The restorative effects of natural sounds on human beings has been reported in many studies. Some of these natural sounds are birdsong, as well as water burbling in rivers, streams, and fountains [1
]. In addition, research by Ma and Shu [4
] revealed that these soothing sounds reduce fatigue and annoyance in people, and that auditory interventions may be more effective than visual stimulations in some circumstances under one particular condition. According to Li and Kang [5
], birdsong in a forest at dawn and an ocean sound on a calm sunny day decreased the levels of the heart rate (HR), respiratory frequency (RF), and respiratory depth (RD) and increased the values of amplitude of the R-wave (ΔR), heart rate variability (HRV), electroencephalography alpha activity (α-EEG), and electroencephalography alpha activity (β-EEG). Among these natural sounds, birdsong was most frequently used as a representative of natural sound [2
], and all the studies revealed that birdsong is more restorative than anthropogenic noise.
These restorative natural sounds can be a useful resource for mental health care in modern society. Azuma and Inoue [10
] implied that natural sounds would play an important role in improving the quality of life of the elderly in old-age homes as soundscapes are easier to manage than landscapes in such facilities. The benefits of natural sounds can also be useful in resting rooms for children with autism. According to Neo and Flaherty [11
], such spaces, called “sensory rooms” in airports, are designed in such a way that children who play there are protected from loud noise. Adding natural sounds in such controlled environments would further improve the comfort provided by these spaces. De Coensel et al. [12
] revealed that people would feel more pleasant when natural sounds were added to traffic noise.
However, the following limitations would have to be considered before implementing the above-mentioned studies into practice. Firstly, the types of natural sounds used in earlier studies are limited. Ma and Shu [4
] concluded that birdsong and sound of water were the most preferred natural sounds based on the results of a laboratory experiment, but these sounds were not compared with other natural sounds, and instead were compared with artificial sounds like footsteps, ventilating noise, and traffic noise. Secondly, not every study contains adequate information on acoustic stimuli presented in its experiment. Hume and Ahtamad [6
] used a realistic mixture of natural sounds and artificial sounds but did not provide detailed information on the acoustic characteristics of each sound-clip (18 types in total), although the mean value, the standard deviation, and the range of the sound pressure level across all the sound-clips were mentioned. Thirdly, the classifications of sounds used in some studies were exaggerated. In Medvedev et al. [8
], each category (birdsong, ocean, construction, motorbike, airplane, and music) had only one type of sound, and the sounds of construction, motorbike, and airplane would understandably be unpleasant and people would not dare to listen to these sounds for stress recovery. For the same reason, Li and Kang’s experiment [5
], which used four types of sounds, namely birdsong, ocean, street noise (an outdoor shopping street full of hurrying pedestrians and hawking), and traffic noise (an intersection at the peak hour in the afternoon on a sunny day), would not have been sufficient to investigate the characteristics of more restorative sounds among the sounds which have already been verified as restorative. However, it should be noted that the study surely confirmed that natural sounds were more restorative than anthropogenic noise. Additionally, these two studies [5
] did not take silence as a control condition, which should have been necessary to examine whether listening to natural sounds was really more restorative than resting in silence.
On the basis of the above argument, an experiment where birdsong with the addition of small external disturbances, such as faint road traffic noise that people are normally exposed to in a park or in a forest to which they come to relax, would be more realistic and be worth investigating. Additionally, it would be necessary to assess the degree of restorativeness under silent conditions as the basis for comparison the effect of road traffic noise mixed with birdsong on the restorativeness and that of birdsong itself. This study aimed to investigate whether the presence or absence of road traffic noise affects the perceived restorativeness and the physiological restorativeness of birdsong. It is based on an exploratory methodology with a combination of physiological measurement and subjective evaluation. The research question of this study is as follows:
Does faint road traffic noise mixed in birdsong affect the restorativeness of the birdsong in terms of physiological evaluation even if the noise does not influence the perceived restorativeness of the place of experiment where people are exposed to the birdsong?