## 5 Conclusions  
1. The results of this study are similar to those obtained previously [10]. The FFPs affected sound pressure levels in the frequency range responsible for 80-90% of the perceived speech intelligibility. The waves of frequency below 1 kHz were not attenuated. 2. Since all the FFPs studied were built of the same synthetic nonwoven fabric, the differences in the sound suppression resulted solely from the differences in the construction of the masks. 3. The moulded masks, M1200 V, M1200 VW, and M1200 VPlus, suppressed the acoustic waves stronger than the folded ones, M1200 VP, M1200 VPW, and M1200 VB, by approximately 2-3 dB in the octave bands of centre frequency from 1 to 16 kHz. Figure 9: Disruptions on the ANSI speech spectra caused by the folded FFPs. Empty symbols—ANSI spectra, filled symbols—spectra with the FFPs corrections: \(L_{f}\) (ANSI) \(-\Delta L_{f}\). Lines are guides for the eye only. Speech levels: Nor.—normal, Ra.—raised, Lo.—loud, Sh.—shouted. 4. The exception was the moulded mask with four foldable parts for tighter fitting of the wearer's face, M1204 V. It attenuated sound similarly as the folded masks did in the octave-wide bands with centre frequency up to 2 kHz. It suppressed waves of higher frequencies stronger than the other studied FFPs did. 5. Persons using FFPs have to raise their voices by one \"loudness level\", as defined in ANSI 3.5-1997, to compensate for the loss of high-pitch tones in verbal communication. This and previous studies [10] led to similar conclusions. The patterns of the acoustic characteristics of the masks are essentially similar to one another. The tighter-fitted masks are stronger than other ones and attenuate the high-frequency waves. We have not considered the influence of contaminations on the acoustic characteristics of FFPs. This problem requires further studies. Such contaminations, e.g., particulate matter, are common in the work environment. Even if not attenuating sound, they would further impede the breathing of the mask wearer [20,21] and make verbal communication harder because louder speech requires more effort in inhaling the air. This is just the second report of the studies performed with our method. Probably, the measurement procedure could be refined and simplified. Parallel studies of speech deterioration predicted by this objective method and the changes in legibility perceived by humans would be helpful to this purpose. Disturbed verbal communication at workplaces may cause danger and even result in accidents. For this reason, occupational safety and health services should be aware of the impact of FFPs on speech intelligibility. However, this question is commonly not recognized as there are no legal regulations on this subject. Our method is sufficiently simple to be widely used in practice, contrary to the methods that require large groups of speakers and listeners. By applying it, manufacturers would collect information necessary for providing their customers with knowledge about this aspect of FFP use. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app13158644/s1](https://www.mdpi.com/article/10.3390/app13158644/s1)]([https://www.mdpi.com/article/10.3390/app13158644/s1](https://www.mdpi.com/article/10.3390/app13158644/s1)), \"FFP2_results.xlsm\". The file contains VBA code which can be misinterpreted as a virus thread by antivirus software. We suggest disregarding this and opening the file with macros. Conceptualization, K.N. and K.L. ; methodology, K.N., W.M. and K.L. ; investigation, K.N. and W.M. ; data curation, K.N. and K.L. ; writing--original draft preparation, K.N. and W.M. ; calculations, K.N., W.M. and L.A.; visualization, K.N., W.M. and L.A. All authors have read and agreed to the published version of the manuscript. Silesian University of Technology (Faculty of Materials Engineering) supported this work as a part of Statutory Research BK-218/RM1/2023 (11/010/BK_23/0045). Not applicable.