# Relationship between Subjective and Biological Responses to Comfortable and Uncomfortable Sounds
## 1 Introduction  
In 2014, a basic survey on industrial safety and health conducted by the Japanese Ministry of Health, Labour and Welfare (MHLW) showed that more than 1.11 million patients suffer from mood disorders such as depression, and that the number has been rising annually. In contrast, on a global level, over 300 million people are estimated to suffer from depression, equivalent to 4.4% of the world's population [1]. One of the major reasons for this situation is the stress caused by various kinds of tension and anxiety in modern life. To make our lives more comfortable, living environments should be designed to be suitable from both physiological and psychological perspectives. Firstly, physiological function is mainly regulated by central nervous system activity and peripheral nervous system activity. While the function of central nervous system responses can be evaluated through background and evoked electroencephalographs (EEGs), which record the electrical activity of the brain, the function of peripheral nervous system responses can be evaluated through cardiovascular activities such as by electrocardiogram [2, 3], reflecting the heart rate, blood pressure [4] and vasomotor activity [5], which indicate autonomic nervous system activity, and other factors such as respiration [6], skin temperature [7], and eye movements [8]. In recent years, many studies on brain science have been conducted, and objective interpretations of how the brain is related to psychological functions have also been studied [9]. A brain wave is an electrical pattern of brain activity. EEGs are typically described in terms of rhythmic activity and transient activity. The former type of brain wave activity constitutes alpha waves, beta waves, gamma waves, and lambda waves. Other than these frequency components, delta and theta waves in the lower frequencies under 8 Hz have also been treated. Alpha waves are found in thefrequency range from 8 Hz to 13 Hz [10]. [PERSON] firstly named this rhythmic EEG activity as an \"alpha wave\" [11]. These rhythmic waves differ in frequency. For example, the rhythmic wave in the alpha domain can be observed when closing the eyes and with relaxation. The mechanism of exciting these various frequency components has been investigated; however, physiological changes including brain activities in response to emotional stimuli are not simple linear responses, but rather are assumed to be complex [3; 12]. The measurement of psychological functions is generally based on a subjective evaluation. However, since the results of subjective evaluations vary widely among individuals, it may be beneficial to combine them with an objective evaluation method using physiological indicators in order to bring universality to evaluations. To use such a combined method with subjective and objective evaluations, it is first necessary to clarify the relationship between physiological and psychological functions. We are constantly exposed to various kinds of environmental stimuli in our daily lives. For example, from an acoustic viewpoint, environmental noise, such as that from a train station or a construction site, can be perceived as \"unpleasant\" if it is too loud, and may thus cause stress. On the other hand, listening to a murmuring river, birdsong, or classical music can be perceived as \"pleasant\". It is thought that the latter type of stimuli may have a stress-reducing effect. For example, the effects of water sounds on reducing stress have been investigated; however, these depend on the individual situation [13]. Even when listening to pleasant sounds, some people may feel uncomfortable due to other factors. However, if the subjective and objective evaluations have a relatively strong relationship, the effects of the surrounding environment on humans can be accurately assessed using a combination of methods.