## 3 Results and Discussion
### Transmission Loss
The effect of sound transmission through the gaps or apertures was investigated theoretically [52; 53; 54; 55]. Leaks introduced through the slits can significantly reduce the sound insulation properties of the panel or enclosures. In our studies, a decreased TL spectrum in the frequencies was observed for GW300. This result reveals that the drop in the TL values might be caused by setting an air space along the gap which exists between the panels and depends on the size of the gaps. However, increasing panel mass lowers resonance frequencies and raises the critical frequencies. For the studied panels, as an orthotropic panel, two critical frequencies were calculated, according to [PERSON], and shown in Table 2.  
For these types of panels, the critical frequency depends on the direction of the incident acoustic wave. The range of critical frequencies is bounded at the lower end by the critical frequency corresponding to a wave travelling in the panel's most stiff direction and at the upper end by the critical frequency corresponding to a wave propagating in the least stiff direction. Figure 6: Measured (experimental) and predicted (theory) transmission loss (TL) for GW270 and GW300 PVC panels. Figure 7: Cont. Figure 8: _Cont._  
Figure 7: Measured (experimental) and predicted (theory) transmission loss (TL) for GW580 and GW700 PVC panels. Figure 8: Measured (experimental) and predicted (theory) transmission loss (TL) for GW610 and GW458 PVC panels. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Frequency Hz**} & \multicolumn{2}{c}{**GW270**} & \multicolumn{2}{c}{**GW300**} \\ \cline{2-5}  & **Calculated** & **Experimental** & **Calculated** & **Experimental** \\ \cline{2-5}  & **dB** & **  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**GW580**} & \multicolumn{2}{c}{**GW700**} \\ \cline{2-5}
**Frequency Hz** & \multicolumn{1}{c}{**Calculated**} & \multicolumn{1}{c}{**Experimental**} & \multicolumn{1}{c}{**Calculated**} & \multicolumn{1}{c}{**Experimental**} \\  
According to [PERSON] and [PERSON] (2009), there are two important points worth noting when using prediction schemes for orthotropic panels. For small panels, the transmission loss of \(0.7f_{\text{-}1}\) is underestimated, the error becoming larger as the frequency becomes lower or the panel becomes smaller. For common fluted or corrugated panels, there is nearly always a frequency between 2000 and 4000 Hz where there is a dip of up to 5 dB in the experimental transmission loss spectrum, which is not predicted by theory. This corresponds either to an air resonance between the corrugations or a mechanical resonance of the flat panel between the ribs. The orthotropic plate theory predicts a shift of critical frequency (especially \(f_{\text{-}2}\)), which would make the TL below the critical frequency higher. This was not observed in the experimental data. The existence of a critical frequency range has an impact on the transmission loss of orthotropic panels. In this case, the coincidence region may extend over two decades for common fluted panels. Therefore, these types of panels should be avoided where noise control is crucial. However, it may be expected that the damping of these panels can improve by  
Figure 5:0 The measured absorption coefficient of studied PVC panels. The studied panels have enough acoustic absorption property in selected frequencies. If the material sound absorption coefficient is above 0.4, it is considered to have good noise absorbing properties. For the studied panel, some dependencies between the absorption coefficient and their construction were found. It can be assumed that the shape of the panel matters here. A similar spectrum for GW700, GW610, and GW580 was observed, with \(\alpha\approx 0.4/0.45\) at 125 Hz. For the GW610 panel, the second peak was observed at 200 Hz (\(\alpha\approx 0.3\)), similar to GW710 (\(\alpha\approx 0.25\) at 315 Hz). An increase in the absorption coefficient was observed between 2000-8000 Hz for the GW710 panel.