## 4 Results and Discussion  
Figure 6 shows the acoustic absorption coefficients available from literature [25; 26; 27; 28] for two thicknesses of PVP plus silica inclusions samples. The figure also includes the absorbing coefficients, obtained by in house made measurements, for a foam widely adopted as material for an aircraft seat in two different masses. Mass and thickness of each material were reported in Table 1. The diameter of samples was equal to 10 cm for all materials. The simple foam was considered as the reference material to compare with the electrospun mats in the BEM simulations. \begin{table}
\begin{tabular}{c c c c} \hline  & **Foam 6 g** & **PVP + Silica 6 g** & **PVP + Silica 24 g** \\ \hline Mass [g] & 6 & 6 & 24 \\ Thickness [mm] & 12 & 11 & 45 \\ \hline \end{tabular}
\end{table} 
Table 1: Masses and thicknesses for each considered material. Figure 5: BEM loading conditions, with highlight of spatial monopole distribution. Figure 6: Absorbing coefficients for the considered materials. It is worth noting that, in the frequency range up to 800 Hz, the acoustic absorption performance of traditional materials is generally weak, as for the \"Foam\" curve in Figure 7. On the other hand, from the literature [25; 26; 27; 28], we know that electrospun mats presents remarkable sound absorption properties in the frequency range (400-1600 Hz), as well as a bell shaped \(\alpha\) curve (Figure 6), in relation to a Helmholtz-type resonance, with a maximum (up to 0.9) (Figure 7). Such a peak of absorption shifts to lower frequencies as long as the number of piled disks increases [25; 26; 27]. Therefore, electrospun mats can also provide tunable sound absorbing properties since the absorbing peak frequency can be shifted with relatively low mass/thickness increments of the absorbing material [25; 26; 27; 28]. Namely, electrospun materials can be designed in such a way to present the highest absorption properties at the frequency of the peak acoustic loads. Moreover, when adding silica particles, self-extinguishing and water-resistant blankets are obtained, preserving the aforementioned acoustic properties [28]. The samples showed also interesting thermal conductivity properties, useful for applications were also thermal insulation is required [28]. Results in terms of SPL measured at the data recovery spherical surfaces (Figure 5) are reported in Figure 8 across the considered frequency range, for the six different configurations of headrest shape and headrest material. Moreover, the OverAll SPLs are calculated for all the configurations and reported in Figure 8. SPLs and OASPLs were directly computed by the code (see Equations (1) and (2)). \[SPL(dB)=20\log_{10}\left(\frac{p_{RMS}}{p_{ref}}\right) \tag{1}\]  
\[OASPL(dB)=10\log_{10}\left(\sum_{i=1}^{n}10^{\frac{SPL_{i}}{10}}\right) \tag{2}\]  
In Equations (1) and (2), \(p_{RMS}\) is the effective acoustic pressure amplitude calculated as spaced average on the spherical data recovery surfaces (representative of passenger's ears in Figures 3 and 4, \(p_{ref}\) is the reference pressure level equals to 20 uPa, \(n\) is the number of considered bands. The addition of lateral caps to the headrest turned out to be generally advantageous at frequencies higher than 800 Hz, whereas they were disadvantageous at lower frequencies, especially if used in combination with low absorbing materials. This can be explained considering that the lateral caps represented surfaces on which noise was scattered and directed towards the ears, so that, such enhanced reflections can only be compensated for by considering a highly absorbent covering material. In conclusion, the adoption of both strategies of headrest optimal shaping and high absorbing covering material can produce noise abatement, especially if used in combination rather than as standalone options. High absorbing materials such as the electrospun mats can be used to reduce noise also at relatively low frequencies (even down to 300-400 Hz), where the classical soundproofing materials have poor performances [25; 26; 27; 28].