**Preface to "Soundscape in Urban Forests"**

In recent years, we have witnessed an increased interest towards soundscape in urban forests. Soundscapes are one of the most important components of the landscape in urban green spaces, natural spaces and cultural landscapes, and soundscapes play an important role in the perception of residents, especially in quiet areas. Fortunately, urban forests are suitable habitats for many plants and animals, which play a key role in producing a soundscape. Therefore, there is increasing research attention focused on soundscapes as people become more concerned about health, urbanization, globalization etc.

We are honored to guest edit this Special Issue and thank all authors and reviewers for their excellent work and unyielding support. Meanwhile, our appreciation goes to the *Forests* editorial team for their enthusiastic dedication and professional editing. Contributions from our colleagues and scholars in different institutions are also sincerely appreciated. Meanwhile, many researches were funded by the National Natural Science Foundation of China (No. 52208052), and the Humanities and Social Science Research Program of Ministry of Education of China (No. 21YJCZH038).

Soundscapes have opened up whole new dimensions in forestry, urban planning and policy, but wide application of them still requires much effort to be made. By searching for multi-disciplinary techniques and strengthening cooperation between academics and industrialists across the world, we will continue to exploit the full potential of soundscape in order to better optimize the urban environment.

> **Xin-Chen Hong, Jiang Liu, and Guangyu Wang** *Editors*

## *Editorial* **Soundscape in Urban Forests**

**Xin-Chen Hong 1,2, Jiang Liu 1,\* and Guang-Yu Wang 3,\***


#### **1. Introduction**

The World Health Organization (WHO) has made considerable efforts to reduce citizens' exposure to community noise in urban and suburban areas. Urban forests contribute to a healthy environment for the public in high-density cities, and the soundscapes in these areas are important health-related resources in urban areas, especially natural soundscapes [1]. The various soundscape standards proposed by the International Organization for Standardization (ISO) [2–4] represent a milestone in the recognition of soundscapes as a legitimate approach to manage and design urban sound environments, especially biophilic outdoor environments [5]. The effects of soundscapes in forested areas are not limited to psychological and physical rehabilitation, contributing to the visiting experience in urban forests together with other perceptions [6], but also have ecological significance, such as protecting the acoustic environment for animal communication [7]. These facts demonstrate that soundscapes in urban forests have profound connotations worth further exploration.

This Special Issue in *Forests* explored the role of soundscapes in urban forested areas. It is comprised of eleven papers involving soundscape studies conducted in urban forests from Asia and Africa. This collection contains six research fields: (1) the ecological patterns and processes of forest soundscapes; (2) the boundary effects and perceptual topology; (3) natural soundscapes and human health; (4) the experience of multi-sensory interactions; (5) environmental behavior and cognitive disposition; (6) soundscape resource management in forests.

#### **2. Summary of Articles Included in the Special Issue**

Soundscapes influencing individuals include physical stages, physiological stages, and psychological behavior stages. Our research collections focus on the experience of multisensory interactions in urban forests. Xu et al. [8] analyzed the audio-visual preference characteristics of exercisers in a park, and found that the forest and its landscape and birdsong avenues are most preferred by exercisers, though the audio-visual preferences of people with different exercise forms differ. Then, they analyzed children's multi-sensory landscape preferences in a park and explored the influence of multi-sensory experiences on children's behavioral experience [9]. They found that visual, auditory, tactile, and olfactory sensations were significantly correlated with children's behavioral experiences. Furthermore, to look into visitors' multi-sensory interactive experiences in an urban forest park [10], these contributors showed that urban forest park visitors' sensory experiences are dominated by visual and olfactory perceptions, followed by audio-visual and visual–tactile interactions. Gan et al. [11] tried to quantify and compare the contribution of audiovisual perception to visitors' overall park landscape preference. They found that visitors' visual landscape preference was generally higher than soundscape preference, while the influence of auditory preference on overall landscape preference was found to be greater than that of visual preference. Shao et al. [12] explored aural–visual interaction attributes that may

**Citation:** Hong, X.-C.; Liu, J.; Wang, G.-Y. Soundscape in Urban Forests. *Forests* **2022**, *13*, 2056. https:// doi.org/10.3390/f13122056

Received: 28 November 2022 Accepted: 30 November 2022 Published: 3 December 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

influence people's perceived overall soundscape comfort in urban green spaces (UGSs). They suggested that a low level of environmental sound does not correspond to higher ratings on the overall soundscape comfort.

