## 3 Results and Discussion
### Listening
The difference of 5.1 dB is statistically significant and suggests that the listeners are prone to setting a significantly higher level of speech when listening to it on headphones, in comparison with listening on loudspeakers. To further analyze this finding, the difference between the A-weighted levels of speech listened over headphones and loudspeakers was calculated for every pair of corresponding observations (the same visual stimulus and the same voice, for each listener). Thus, a total of 420 pairs of observations were obtained, and the same number of difference values. These values are shown in Figure 2 as a histogram. The histogram in Figure 2 shows the distribution of the difference peaking at 5 dB, as already confirmed with statistical analysis. Additionally, it shows that the subjects were prone to setting a higher level of speech when listening on headphones, with the extremes exceeding 10 dB and going up to 25 dB. On the other hand, negative values of the difference, i.e., higher level set on loudspeakers than on headphones while listening appear in a marginal number of observations. Assuming that the listeners expect a certain loudness of normal speech regardless of the sound reproduction system used for reproducing that speech, the obtained results are in line with the phenomenon of the \"missing 6 dB\", i.e., a higher reproduction level required on headphones than on loudspeakers to evoke the same sense of loudness. The phenomenon has been reported in numerous work [17; 18; 19; 20]. The level mismatch can be avoided if individual equalization is performed to produce the same waveforms at the eardrums using both reproduction systems [23; 24]. However, even then, the mismatch \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sound Reproduction System** & **Loudspeakers** & **Headphones** \\ \hline Mean A-weighted level (dBA) & 49.9 & 55.0 \\ Standard deviation (dB) & 6.4 & 7.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Marginal means and standard deviations of the groups at two different levels of the sound reproduction system in the listening part of the experiment. Figure 2: The histogram of the difference of the set A-weighted level of speech listened over headphones and loudspeakers. is still present for sounds with pronounced low-frequency content and in moderately reverberant rooms [21; 22]. In the context of this research and its possible application, the described mismatch is to be kept in mind and accounted for, as there is no other way to address this issue in non-laboratory environments, i.e., in acoustically ordinary rooms in dwellings or at the workplace, with the subject equipped with a pair of headphones and/or loudspeakers of arbitrary type and model. Table 5 shows the marginal means and standard deviations for the three investigated conditions of the visual stimulus. The data shows that all three marginal means fall within the interval of less than 1.5 dB. The results of the ANOVA test do not show a statistically significant effect of the visual stimulus on the set level of speech. The visual stimulus put at 280 cm away from the listener yields a 1.3-1.4 dB lower average level than the other two conditions (no dummy head and dummy head at 85 cm from the listener) that yield virtually identical average levels. This indicates that the visual stimulus, as set up in this experiment, has virtually no meaningful effect on the chosen level of reproduced speech, as the indicated differences between marginal means barely exceed the just-noticeable difference in level (1 dB) that leads to a perceivable difference in loudness. The very small effect size in terms of the calculated eta-squared goes in line with this finding. Table 6 shows the marginal means and respective standard deviations for four different speaker voices. The data shows that all four marginal means fall within the interval of 1.8 dB. The results of the ANOVA test do show a statistically significant effect of the used speaker voice on the set level of speech.