# Relationship between Subjective and Biological Responses to Comfortable and Uncomfortable Sounds
## 1 Introduction
Regarding the mechanism of emotional and biological responses to sounds, it has been reported that music perception involves complex brain functions, potentially affecting emotion and influencing the autonomic nervous system [14]. In a study of the EEG responses of subjects listening to their preferred and non-preferred music, the power of alpha waves increased when listening to their preferred music [15]. Many studies on the relationship between auditory stimuli and EEG, such as a study that measured EEG when [PERSON]'s music was presented [16], can be seen. In the other study, the effect of music preference on brain waves was studied using classical and rock music [17]. As described above, the auditory effect of comfortable music on brain waves has been investigated, but the auditory stimuli of music consist of many sounds and complex structures, and their effect on humans is also considered to be complex. In contrast, natural sounds such as river sounds are simple, and they may provide relaxation to humans [18]. However, the auditory effect of such a comfortable sound on a human's brain wave has barely been discussed in detail. On the other hand, heart rate variability (HRV) has also been utilized as an indicator of many physiological and behavioral factors [19] such as stress [20], fatigue [21], and performance [22]. The relationship between HRV and other biological conditions such as autonomic nervous function has been investigated [23]. While the relationship among various biological indicators has been generally discussed, HRV and EEG are often investigated together [24] since both provide cyclic time-transient data and are relatively easy to measure. The synchronicity of HRV and EEG is often analyzed and applied to various cases [25]; however, the relationship between HRV and EEG in response to sound stimulation has rarely been studied. As mentioned above, it is clear that sound has an emotional and biological impact on humans, and it is worthwhile to investigate such a mechanism; it will be beneficial to investigate the relationship between the subjective impressions and biological responses caused by sounds using HRV and EEG. The purpose of this study is to evaluate the effects of auditory stimuli comparing two kinds of sounds, white noise and a murmuring river sound, using psychological and physiological responses. Through such a study, a basic understanding of how auditory stimuli affect both psychological and physiological states of humans can be confirmed. So,in this study, using EEG and HRV as the means of physiological evaluation, the relationship between subjective impressions and biological responses is discussed. Firstly, the experimental methods of subjective and biological measurements are described in Section 2. The subjective impressions of the white noise and murmuring river sound were measured by a subjective evaluation test using the semantic differential (SD) method. After this, the biological parameters related to EEG and HRV were physiologically measured for each of the subjects. In Section 3, the subjective and physiological results are comparatively discussed. Then, we determine whether some of the biological parameters of EEG and HRV are significantly related to the comfortableness of the reproduced sound. However, some subjects decreased their alpha wave energy of the EEG and indicated different responses to the comfortable sound of murmuring river compared to the majority of the subjects who showed an increase in alpha wave energy. A minority of subjects had slightly different auditory impressions of the murmuring river sound where they felt some negativity toward it. Finally, it has been indicated that individuals have different impressions of sound, but the relationship between subjective impressions and biological responses shows a consistent trend.