The natural properties of forests contribute to the occurrences of natural soundscapes which are beneficial to physiological stages. Our research collections focus on various scenes and contexts. Hong et al. [5] focused on the different forest structures, and explored the relationships between perceived soundscape and acoustical parameters, and observe physiological indicators. They suggested that LAeq and L<sup>10</sup> are important factors that influence questionnaire responses, and electromyogram (EMG) signals were the most obvious and sensitive in physiological parameters. Wang et al. [13] explored the effects of soundscapes on human physiology and psychology from the perspective of auditory senses in national parks. The authors found that the sound of water had the most significant effects on the heart rate and respiratory rate of the subjects. Yu et al. [14] investigated the psycho-physiological effects of traffic sounds in urban green spaces. They found that traffic sounds had significant detrimental psychological and physiological effects. In terms of psychological responses, the peak sound level outperformed the equivalent sound level in determining the psychological impact of traffic sounds.

Furthermore, urban forested areas represent an important part of urban ecosystems. Our research collections focused on the ecological patterns and processes of forest soundscapes as well. Zhao et al. [15] explored the spatial and temporal characteristics of the urban forest area soundscape. They found that the power spectral density (PSD) and the soundscape diversity index (SDI) of the park were greatly affected by public recreation behaviors. Nwankwo et al. [16] explored how the soundscape of the natural environment could affect humans with respect to the different densities of vegetation, as well as the frequency of singing events and the sound pressure levels of common birds that generate natural sounds in a commonly visited urban park. Chen et al. [17] explored the spatial dependency of the soundscapes in Kulangsu, based on the spatiotemporal dynamics of soundscape and landscape perceptions. The authors found that the perceptions of soundscape and landscape were associated with significant spatiotemporal dynamics, and revealed the dominance of biological sounds in all sampling periods and human sounds in the evening.

#### **3. The Researchers' Perspectives on Soundscapes in Urban Forests**

To gather the perspectives of contributors for our Special Issue after completing their researches, we invited one contributor from each paper to answer the following open-ended question: "What are the current priorities and challenges for soundscape research in urban forested areas?". We required the contributors to respond with short comments, typically 100 to 150 words. We report below the full report of the contributors which sorted by title and family name initials.

#### *3.1. Interviewee*

Soundscape includes both the sound of natural environment and the played music, especially the natural sound which always attracts a tremendous audience. In the shaping of soundscapes, the sound characteristics of the natural environments are often skillfully used to craft a beautiful and diverse sense environment for human beings. It has been a long time since landscape architects have innovated natural sound into the traditional Chinese garden environment as a special landscape element, which have been abundantly documented in the history, such as rain drizzling down on the plantain leaves with a staggering sound, the sound of a reverberating breeze blowing across the lotus pond, and the whistling of the wind in the pines. In these time, research of soundscapes focuses more on high-quality built environments and the natural environments, which aim to satisfy people's auditory and perceptional demands, generally creating a relaxing, comfortable, and pleasant environmental atmosphere through landscape design.

(Prof. Dr. Yuning Cheng—Southeast University)

#### *3.2. Interviewee*

As a new perspective of experience and cognition, certain theoretical and practical achievements have been developed in the field of soundscape and multi-sensory research. However, the best way of maximizing the function of urban forest parks to serve children and improve their multi-sensory experience remains an open question. Compared with adults, children are natural sensorial explorers and are more sensitive to external sensory stimuli. Their sensory experience in urban forests is unique and energetic, which deeply influences the healthy growth and development of children. Therefore, it is vital to pay attention to specific groups of people in the study of urban forest soundscapes and analyze their experiences and preferences. Children's multi-sensory perceptions in forest parks are worth exploring, and more in-depth additional research is needed through increased research time, more case sites, and greater sample sizes.

