1. Introduction
In recent decades, intensive urbanization has led to isolation and disconnection between humans and nature, resulting in mental health issues. Global mental health issues have worsened over the past decade, with a dramatic 25% increase in the global prevalence of anxiety and depression following the COVID-19 pandemic [
1]. The economic downturn, lockdowns, and quarantines resulting from the post-COVID-19 era have placed people under considerable pressure to survive [
2,
3,
4,
5,
6,
7,
8]. Facing mental health issues, mental health systems worldwide are not adequately equipped to address the current burden of mental disorders. Traditional psychiatric medications have established side effects [
9,
10,
11]. However, pro-nature non-invasive preventive, complementary, or alternative therapies are widely accepted, such as Forest Bathing [
12], Aromatherapy [
13], Horticultural Therapy [
14], and Five Senses Therapy [
15]. Numerous studies have shown that these methods bring many benefits to humans through contact with or perception of nature, such as reducing stress and depression, helping to recover from fatigue [
16], improving emotional state, shortening recovery time after surgery [
17], promoting physical activity [
18], reducing anxiety, and improving mental health [
19].
Wilson et al. proposed the “pro-life hypothesis”, suggesting that the natural environment is vital for physical and mental health and that humans are born with green genes [
20]. Although there is evidence that Forest Bathing, Horticultural Therapy, and Vertical Greening effectively improve human physical and mental health, few people today benefit from these environments because forests are less accessible and less frequently used [
21]. Home gardens need to work better in densely populated low-income cities. Vertical greening is still in its infancy for cities [
22]. Many scholars have begun to notice the role of urban healing systems in spiritual healing and healing systems. Liu et al. proposed taking the “prevalence rate of mental disorders among people aged 18 and above” as the evaluation index of park city in constructing the park city evaluation system [
23]. Li et al. have systematically expanded traditional healing environments into healing streets, healing buildings, virtual healing, and other areas as part of the healthy city healing system [
24]. The team of Ha also proposed in the connection between cities and health that more than urban blue and green space is needed. The positive impact on mental health requires active planning in the spatial distribution of multiple landscapes [
25].
Vision and hearing are the primary sources of information for the human body. From the perspective of audio-visual integration, exploring ways to improve the healing potential of city park systems can enhance the city’s prevention and healing capabilities against public mental disorders and reduce the burden on the mental health system. Furthermore, studies on the healing potential of environments through visual perception alone cannot provide comprehensive guidance for actual landscape design [
26]. The soundscape serves as an innovative design tool that holds great potential in concealing negative ambient sounds and enhancing positive auditory experiences within a given environment. In contrast to visual stimuli, which experience momentary interruptions during each blink, resulting in the loss of approximately 450 milliseconds of information per blink and approximately 6 s per minute, the auditory channel remains continuous and uninterrupted. Consequently, many scholars began to investigate the relationship between visual and acoustic landscapes. For example, better therapeutic effects have been observed when natural water bodies and dense vegetation landscapes are complemented by birdsong [
27]. This includes Alvarsson et al.’s work, which showed that natural sounds help with recovery from sympathetic activation after psychological stressors [
28]. Through research, Renterghem found that the level of noise perception reduction with green vision was easily more than 10 decibels [
29]. Natural-related audio-visual stimuli are generally believed to have greater health benefits compared to singular visual stimuli [
30]. However, simultaneously, Li et al. discovered that audio-only stimuli may produce more therapeutic EEG responses than audio-visual stimuli [
31]. Annerstedt et al. demonstrated that adding natural sounds to a virtual reality environment can even promote stress reduction [
32]. Furthermore, in park settings, demographic factors such as age, education, and stress levels have been found to significantly influence the therapeutic effects of bird songs [
33]. In a recent study, it was discovered that individual traits have a greater impact on the potential restorative effects of urban sounds compared to innate characteristics [
34]. In summary, the acoustic environment is a critical factor that affects overall comfort. Visual and audio elements are always in the same interaction, highlighting the importance of studying audio-visual blending environments, including soundscapes, to improve the impact of the physical environment on people’s bodies and minds [
35,
36]. However, research on the mechanism of the effects of audio-visual composite scenes on the human body in urban parks needs to be more robust.
To enhance the therapeutic potential of urban parks, this study aims to explore the expected relationship between visual and auditory stimuli and their therapeutic potential by measuring people’s brainwave signals when experiencing different park visual and auditory scenes and evaluating the psychological relaxation effect of restorative components. This study intends to provide guidance for improving and optimizing the therapeutic function in urban park planning and design based on evidence of visual and auditory therapeutic potential.
