Construction of Half Masks for the Respiratory Tract Protection and the Speech Intelligibility Assessed from the Measured Suppression of Sound  
[PERSON]  
1 Department of Production Engineering, Faculty of Materials Engineering, Silesian University of Technology, Akademicka 2A, 44-100 Gliwice, Poland; [EMAIL_ADDRESS]  
2 Faculty of Science and Technology, Jan Dlugosz University, Al. Armii Krajowe 13/15, 24-200 Crestchowa, Poland; [EMAIL_ADDRESS]  
3 Institute for Energy Security and Environmental Safety, Centre for Energy Research, Konkoly-Thege Miklos ut 29-33, 1121 Budapest, Hungary; almasy.laszlo@bek-cerhu  
Correspondence: [EMAIL_ADDRESS]; Tel: +48-32-603-44-12  
[PERSON]  
2 Faculty of Science and Technology, Jan Dlugosz University, Al. Armii Krajowe 13/15, 24-200 Crestchowa, Poland; [EMAIL_ADDRESS]  
3 Institute for Energy Security and Environmental Safety, Centre for Energy Research, Konkoly-Thege Miklos ut 29-33, 1121 Budapest, Hungary; almasy.laszlo@bek-cerhu  
3 Institute for Energy Security and Environmental Safety, Centre for Energy Research, Konkoly-The  
The COVID-19 pandemic regulations introduced masks as mandatory protective equipment in public spaces. The wider use caused broader interest in mask-wearing comfort, including the so far overlooked problem of speech suppression. Still, the studies dealt mainly with subjective assessments of the speech deterioration caused by the masks [5; 6; 7; 8; 9]. Indeed, the legibility perceived by humans is the ultimate test of speech disruption. However, large groups of speakers and listeners must be involved in such experiments to keep the uncertainty of the results within reasonable limits. Another problem is the repeatability of the results obtained in this manner. Objective methods based solely on acoustic measurements rather than on human perception seem to be a good alternative. To the best of our knowledge, the first objective method was suggested in our previous paper [10]. We have not found information about similar ideas in the literature. For this reason, the basics of the method and the validation procedure are summarised below for the readers' convenience. The approach relied on the measurements of white noise attenuation by FFPs. The measured attenuation of sound in the octave-wide frequency bands, \(\Delta L_{f}\), constituted an \"acoustic characteristic\" of the mask. With the acoustic characteristics, one could easily predict the speech disruption caused by the FFP just by subtracting the \(\Delta L_{f}\) values from the acoustic spectrum of the speech, e.g., that defined in the ANSI 3.5-1997 standard [11]. We verified the method using a speech signal played by a calibrated acoustic source. Two time series of sound pressure levels (SPL) were compared for each FFP. The first series contained the SPLs recorded for the source speaker covered with the mask. The data in the second series were differences between the SPLs recorded for the uncovered speaker and the respective \(\Delta L_{f}\) values. We observed almost perfect agreement for all tested FFPs. Thus, the method ensured objective characteristics of speech suppression by the masks. The studies [10] led to several conclusions of practical importance. All tested half-masks attenuated acoustic waves in the frequency range responsible for 80% of perceived speech intelligibility, especially in the octave band with the centre frequency of 2 KHz. Masks of higher filtration efficiency stronger attenuated the sound. Moreover, FFPs changed the voice timbre because the attenuation depended on the sound wave frequency. In the present paper, we report the acoustic characteristics of seven FFPs. All of them belonged to the FFP2 type and were made of the same materials by one manufacturer but differed in construction. We studied how these differences influenced the suppression of sound and, consequently, disruption of the standardized speech spectra. The paper has a structure typical of such works. The research material, measuring apparatus, and methodology were described in Section 2.