## 4 Discussion
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**FFP** & **M1204 V** & **M1200 VP** & **M1200 VPW** & **M1200 VB** & **M1200 VB** & **M1200 V** & **M1200 VW** \\ \hline M1204 V & & 16.77 & 15.83 & **14.27** & 18.21 & 15.77 & 16.70 \\ M1200 VP & 16.77 & & **3.01** & 6.07 & 11.75 & 10.87 & 11.91 \\ M1200 VPW & 15.83 & **3.01** & & **5.80** & 10.34 & 9.27 & 10.16 \\ M1200 VB & **14.27** & 6.07 & **5.80** & & 12.02 & 9.80 & 10.07 \\ M1200 VPlus & 18.21 & 11.75 & 10.34 & 12.02 & & 6.16 & **5.85** \\ M1200 V & 15.77 & 10.87 & 9.27 & 9.80 & 6.16 & & **3.39** \\ M1200 VW & 16.70 & 11.91 & 10.16 & 10.07 & **5.85** & **3.39** & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The Euclidean distances \(d\), in dB, between the \(\Delta L_{f}\) datasets for seven FFP types. \(\Delta L_{f}\) values in fifteen 1/3 octave-wide bands with centre frequencies 800–20 kHz. 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. Figure 4: A hierarchical diagram of relative distances between \(\Delta L_{f}\) datasets for seven types of the FFPs studied. Each dataset contained fifteen \(\Delta L_{f}\) values for the 1/3 octave-wide bands with centre frequencies from 800 Hz to 20 kHz. The single linkage method (“nearest neighbour”) was the amalgamation rule. Figure 5: Attenuation of sound by the seven studied FFPs in octave-wide frequency bands. Error bars are omitted for the picture clarity; uncertainty is roughly \(\pm\)1.5 dB. Lines are guides for the eye only. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**FFP** & **M1204 V** & **M1200 VP** & **M1200 VPW** & **M1200 VB** & **M1200 VB** & **M1200 V** & **M1200 VB** \\ \hline M1204 V & & 8.13 & 7.33 & **7.13** & 7.92 & 8.40 & 8.81 \\ M1200 VP & 8.13 & & **1.38** & 2.39 & 4.77 & 4.87 & 5.78 \\ M1200 VPW & 7.33 & **1.38** & & **2.22** & 3.98 & 4.18 & 5.05 \\ M1200 VB & **7.13** & 2.39 & **2.22** & & 3.42 & 3.39 & 4.30 \\ M1200 VIPlus & 7.92 & 4.77 & 3.98 & 3.42 & & **0.83** & 1.30 \\ M1200 V & 8.40 & 4.87 & 4.18 & 3.39 & **0.83** & & **0.93** \\ M1200 VW & 8.81 & 5.78 & 5.05 & 4.30 & 1.30 & **0.93** & \\ \hline \end{tabular}
\end{table} 
Table 7: The Euclidean distances \(d\), in dB, between the \(\Delta L_{f}\) datasets for seven FFP types. \(\Delta L_{f}\) values in four octave-wide bands with centre frequencies 1, 2, 4, and 8 kHz. 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. Figure 6: A hierarchical diagram of relative distances between \(\Delta L_{f}\) datasets for seven types of the FFPs studied. Each dataset contained four \(\Delta L_{f}\) values for the octave-wide bands with centre frequencies 1, 2, 4, and 8 kHz. The single linkage method (“nearest neighbour”) was the amalgamation rule. The moulded FFPs attenuate sound stronger than the folded ones in the whole frequency range (Figure 5). Contrary to other FFPs, they suppress waves in the octave-wide band with a centre frequency of 1 kHz. It should be noted that frequencies above 1 kHz are essential for vowels articulation [17], thus for the understanding of human speech. The difference reaches ca. 3 dB for the octave-wide band with a centre frequency of 2 kHz. [PERSON] and [PERSON] [18] estimated that this frequency band carries about 28% of the articulation index; thus, it is essential for the understanding of speech. For other frequency bands, the values are 18, 30, and 13% for 1 kHz, 4 kHz, and 8 kHz, respectively. The percentages were calculated from the numbers reported in Table III of [PERSON] and [PERSON]'s paper [18]. They slightly differ from those reported earlier [10] because of the improved calculation method. The FFP equipped with four foldable parts for better fit, M1204 V, significantly attenuates sound waves of higher frequency.