3. Results
3.1. Effects of Visual Sample and Audio Sample on Meditation
The results of the one-way analysis of variance indicated that the visual stimuli in different urban park spaces showed heterogeneity of variance for the eSense “meditation level” index of subjects’ brainwaves, while the audio stimuli had a significant impact on the index (
p < 0.05) (
Table 2). The results of the multifactor analysis of variance revealed significant differences in the meditation level of brainwaves for the visual stimuli, audio stimuli, interaction of visual and audio, and gender (
p < 0.05) (
Table 3).
Figure 5 demonstrates that after the addition of the S0 (100% birdsong) audio stimuli, the meditation level improved significantly except for the waterfront recreation space, indicating that S0 can effectively increase the comfort of the scene. However, the meditation level of the waterfront recreation space showed a significant decline. The S0 does not seem to be suitable to be used in waterfront recreation space.
Figure 6 shows that among audio stimuli, S0, S11, and S12 audio samples adapted to the waterfront recreation space, among which S12 > S11 > S0. The meditation level of the visual stimuli of the waterfront recreation space was taken as the control group. It was found that the meditation level of the waterfront recreation space was significantly improved after the addition of S11 and S12. In contrast, the meditation level of the waterfront recreation space significantly decreased after the addition of S0.
Figure 7 presents the results of the audio samples (S41, S42, S43) adapted to the square recreational space, where gender determines some of the differences. The meditation level of audio stimuli in male subjects was S43 > S42 > S41, while that in female subjects was S41 > S42 > S43. The meditation level of the visual stimuli in the square recreation space was taken as the control group. The meditation level of male subjects decreased after the addition of S41, while it increased significantly after the addition of S43 and S42. On the other hand, the meditation level of female subjects decreased significantly after the addition of S43, increased after the addition of S42, and also increased significantly after the addition of S41.
Figure 8 illustrates the results of ten audio samples of S0, S61, S62, S63, S64, S65, S66, S67, S68, and S69 adapted to the interactive amusement space. The meditation level of the first five audio stimuli (S0, S61, S62, S64, S65) was significantly greater than that of the last five. The top five audio samples contained 50% or more birdsong, but there was no positive correlation between birdsong and meditation level. When birdsong was matched with children laughing, S61 > S62 > S63, the birdsong ratio was positively correlated with meditation, while children laughing was negatively correlated. We also observed an interesting phenomenon where there was no significant difference between S64 and S65, as well as between S68 and S69. This indicates that when the proportion of traffic noise is either 30% or 50%, there is no significant difference in the impact of noise on the healing effect. Additionally, we found that S65 > S68 and S64 > S69, indicating that the meditation level when paired with birdsong is significantly higher than when paired with children laughing, for both 30% and 50% proportions of traffic noise. However, when the traffic noise reaches 70%, there is no significant difference between S66 and S67, suggesting that even with 30% birdsong inclusion, it does not have a positive effect. In this regard, children laughing shows a different pattern, where regardless of the proportion of children laughing (S61, S62, S63, S67, S68, S69), pairing it with birdsong always results in significantly higher meditation levels compared to pairing it with traffic noise.
The meditation level of the visual stimuli of the interactive amusement space was taken as the control group. It was found that audio-visual stimuli showed the same trend as pure audio stimuli. The first five meditation levels were significantly greater than the last five. It was found that the meditation level after adding the first five pieces of audio was significantly higher than that of the visual stimuli. The meditation level after adding the last five audio pieces was significantly lower. It is worth noting that the increase in meditation was not related to the proportion of birdsong in the audio. Although both S63 and S66 contain 30% birdsong, the audio stimuli still have a negative effect on the scene. The combined audio-visual stimuli of S0 and S64 results in a higher meditation level compared to pure visual or pure audio stimuli. The meditation level of the audio stimuli S61 is higher than that of S64; however, when visual elements are introduced, the increase in meditation level for the audio-visual stimuli of S61 is significantly lower than that for S64.
