# A Metawindow with Optimised Acoustic and
## 3 Results
### Broadband Potential Optimisation of the AMW Unit's Acoustic Performance  
A further broadband potential optimisation of the AMW unit's acoustic performance is investigated next. The numerical characterisation study of the AMM unit is indeed performed due to the numerical and experimental method agreement for the \(IL\) calculation \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Frequency Ranges (Hz)**} \\ \hline
300–1000 & 1000–2000 & 2000–3000 & 3000–4000 & 4000–5000 \\ \hline \(CV\) (A) & 36.34\% & 28.05\% & 39.55\% & 22.63\% & 28.62\% \\ \hline \(CV\) (B) & 89.45\% & 33.07\% & 32.97\% & 21.80\% & 39.92\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: \(CV\) value calculated between the numerical results and the experimental results of specific frequency ranges. of such an AMM window system and the need for acoustic performance improvements on the lower frequency range. The resonating volume extension through the perforation of AMM unit cell panels is investigated to reach a resonance that affects the low-frequency bandwidth wavelength. The study this time is performed within a frequency range of 10-5000 Hz. As a comparison, three models with different perforation percentages and dispositions are studied and compared to the _IL_ performance of the original frame model concerning Point A measurements. Only the inner sides of the frame's corners are modified accordingly with a 14% or 7% perforation ratio (where the percentage is calculated with the ratio between the whole inner panel area and the perforated area). As illustrated in Figure 4a, Configuration A (with 14% of perforation of the area of the overall panels) has a resonating volume of 13.85 \(\times\) 10\({}^{-4}\) m\({}^{3}\). Configuration B has 7% of perforation on the panels belonging to 2/4 of the same AMM units, while Configuration C has 7% of the overall panel area's perforation applied on one panel of each AMM unit. In these two last models (B and C), the resonating volume reaches 15.40 \(\times\) 10\({}^{-4}\) m\({}^{3}\), and the perforated panels within the AMW unit geometry are placed differently to investigate whether the perforation position is a determinant parameter for the AMW unit _IL_. Figure 4: (**a**) Sketches of the upgraded AMW unit models through the perforation of the side panels of the AMM unit frame (the position of the perforated panels is shown in the section schematic with thicker blue lines); (**b**) _IL_ numerical comparison between the original AMW unit model and the perforated ones (A–C); (**c**) contribution assessment of the flanks in combination with the resonant volume extension (through perforation). Figure 4b shows that the _IL_ performance of the acoustically optimised Configurations B and C improves from 50 to 5000 Hz, with an average increase of 11 dB from the original frame window model. This is because the resonant volume is increased significantly and can control the wavelength of lower frequency bandwidths. The _IL_ curve in Figure 4b also shows that for Configuration A there is no significant difference with the normal frame performance except for a significant average improvement of 5 dB in the frequency range of 700-1200 Hz. Consequently, such a small resonance volume addition does not improve the AMW unit performance significantly on broadband. Therefore, Configurations B and C work more efficiently when compared to either the original configuration or Configuration A, because they allow the AMM unit to have the biggest resonating volume, showing that the perforation percentage is a crucial factor for the design of such an AMM window, especially at low frequencies (50-800 Hz) where they reach an _IL_ average of 15 dB. On the other hand, it is important to note that the perforated panels' disposition does not seem crucial, meaning that the _IL_'s most influencing parameter is the additional resonant volume (and therefore the panels' perforation percentage).