(Prof. Dr. Muchun Li—South China University of Technology)

#### *3.3. Interviewee*

Natural sound, biological sound, and synthetic sound make up the majority of the soundscape, which is a crucial component of the composition of an environment. The urban forest soundscape contains a variety of biological sounds, including birds' and insects' sound. This type of soundscape has characteristics which vary over time and space. The activity of different biological communities, plant community structures, and other factors influence the soundscape diversity and characteristics of urban forest biological sound. The urban forest's acoustic environment can be improved by creating a habitat space to increase the richness of the soundscape. The relationship between biological activities, the diversity of soundscapes, and the vertical structure characteristics of urban forest plants can also be explored in future research.

(Prof. Dr. Qi Meng—Harbin Institute of Technology)

#### *3.4. Interviewee*

The current priorities of soundscape research in urban forested areas are as follows. Firstly, combing soundscape research with new tourism formats and tourist' requires in-depth experience. Future research should focus on diverse perspectives, such as "Soundscape + Culture" research, because different places have different soundscape characteristics and change processes, behind which is the expression of local cultural differences. Secondly, previous researches have tended to focus on a single perspective, such as vision. Focusing on a single research perspective is insufficient in the context of a normalized epidemic. More attention should be paid to multi-sensory dimensions such as "Soundscape + Olfactory".

The challenges of soundscape research is the quantitative analysis of the soundscape. As the concept of soundscape becomes more widespread, there is a need to experiment more with experimental methodologies of measuring the healing effects of soundscape to collect objective data, but also to focus on subjective factors such as changes in the psychological attributes of the study participants.

(Prof. Dr. Zhicai Wu—South China University of Technology)

#### *3.5. Interviewee*

Research on the interaction between auditory and non-auditory perceptions, including audiovisual interaction, and the sensory contribution mechanism of the urban forest landscape evaluation should be strengthened, and through this, soundscape perception can be expanded into a multi-sensory landscape experience. Moreover, the physiological and psychological healing functions and

health promotion effects of urban forest multi-sensory landscapes on visitors should be further studied.

(Assoc. Prof. Dr. Yonghong Gan—Minnan Normal University)

#### *3.6. Interviewee*

The importance of multi-sensory perception in constructing human landscape experiences has been increasingly emphasized in contemporary urban life. The major priorities and challenges for soundscape research in urban forested areas is to broaden the understandings of environmental attributes with aural–visual interactive effects, and their impacts on human perceptions. These investigations should also consider whether their attributes and influences vary with sound-level ranges. Another important aspect that should be emphasized is the functional values of urban forested areas. Though numerous efforts have been made, the relations constructed between human perception and environmental aural–visual characteristics are only used as auxiliary tools in landscape practices. Future studies should further explore the effective ways of translating people's aural–visual landscape experience into design languages, which can further contribute to enriching human spatial experiences from the perspective of multi-sensory design interventions.

(Assoc. Prof. Dr. Yuhan Shao—Tongji University)

## *3.7. Interviewee*

Combining the current major publication trends and research, I believe that the current priorities of urban forest soundscape research comprise mainly two aspects. On the one hand, the study not only focuses on human perception and soundscape experiences, but also includes psychological effects, such as recovery effects and stress reduction effects. On the other hand, the study of soundscape and other spaces is gradually integrated. The current related research mainly involves cultural, landscape, virtual, and color spaces. This extends the research direction and field of soundscape and enhances the application value of soundscape research results.

Research on urban soundscapes is also currently facing some challenges. Some studies have shown that noise in cities reduces people's sensitivity to sound changes, which is a large bias for researchers using urban samples. Second, new research techniques, such as virtual reality, as a way to improve experimental accuracy, have been slowly applied in the field of soundscape.

(Assoc. Prof. Dr. Jian Xu—South China University of Technology)

## *3.8. Interviewee*

Urban green spaces provide important spaces for relaxation, recreation, social interaction, and sports. Many studies have proved that the soundscape had significant effects on visiting experiences. Our study revealed that soundscapes also had psychophysiological effects on people in urban green spaces. However, most existing studies focused on the short-term effects of soundscapes on people. However, for policy making and design practice, the long-term effects, including the effect on behavior and further health effects, are more important, which remains in question. Therefore, in my opinion, the quantitative investigation of the relationship between soundscape characteristics and the health effects is one of the most challenging problems for soundscape research in urban forested areas.

