## 3 Results and Discussion  
The airflow resistivity values experimentally obtained using the alternate airflow meter for all considered densities are shown in Table 1. The lowest value of the flow resistivity, \(\sigma\) = 3800 \(\mathrm{Ns/m^{4}}\), was obtained for specimens with a density \(\rho\) = 40 \(\mathrm{kg/m^{3}}\), while the highest value, \(\sigma\) = 10,200 \(\mathrm{Ns/m^{4}}\), was obtained for specimens with a density \(\rho\) = 80 \(\mathrm{kg/m^{3}}\). Airflow resistivity is one of the 5 parameters needed for a simulation of the porous layer. The curves of the measured sound absorption coefficients in the impedance tube, obtained as an average of three measurements, are depicted in Figure 6. When the thickness was increased, there was very good improvement in the low to middle frequencies. This was due to the shift of the first peak towards lower frequencies. Unfortunately, the drawback of this phenomenon was a reduction in the sound absorption coefficient for some of the higher frequencies. Thus, the sound absorption coefficients for the RPUF with density \(\rho\) = 40 \(\mathrm{kg/m^{3}}\) and thickness \(\mathrm{h}\) = 4.5 cm were higher than those of samples with \(\mathrm{h}\) = 7 cm and \(\mathrm{h}\) = 9.5 cm in the region 1515-1730 Hz. This occurred in a wider frequency region (1295-1740 Hz) for the RPUF with \(\rho\) = 55 \(\mathrm{kg/m^{3}}\) and was even more pronounced (1030-1650 Hz) for the RPUF with \(\rho\) = 80 \(\mathrm{kg/m^{3}}\). Although there was a decrease in the sound absorption coefficient in these regions, the values remained above 0.8, which represented good sound absorption. Additionally, this effect will be less pronounced for a diffuse sound field. One way to improve the sound absorption coefficient is by creating a layered composite acoustic absorber that can be optimized in both the low and high frequency regions [37]. For the frequency of 500 Hz, considering the RPUF with density \(\rho\) = 40 \(\mathrm{kg/m^{3}}\), the sound absorption coefficients were 0.33, 0.60, and 0.86 for thicknesses \(\mathrm{h}\) = 4.5 cm, \(\mathrm{h}\) = 7 cm, and \(\mathrm{h}\) = 9.5 cm, respectfully. The sound absorption coefficients for the RPUF with density \begin{table}
\begin{tabular}{c c} \hline \hline
**Density [kg/m\({}^{3}\)]** & **Airflow Resistivity [Ns/m\({}^{4}\)]** \\ \hline \(40\pm 10\)\% & 3800 \\ \(55\pm 10\)\% & 5000 \\ \(80\pm 10\)\% & 10,200 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimentally obtained values for the flow resistivity of the considered RPUFs. Figure 6: The measured sound absorption coefficients (average of 3 measurements). \(\rho\) = 55 kg/m\({}^{3}\) were 0.42, 0.74, and 0.91 and the sound absorption coefficients were 0.50, 0.83, and 0.96 for the RPUF with density \(\rho\) = 80 kg/m\({}^{3}\) for thicknesses h = 4.5 cm, h = 7 cm, and h = 9.5 cm, respectfully. In other words, for all specimens with thickness h = 9.5 cm, for f = 500 Hz, a sound absorption coefficient above 0.8 was achieved, as well as for the RPUF with density \(\rho\) = 80 kg/m\({}^{3}\) and thickness h = 7 cm. For the RPUF with density \(\rho\) = 40 kg/m\({}^{3}\), a sound absorption coefficient above 0.8 was achieved after the frequencies 410 Hz, 740 Hz, and 1330 Hz for thicknesses h = 9.5 cm, h = 7.0 cm, and h = 4.5 cm, respectfully. Similarly, for the RPUF with density \(\rho\) = 55 kg/m\({}^{3}\), a sound absorption coefficient above 0.8 was achieved after the frequencies 380 Hz, 575 Hz, and 910 Hz and after the frequencies 310 Hz, 470 Hz, and 820 Hz for the RPUF with density \(\rho\) = 80 kg/m\({}^{3}\). For higher frequencies than those stated, the sound absorption coefficients ranged between 0.8 and 1. The efficiency of the specimen with h = 95 mm and \(\rho\) = 80 kg/m\({}^{3}\) stood out in particular, since its sound absorption coefficient for 250 Hz was near 0.6 and all of the sound absorption coefficients were in the range 0.85-1 after 325 Hz.