## 1 Introduction  
Urbanization leads to many environmental problems, among which noise is a major one [1]. In European Union countries, about 20% of the population suffers from noise levels above 65 dB [2] and this is more serious in developing countries. Recently, with the rapid increase in automobiles, noise has become the top environmental complaint in China's metropolises (e.g., Shenzhen) [3]. In comparison to other environmental problems, the effect of noise is greater in high-density cities. However, when alleviating noise nuisance, the conventional approach of reducing ambient sound levels has been proven to not always be effective. According to the International Organization for Standardization (ISO), a soundscape is defined as an \"acoustic environment as perceived or experienced and/or understood by a person or people, in context\" [4]. It emphasizes a perceptual structure concentrated on individuals and this is influenced by the context [5,6]. Many studies have confirmed that soundscapes have a vital impact on the perceived quality of urban environments [7]. The findings of a study on community noise showed that better availability of nearby green areas in noise-exposed sites can mitigate noise's effects as they can deliver the required improvements in aural perception and mitigate feelings of annoyance [8]. In a large survey conducted by the Greater London Council, it was found that the presence of parks and green spaces lowered traffic noise's impact on neighborhoods and improved the quality of life in communities [9]. Given the multitude of factors influencing noise perception, the soundscape approach concerning satisfaction with the whole environment via aural perception is considered more useful than purely controlling the equivalent sound pressure level [10; 11; 12; 13]. In residential areas, the concept of a soundscape has been emphasized for its role in enhancing public health [14; 15; 16]. Hence, soundscapes are studied within high-density residential sites in particular and are discussed in this paper. A soundscape makes vital contributions to individuals' aural perceptions of a certain place (e.g., an urban public open space). According to the definition of a soundscape [4], a soundscape pays more attention to noise annoyance evaluations, in which aural perception is strengthened. In a study on public open spaces, the sound source was found to be more important in noise annoyance evaluations than the sound level when it was below a certain value [17]. For residential areas, the Guidelines for Community Noise suggest that open spaces provide recreational functions to inhabitants [2] while the accessibility of nearby green areas provides important benefits for social health [8]. A study on sustainable communities indicated that well-designed public open spaces can enhance resident satisfaction and promote place attachment [18]. Undoubtedly, public open spaces play an underlying role in promoting a sustainable environment in which acoustic comfort, defined as one of the essential feelings toward an acoustic environment, is required [19]. Factors from either acoustic or non-acoustic facets can influence soundscape evaluations [20]. A soundscape is a perceived acoustic environment [4] in which not only acoustic indices but also non-acoustic factors related to the audience's perception and social attributes may have impacts. Soundscape studies have indicated that sound levels, frequency, and psychoacoustic parameters are important for determining soundscape evaluations [21]. The findings of a study on soundscapes in public open spaces show that sound sources are essential for determining annoyance perceptions [22]. Natural sounds have been proven to be delightful and they can reduce noise nuisance, while exercise sounds from open-space events can be either pleasant or noisy, depending on the roles that people assume in a scenario [23]. In addition, visual factors have also been proven to be important in determining noise annoyance.