## 4 Discussion  
The attenuation spectra in two FFPs pairs are similar to one another: M1200 V to M1200 VW and M1200 VP to M1200 VPW, cf. Figures 1 and 2. This result evidences that modification of the filter for protection against unpleasant odours does not change the sound attenuation characteristics of the FFP. Euclidean distances \(d\) (Equation (5)) between the sets of \(\Delta L_{f}\) values quantitatively describe differences between the FFPs studied. They are reported in Table 6. The hierarchical tree plotted in Figure 4 illustrates the relative distances between the datasets. Three moulded masks, M1200 V, M1200 VW, and M1200 VPlus, make one group, while three folded ones: M1200 VP, M1200 VPW (both folded horizontally), and M1200 VB (folded vertically), make another. The moulded M1204 V stands apart from the two groups probably because of different construction. It has four foldable parts providing better fit on faces of various sizes or shapes. Figure 3: A hierarchical diagram of relative distances between ten \(\Delta L_{f}\) datasets for five masks of M1200 VPlus type. The single linkage method (“nearest neighbour”) was the amalgamation rule. The datasets are identified by the names #i-j, where \(i\) and \(j\) denote mask and measurement, respectively. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Dataset** & **\#1-1** & **\#1-2** & **\#2-1** & **\#2-2** & **\#3-1** & **\#3-2** & **\#4-1** & **\#4-2** & **\#5-1** & **\#5-2** \\ \hline \#1-1 & & 7.93 & 4.24 & 5.34 & 5.82 & 5.70 & 4.69 & 5.23 & **4.23** & 6.3 \\ \#1-2 & 7.93 & & 7.79 & 5.75 & 6.19 & 6.43 & 4.59 & 4.28 & 6.33 & **4.21** \\ \#2-1 & 4.24 & 7.79 & & **3.21** & 5.70 & 4.24 & 6.24 & 6.23 & 5.61 & 6.85 \\ \#2-2 & 5.34 & 5.75 & **3.21** & & 4.27 & **3.14** & 5.22 & 4.96 & 5.43 & 5.23 \\ \#3-1 & 5.82 & 6.19 & 5.70 & 4.27 & & **3.83** & 4.66 & 4.94 & 5.60 & 5.10 \\ \#3-2 & 5.70 & 6.43 & 4.24 & **3.14** & **3.83** & & 5.13 & 5.59 & 5.94 & 5.46 \\ \#4-1 & 4.69 & 4.59 & 6.24 & 5.22 & 4.66 & 5.13 & & **1.77** & 3.53 & 3.16 \\ \#4-2 & 5.23 & 4.28 & 6.23 & 4.96 & 4.94 & 5.59 & **1.77** & & **3.04** & **2.76** \\ \#5-1 & **4.23** & 6.33 & 5.61 & 5.43 & 5.60 & 5.94 & 3.53 & **3.04** & & 3.99 \\ \#5-2 & 6.30 & **4.21** & 6.85 & 5.23 & 5.10 & 5.46 & 3.16 & **2.76** & 3.99 & \\ \hline \end{tabular}
\end{table} 
Table 5: The Euclidean distances \(d\), in dB, between the \(\Delta L_{f}\) datasets for five masks of M1200 VPlus type. Numbers in bold print denote the shortest distance for at least one set in the pair; underlining marks the shortest distance for both sets in the pair. The datasets are identified by #i-j, where \(i\) and \(j\) denote mask and measurement, respectively. In the previous paper [10], we reported speech intelligibility disrupted by FFPs. In this study, we applied the same approach based on perceived speech intelligibility. According to [PERSON] and [PERSON] [17], sound waves of frequency ranging from 250 to 7000 Hz transmit all the information necessary for the understanding of speech, at least in non-tonal languages. Six octave-wide bands with centre frequencies of 0.250, 0.5, 1, 2, 4, and 8 kHz fully encompass this range. Three moulded masks without elements for tighter fitting the wearer's face suppress sound waves belonging to the octaves with a centre frequency of 1 kHz and higher. The frequency limit of suppression is 2 kHz for other FFPs. Figure 5 illustrates the differences between the masks. The band with a centre frequency of 16 kHz is not important for the understanding of speech. Thus, we compared the attenuations in four octave-wide bands with centre frequencies 1, 2, 4, and 8 kHz. The Euclidean distances between the sets of \(\Delta L_{f}\) values in octave-wide bands are reported in Table 7. The hierarchical diagram plotted in Figure 6 shows that the relative distances between the datasets within the two groups of FFPs are even shorter than those between the sets of \(\Delta L_{f}\) values in 1/3 octave-wide bands. In general, the two analyses led to the same conclusion about the FFPs' similarity.