Study protocol
The physiological experiment was conducted in an isolated room of Ohishi Clinic for 3 days. The mean temperature, relative humidity, and illumination of the indoor environment for 3 days were 23.4°C ± 1.5°C (mean ± standard deviation [SD], hereafter the same), 46% ± 6%, and 150 ± 70 lx at eye level, respectively.Sensors for heart rate variability (HRV) were attached to the participants. Then, two participants entered the room each time and sat on their assigned chairs. An HRV measurement device, near-infrared spectroscopy (NIRS) system, and sound-producing headphone were attached to the participants (Measurement scene. First, the participants put on the headphones and maintained in a resting state, while in sitting position with eyes closed for about 2 min. The experimenter turned on the audio player and played the auditory stimuli for nature or city sounds for 60 sec. Next, the participants answered the subjective evaluation questionnaire (