## 3 Results and Discussion  
First, the measurements without face protection were performed, in order to obtain the absolute voice spectra of the different speakers that participated in the experiment. The average results for the two kinds of texts, i.e., with/without sibilants, are shown in Figure 3. The figure shows the sound pressure level of voice recordings in the third octave bands, distinguishing male (light-grey bars) and female (dark-grey bars) speakers. Figures show recorded speakers speaking freely, without the use of face protection. The two graphs in Figure 3 also include the standard deviation per third octave band between measured data on people, together with a result measured by means of artificial mouth (solid line). Note that theoretically, this would only be a continuous horizontal line, since the produced sound signal was pink noise. In practice, the result is influenced (as shown in Figure 3) by the loudspeaker placement in the opening and its spectral and directivity characteristics. In our experiment, the perfectly flat response of the artificial head is, however, not an issue, as we are interested in relative differences of sound pressure level in the situations with and without mask protection. The dashed line in Figure 3 represents the background noise level in the laboratory during the recording sessions. There is a visible difference in the voice spectrum of text including sibilants (right) and without sibilants (left): their presence is mainly pronounced at high frequencies above 3150 Hz. The overall difference between male and female voices is confirmed at low frequencies in the case of the speech including sibilants. For this reason, further analysis was also performed separately for men and women. Figure 4 and Table 2 summarize the data measured on the artificial head, showing the general effect of various face masks on the sound pressure level spectrum in the third octave bands. There is no effect of the mouth mask at frequencies below 630 Hz. For plastic shields, there is no effect below 400 Hz. Effects are most pronounced above 2000 Hz. Figure 3: Mean sound pressure level spectrum of measured male and female voices without wearing face protection for text without sibilants (**left**) and with sibilants (**right**) compared with the one of recorded pink noise emitted by the artificial head, and with the background noise. There is also a clear influence of the two face shields (thick solid line and thick dashed line), resulting in an increase in the sound pressure level of around 800 Hz. This is consistent with the data measured by [PERSON], 2020 [18]. This effect is a consequence of cavity resonance between the face and the shield. Table 2 provides a detailed overview and comparison between all investigated scenarios. It highlights the effect of face protection at different chosen frequency ranges. Figure 5 shows detailed results for all investigated face coverings. The impact of wearing a mouth mask is shown in the frequency domain. It is expressed by differences in the sound pressure level without and with mouth protection in each third octave band. Results measured from people were compared with data obtained from the artificial mouth experiment. The presented results are the average values of all participants (for each particular mask), structured in two columns, the recited text containing sibilants and the text without sibilants, respectively, in the left and right column. A distinction is made between male (light-grey bars) and female readers (dark-grey bars). The results are accompanied by the spectra obtained by transmitting pink noise from the artificial head (solid line). Figure 4: Effect of different face protections on sound pressure level spectrum measured at the listeners position, for an artificial mouth sound source (\(\Delta L_{\text{p}}=L_{\text{p,mask}}-L_{\text{p,no mask}}\)).