(Assoc. Prof. Dr. Boya Yu—Beijing Jiaotong University)

#### *3.9. Interviewee*

The soundscape in urban forested areas is complex, and factors such as forest structure, topography, biodiversity, seasonal changes, and environmental context should be considered. At the same time, the monitoring of forest soundscape should also change its ideas from specific monitoring points to the monitoring of the whole spatial and temporal dimensions, and from emphasizing the study of individual specific indicators to exploring the correlation of each indicator (e.g., biological vocalization is influenced by various factors such as microclimate, season, vegetation, etc.), so that the study of forest soundscape can be more systematic and perfect in the future.

(Dr. Weicong Fu—Fujian Agriculture and Forestry University)

#### *3.10. Interviewee*

The first point is the scientific evaluation of soundscapes in urban forested areas. According to the existing research methods, both physical data and perceptual data are closely related to the way people perceive the acoustic environment, which indicates that the measurement method, sampling method, sample size, interview form, and other soundscape contents may affect the objective quantification of soundscapes. The second point is the relationship between soundscape perception and visual aesthetics. Although the academic circle has conducted some studies on soundscapes and their interaction with visual landscapes, most of them are still limited to the theoretical level, and the existing results cannot effectively guide the planning, design, and optimization management of soundscapes in urban forested areas. The third point is the impact of soundscape perception on public health. Some studies have shown that the natural environment has positive effects on physical and emotional health, but more systematic empirical research and mechanism analysis are still lacking in the field of soundscape research.

(Dr. Peng Wang—Chinese Academy of Forestry)

#### *3.11. Interviewee*

I believe the most challenging work for soundscape research in either urban forests or other types of landscape is the database that could reflect both the highly spatiotemporal dynamic characteristics of soundscapes themselves and human collective values (or intersubjective values). At present, the data and evaluation in soundscape studies are mainly based on the short-term human responses to the soundscapes that could partially reflect and indicate the local perspective. However, they are very time-consuming and user- or weather-dependent, which is not very appropriate to be directly applied in landscape planning and management. To overcome these deficiencies, more efforts should be devoted to long-term acoustic monitoring (e.g., passive acoustic monitoring), which could be facilitated by intelligent sensors that could automatically record soundscapes and other associated environmental data, as well as Big Data sources that could reflect human perception and opinions. More effective and user-independent soundscape indicators and proxies that combine public intersubjective values and the acoustic environment are still needed.

(PhD Candidate Zhu Chen—Leibniz University Hannover)

#### **4. Concluding Remarks**

As mentioned by many of the contributors of this collection, multi-sensory experience in urban forests or green spaces have attracted increasing attention in the landscape field. The fields of landscape planning, design, and management also highlight the role of multi-sensory perception led by traditional visual perceptions in landscape experience,

with an aim of creating high-quality landscapes to meet the current needs. Theories and technical approaches are becoming increasingly multi-disciplinary in light of how landscape experiences can affect human mental and/or physical status and well-being. However, there is still a long way to go to reveal the mechanisms on how soundscapes are formed in different landscapes, and how internal and external factors could affect the perception process which is related to the quality of landscape experience. Crucial influential factors, especially those previously mentioned, should be manipulated in planning and design processes should be identified for practical purposes. It is also good to see that the data sources used for soundscape researches are becoming diverse. Except for data acquired from traditional public investigation, on-site monitoring methods, etc., Big Data acquired from the Internet and human reaction parameters acquired from physiological apparatus are expanding the range of multi-sources. This will definitely facilitate soundscape research progress, and will clarify multi-sensory interaction mechanisms and their associated effects. We also believe more studies should address the following topics in the future:


**Author Contributions:** Conceptualization, X.-C.H., J.L. and G.-Y.W.; writing—original draft preparation, X.-C.H.; writing—review and editing, X.-C.H. and J.L.; supervision, J.L. and G.-Y.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was supported by the National Natural Science Foundation of China (52208052, 51838003), the National Key Research & Development Program of China (2022YFF130130303), and the Humanities and Social Science Research Program of Ministry of Education of China (Grant No. 21YJCZH038).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

