# Relationship between Subjective and Biological Responses to Comfortable and Uncomfortable Sounds
## 3 Results and Discussion
### Physiological Response  
The change rates of \(\alpha\)-EEG, \(\beta\)- EEG, percent \(\alpha\)-EEG, LF/HF, and SD2/SD1 are shown in Figure 8a-e, respectively. Note that, in this section, the biological data obtained from all three trials in Figure 1b are averaged and discussed. The change rates of \(\alpha\)-EEG showed a significant difference between SA and SB (\(p<0.05\)). This indicates that the \(\alpha\)-EEG significantly decreased when SA was presented, while the \(\alpha\)-EEG did not significantly increase when SB was presented. This is consistent with past research [49], where the brain wave in the alpha domain was also decreased by exposure to white noise. In some references, such as [50], the strength of \(\alpha\)-waves was proportional to the degree of relaxation of the subject. However, in this study, the averaged result of \(\alpha\)-EEG was not increased by the murmuring river sound. The reason for the unchanged \(\alpha\)-EEG will be discussed in the next section. In the case of change rates of \(\beta\)-EEG and percent \(\alpha\)-EEG, there was no significant difference between those in the SA and SB conditions. The presence of stress has been shown to increase the \(\beta\)-EEG [51], but in this case, it is considered that the environmental stress on humans caused by the white noise and murmuring river sound did not have different influences on human, while the \(\alpha\)-EEG slightly increased in the SA condition. It is said that components in the beta frequency band are distributed in the front of the head [52], and the amount of frontal beta is correlated to task difficulty. The reason for the lack of a significant difference may be due to the fact that the stimulus was not a task involving difficult judgements, but rather just exposure to sounds. Moreover, in the literature [53], no significant differences have been observed between conditions with and without acoustic stimuli. However, the results of the present study had a greater standard deviation, especially under the SB condition, indicating large inter-individuality. This may be caused by inter-individual differences in the perception of the sound of a murmuring river. Next, LF/HF slightly increased in the SA condition compared to the SB condition, but there was no significant difference between the conditions with and without acoustic stimuli. Since numerous papers have suggested that the increase in LF/HF is related to the increase in sympathetic nerve activity [28], the increase in LF/HF may be due to the stress that occurs when listening to uncomfortable sounds such as white noise. It is also related to previous results [54] where the HF component decreased with white noise exposure. SD2/SD1 showed a decreasing trend during the SA condition and an increasing trend during the SB condition, with a significant difference between the two sound sources (\(p<0.05\)). It is said that SD2/SD1 is related to autonomic balance where SD2 reflects sympathetic cardiac regulation and SD1 reflects parasympathetic activity [55; 56]. In addition, in yet another study [57], it is shown that when white noise is greater 50 dBA, significant sympathetic activation is induced, producing significant cardiovascular stress. Thus, it  
Figure 7: Relationship of the averaged factor scores and SDs between (**a**) beauty and timbre factors, and (**b**) timbre and power factors, respectively. These scores of SA (white noise) and SB (murmuring river sound) contrast with each other, and white noise and murmuring river sounds can be separately perceived as uncomfortable and comfortable sounds. may be reasonable that SD2/SD1 decreased with white noise exposure and increased with exposure to the more comfortable murmuring river sound. Herein, both \(\alpha\)-EEG and SD2/SD1 decreased in the white noise condition, while increased or unchanged in the murmuring river sound condition. So, the correlations between (a) \(\alpha\)-EEG and SD2/SD1 and (b) \(\beta\)-EEG and SD2/SD1 are shown in Figure 9a,b, respectively.