## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c c} \hline \hline  & & \multicolumn{10}{c}{**125 Hz-10 kHz**} \\ \cline{2-13}  & & \multicolumn{3}{c}{average} & \multicolumn{3}{c}{surgical} & \multicolumn{3}{c}{surgical + BR} & \multicolumn{3}{c}{2-ply cloth} & \multicolumn{3}{c}{2-ply cloth + BR} & \multicolumn{3}{c}{FFP2} & \multicolumn{3}{c}{FFP2 + BR} & \multicolumn{3}{c}{thin shield} & \multicolumn{3}{c}{thick shield} \\ \cline{2-13}  & & \(\Delta l_{p}\) & STD & \(\Delta l_{p}\) & STD  
Figure 5: _Cont_. Figure 5: _Cont._  
No significant difference between the speech with and without sibilants was found; however, the standard deviation in cases with sibilants is much larger (Figure 5). This means that the speech intelligibility of people wearing face masks is highly individual in Slavic languages. In both cases (with and without sibilants), the impact of mouth masks above 3000 Hz is quasi constant for all high frequencies, with cases with a bracket even slightly dropping to around 6-8 kHz. In experiments with people wearing plastic shields, a cavity resonance of around 800 Hz was confirmed (the same for both shields) for both cases (with/without sibilants). The agreement between the artificial head-based results and people is better in cases without a bracket. In cases with a bracket, the effect measured by the artificial mouth was overestimated, mostly at high frequencies above ca. 5 kHz. One of the addressed research questions was to understand whether people increase their vocal intensity when wearing mouth-face protection. This was checked by means of accelerometer. Figure 6 shows the results from accelerometer measurements (in terms of measured velocity) performed on two male and one female. The differences between individuals are relatively large, and three types of behavior can be seen. However, an increase in the fundamental frequency can be seen in all cases. In the case of female speech, this had a much stronger effect. In all cases, people's vocal output was lower when wearing face shields in comparison with either cloth or fabric mouth mask protection. The results indicated that the voice level and also the voice increase due to mouth protection is to some extent individual. Therefore, in addition to the presented data, the effect of sound level normalization was also investigated for a few people. Figure 7 shows an example of the effect of the mask: 2-ply cloth + BR. The presented data were normalized according to measurements of vibration by an accelerometer attached to the person reading the text. The influence of the normalization of recordings is visible mostly in a frequency range between 200 and 2500 Hz. This range is a substantial part of the human speech frequency spectrum [28]. It is clear that more research should be conducted by means of listening tests involving the perceptual assessment of speech intelligibility with and without a mask. Using only the objective spectral filtering of sound, without taking into account phenomena such as articulation and visual feedback, might not be sufficient when real speech intelligibility needs to be assessed. Figure 5: Effect of different face protections on sound pressure level measured at listener position for text with sibilants (**right**) and without sibilants (**left**) used as sound stimuli. Difference in influence on male and female voice compared to artificial mouth. Light grey bars represent results for male talkers, and dark grey bars represent results for female talkers. The difference observed in the case of the artificial head is shown with a solid black line. Figure 6: Results from accelerometer measurements of individual chosen persons. Top and middle pictures are from male subject and the bottom picture from a female. Results are the relative values of the sound pressure level derived from surface oscillations based on the theory of propagation of plane acoustic waves.