatterers are characterized by a 20 mm diameter that can be perforated (type C) or with an enclosed cavity (type B), whereas the distance between scatterers in the same x-y plane is 7 mm. By directly conducting this type of comparison, the effects of perforation on the first row of scatterers are evaluated. The holes are created with the purpose of obtaining a resonant system [35]. With configuration B1, the maximum attenuation is achieved at 5 kHz, equal to 5 dB, while with configuration C1, the maximum attenuation is shifted to 6.3 kHz, equal to 7 dB. A common factor between configurations B1 and C1 occurs across the other octaves, which is very comparable around 1 dB at frequencies below 4 kHz and has the same trendline at 10 kHz. The same shift in peak is found with configurations B2 and C2, where type B scatterers give an attenuation of 8.5 dB at 5 kHz, while type C gives an attenuation of 10 dB at 6.3 kHz. With the exception of these two peaks, the trendline between configurations B2 and C2 is very comparable. With three rows of scatterers, the maximum peaks are tuned in the same octaves, recorded at 2.5 kHz and 6.3 kHz, with a very similar trendline. The difference between maximum peaks at the aforementioned octaves is only 1 dB, which is higher with type C scatterers.