The term "soundscape" was first coined by Murray Schafer in his book "The tuning of the world" published in 19971,2. In 2014, the International Organization for Standardization systematically elaborated the definition of the soundscape: "acoustic environment as perceived or experienced and/or understood by a person or people, in context"3. It also defined the constituent elements of the soundscape as sound elements, environmental elements and audio receivers. The physical characteristics of sound include loudness, pitch, and timbre. In the discipline of soundscape ecology, sound is classified into three distinct types: biophonies, geophonies and anthrophonies4,5.

Classical Chinese gardens reflect the profound metaphysical beauty of Chinese culture in scrupulous garden design, and they are a significant component of world cultural heritage. The creation of classical Chinese gardens places a strong emphasis on crafting a multisensorial experience through the sensescape, in which soundscape plays a critical role6. Early studies have revealed a remarkable consistency in the adoption of similar soundscapes and sound sources in classical Chinese gardens7. However, most of these studies provided only a summary and categorization, without further analysing the physical characteristics of these sound sources and the reasoning behind the preference matrix.

Most of the soundscape research focuses on loudness, despite the fact that physical characteristics of sound also include frequency, timbre, and duration. Scholars have pointed out that once the loudness of sound remains within peoples comfort zone, evaluation of soundscape will mainly depend on the type of sound source and personal subjective preference, while ignoring the influence of sound frequency, timbre and other physical properties on preference8,9. Despite the profound impact of frequency attributes on human sound perception and the growing recognition of their therapeutic effects on physical and mental well-being10, relevant research in this domain remains relatively sparse. Many scholars use soundscape data, combined with theories and methods of sociology, psychology or physiology to evaluate soundscape. On the one hand, such as Hunte and Jo, etc. concluded that the sound of water, wind, birds and other natural sounds have healing effects on human beings, but did not explore the healing mechanism of these soundscapes in depth; on the other hand11,12, Casc detailed standardizes the standard of using the frequency characteristics of sound to measure species diversity, and discusses the role of sound frequency in the study of species diversity13. This study takes the most recorded 12 soundscape in classical Chinese gardens as the research object, analyzes its frequency characteristics, discusses its healing mechanism, and provides a frequency perspective for the study of soundscape healing mechanism. Audio data for these identified soundscapes is sourced from the BBC Sound Effects website, allowing for an acoustic analysis that focuses solely on the frequency dimension, while mitigating the influence of the physical variable of loudness. By analyzing sound spectrograms, Bai et al.14 divided sound into two types based on sound duration: discrete and continuous. Discrete sounds tend to be associated with musical melodies; a melody is composed of two or more tones, which are discrete vocal events with pitch15. Continuous sound, on the other hand, forms a spectrogram through methods such as Fourier transform16. The spectrogram is the basis for distinguishing between colored noise and white noise17. Not all noises are harmful to human health. Pink noise and white noise can provide a soothing quality, by masking out disturbing sounds from the external environment to inhibit the activation of brain activity, which is evident from the reduced complexity of electroencephalogram (EEG) recordings18.

The ancient Chinese culture attached great importance to sound perception. In ancient Chinese idioms, when the words "ear" and "eye" are mentioned together in an idiom, the word "ear" is always placed before the word "eye"; the sound comes first, and the form follows. Soundscapes have also been recorded in various ancient texts. Wu et al.6 have conducted an extensive literature review on the soundscape present in the "Book of Songs" (also known as Shijing), and found that 86 of the 305 poems in the "Book of Songs" involve soundscapes.

In classical Chinese gardens, the creation and crafting of soundscapes is a meticulous and significant art form with a longstanding history. Tang et al.19 have conducted an extensive literature review on the soundscape present in "Yuanye", the first comprehensive garden art monograph in China. They found 21 descriptions of soundscapes, with biophonies (e.g. bird calls) as the most commonly recorded, followed by geophonies (e.g. water and wind sounds), and lastly anthrophonies (e.g. singing and musical performance). Xie and Ge7 also conducted a literature review on the important contemporary works of classical Chinese gardens, and found a similar ranking of the soundscapes based on their frequency of occurrence20,21,22,23,24,25.

Although the ancients did not systematically acquire relevant acoustic theory and technology, their understanding and experiences have been demonstrated in the mastery of soundscapes in the making of the garden landscape20,23. There are many well-known soundscapes in existing classical Chinese gardens. In the Humble Administrators Garden, the "Pine Wind Pavilion" (Songfengting) has a horizontal banner inscribed with the phrase "Listen to the Pine Wind", which is an extract from the poem "I love the pine wind, and the courtyard is planted with pine trees. Every time I hear the sound, I am delighted." Fig.1a "Rain Pavilion" (Tingyuxuan) has a pond filled with lotus flowers in the front, and with Musa trees and bamboo planted by the side. When raindrops fall on these different plants, they produce responses in listeners; often, these responses will be highly individual (i.e. differ between people) (Fig.1b and c). Chengde Mountain Resort has a building dedicated to listening to the wind blowing through an old-growth pine forest (Fig.1d). The "Orioles Singing in the Willow" (Liulangwenying) is a lakefront park located on the southeast bank of the West Lake, which is famous for bird calls from orioles (Fig.1e). The Octave Stream (Bayinjian) in Jichang Garden demonstrates skilful stonework, creating an enclosed environment which isolates noise from the outside world and amplifies the trickling sound of spring water (Fig.1f).Soundscape research centers on two major aspects: the objective assessment of the physical characteristics of sound, and the subjective evaluation of the perception of sound. However, most research on soundscape perception focused mainly on the perceived loudness, despite the fact that frequency and timbre are equally important components of the physical characteristics of sound26. In this paper, we conducted an extensive systematic literature review of related soundscape research and selected 12 sound sources that are most typically present in classical Chinese gardens. We then collected the respective audio samples from the BBCs library of Sound Effects to classify these 12 audio samples into discrete and continuous sounds based on spectrogram analyses. Their frequency distribution was further analyzed by the Pitch Estimation Algorithm and LSTM neural network noise type judgment method based on the theories of musical tones, melody and colored noise classification. According to the results, the frequency distribution characteristics of discrete sounds indicate a pitch change, while continuous sounds show white noise or pink noise. The preference mechanism for these sound sources from the perspective of healing and health benefits will be discussed in this paper. This study presents a novel approach by explaining the physical attributes and preference mechanisms of soundscapes in classical gardens from the perspective of sound frequency. The innovation of this study can be observed in two aspects.

Site photos of existing classical Chinese gardens with distinct soundscapes. Inset images show soundscape representatives of the 6 in existing classical Chinese gardens.From left to right: (a) Tingyuxuan; (b) Songfengting; (c) Liutingge; (d) Wanhesongfengdian; (e) Liulangwenying; (f) Bayinjian.

Firstly, methodological innovation: The utilization of LSTM neural networks and principles from music theory to analyze audio data led to the discovery of frequency-based features that are favored by individuals. Specifically, it was found that the frequency distribution of continuous sound sources conforms to the distribution patterns of white noise and pink noise. This approach provides a unique methodology for understanding and interpreting the frequency characteristics of preferred sounds, that is discrete sounds exhibit two or more variations in pitch. Secondly, theoretical innovation: The study offers an explanation from a therapeutic perspective as to why people are drawn to sounds with these frequency features. By highlighting the potential healing effects of these sounds, the research contributes to the improvement of urban green spaces and the enhancement of residents physical and mental well-being. The study provides a frequency-based perspective and serves as a valuable reference for the enhancement of acoustic environments in urban green spaces and the promotion of residents overall health.

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