## 1 Introduction  
The context of urban open spaces, especially the acoustic environment, has played an indispensable role in the experience of users of those spaces since residents and tourists pay more attention to acoustic perception than visual satisfaction [1]. With the spread of urbanization, urban open spaces will be increasingly multifunctional and integrated, and crowds will be exposed to noise because of complex acoustic contexts [2; 3]. Many previous studies focused on how to prevent and control noise, using tools such as predictive models, noise maps and noise barriers [4; 5; 6; 7; 8], while the quality of the sound environment had not been improved effectively. Currently, there is a tendency to shift the focus from purely noise control to overall soundscape design, which brings a change to the field of sound environment design. Soundscape, an important aspect of evaluating an acoustic environment, has been widely applied to studies of acoustic environments in urban open spaces [9; 10; 11; 12]. Different from noise control engineering, the soundscape is the acoustic environment as perceived or experienced and/or understood by a person or people, as defined by the ISO. It represents a timely paradigm shift in that it considers environmental sounds like a'resource' rather than a 'waste' [13]. Acoustic factors such as sound sources, absorption coefficient, sound pressure level and reverberation time are of primary importance in a wide range of studies [13; 14; 15; 16]. Regarding sound sources, some studies prefer to discuss the types, the masking effects, and the perception of sound sources as a general rule. [PERSON] and [PERSON] considered that the sound sources mainly included 4 types: natural, human, mechanical, and instrumental sounds based on 19 urban squares [17]. Among these types of sound sources, natural sounds and traffic noise have been increasingly considered in recent years, especially their effects on perceptions from citizens [2; 18; 19; 20]. Several works still attempted to explore the effect of using natural and artificial sounds to reduce the impact of traffic noise [14; 21; 22]. Additionally, other studies aimed to improve the acoustic environment by using vegetation to absorb sound in urban open spaces. [PERSON] and [PERSON] carried out a study to determine the effect of roadside vegetation on the reduction of road traffic noise under varying planting intensities [23]. Traditionally, sound pressure level and reverberation time were frequently used as acoustic factors to estimate a given acoustic context in urban open spaces [10; 24; 25; 26]. [PERSON] noted that the reverberation time and the early decay time are significantly longer and the sound pressure level attenuation with distance is generally smaller in urban squares unless the height/side ratio is high compared to diffuse boundaries with geometrical boundaries [24]. Soundscape studies have contained both acoustic and contextual factors affecting human perception of the acoustic environment. Now, contextual factors such as spatial, temporal, and functional factors are receiving great attention [10; 22; 27; 28]. A number of studies have noted that spatial locations and regions can change the diversity of sound sources, acoustic environments and sequentially affect citizens' evaluation of acoustic quality [4; 8; 29]. Temporal factors could refer to the time of a day or the seasons of a year. [PERSON], [PERSON] et al. proposed that subjective and objective evaluations of acoustic environments were different in different seasons [18; 30]. Some related studies explored the seasonal effect of the temperature difference on soundscape [31; 32; 33; 34; 35]. In terms of functional factors, many previous studies indicated that soundscape assessments were influenced by the functions of places, including commercial, residential, business, and recreational places. [12; 29; 36]. [PERSON] and [PERSON] pointed out that places with different functions might be evaluated differently, even if the same sound sources existed in a place [29].