3.2. Effects of Visual Samples and Audio Samples on the Restorative Component Scale
Reliability and validity analyses were used to test the internal consistency of the 20 restorative scale data obtained in the experiment. The results indicated that the overall Cronbach α coefficient was 0.925, with the RCS (Recovery Component Scale) scale at 0.901, the VAQ (Visual Aesthetic Quality) scale at 0.904, and the TR (Tranquility Rate) scale at 0.915, all of which were higher than 0.9. Additionally, the KMO and Bartlett’s tests were passed, indicating that the data quality of the scale was very high. After a standardized entropy analysis of the RCS scale data, the weights of the four dimensions were being away (19.21%), extent (25.5%), fascination (26.58%), and compatibility (28.71%). The weighted average of these dimensions provided the RCS value, while the TR and VAQ scales were averaged as the TR and VAQ values, respectively. The Kruskal–Wallis’ analysis showed that different combinations of visual and audio scenes significantly affected the RCS, TR, and VAQ values of urban park scenes, respectively.
As shown in
Figure 9, when the audio conditions remained unchanged (S0-100% birdsong), with the exception of the interactive amusement space, the average RCS score in all other spaces was significantly greater than 2, indicating a good overall healing effect of urban parks, except for the interactive amusement space. The lawn recreation space had the highest RCS value (3.41), followed by the underforest recreation space (3.14), while the interactive amusement space had the lowest RCS value (2.01). The TR value and VAQ value showed a similar trend. Notably, the TR value of the park rest space and square recreation space was higher than that of the waterfront recreation space.
In the classification discussion, the visual conditions were unchanged, and audio-visual scenes were represented using audio sample coding for easy reading. The visual conditions were waterfront recreation space, interactive amusement space, and square recreation space. In the waterfront recreation space, the RCS values of the composite audio S11 and S12 were significantly greater than that of the original audio S0, with no significant difference between S11 and S12. The TR and VAQ values showed similar trends, but S11 had a significantly larger TR value than S12, with the difference being mainly provided by women (
Figure 10). In the square recreation space, the data difference of the RCS questionnaire was mainly provided by women, and the data of women and men showed opposite trends. The combined questionnaire was not representative, so gender classification was discussed (
Figure 11). In the results of the female RCS value, S0 > S41 > S42 > S43, with the decrease in the RCS value being related to the decrease in the proportion of birdsong. In the male RCS value results, S43 > S42 > S41, with the decrease in the RCS value being related to the decrease in the proportion of conversation.
As depicted in
Figure 12, the RCS values of all audio-visual scenes, except for S0 (2.01), in the interactive amusement space were less than the scale mean value of 2. Similarly, TR and VAQ values also showed poor healing perception in this audio-visual space. These results suggest that the overall healing degree of the interactive amusement space is not satisfactory and optimization plans are necessary.
In the ten audio-visual scenes of the interactive amusement space, subjective rating data revealed features the same as physiological data:
- (1)
The RCS, TR, and VAQ values of the first five audio-visual combinations were significantly higher than those of the last five audio-visual combinations.
- (2)
In audio-visual stimuli, S64 was significantly greater than S61, and S65 was significantly greater than S62. Children laughing at 50% or less had a more negative impact on the scene than traffic noise at 50% or less.
- (3)
In audio samples containing 30% and 50% traffic noise, S64 was greater than S69, and S65 was greater than S68. However, the audio samples containing 70% traffic noise (S66, S67) showed no significant difference when paired with birdsong or children laughing.
5. Conclusions
This study investigates the therapeutic effects of urban parks on students under chronic stress through Mindwave monitoring and the Recovery Component Scale. The following conclusions were drawn: (1) There may be significant differences in the visual sky above the blue space and the visual sky above the green space when interacting with birdsong. (2) Birdsong and conversation from seemingly contradictory dimensions of “Quiet” and “social” seem to provide healing, with gender differences among young students being a key factor, providing new evidence for their healing mechanisms. (3) Visible children laughing at low levels (30% and 50%) has a more negative impact than invisible traffic noise at the same sound pressure level. (4) Depending on the visibility of the sound source, audio-visual interaction may not necessarily have a positive effect. (5) In the experiment, no single sound source was found to have a positive correlation with the RCS value as its proportion increased. Instead, it was closely related to the matching sound source, indicating the complexity of acoustic landscape therapy.
In fact, multi-sensory design, such as audio-visual integration, has great potential in enhancing healing effects, enabling urban parks to play a role in “substitute therapy” and reducing the burden on the mental health care system. Therefore, future park designers should consciously consider strategies for improving the healing potential from the audio-visual perspective by controlling the visibility of sound sources to reduce the negative effects of children laughing and traffic noise, enhancing the positive effects of birdsong and flowing water sounds; by adjusting the matching of sound sources to transform children laughing, which is originally a negative effect, into an overall positive effect; and by enhancing positive sound sources that may appear in visual stimulation to achieve healing, thereby maximizing the audio-visual healing effect of urban park spaces.