Soundscape in religious historical buildings: a review

Over the past decade, a substantial body of research exploring soundscapes in religious historical buildings has emerged, yet a comprehensive summary of this work is lacking. This paper selects 74 typical studies of sound-scapes in religious historical buildings published between 2011 and 2022 to conduct quantitative statistics and visualization analysis from a bibliometric perspective. The selected literature is categorized based on the type of religious building, Christian, Islamic, or Buddhist. The literature is further segmented according to the research subject, including sound field studies, sound analyses, and evaluations of the acoustic environment. The research methods employed are also differentiated and include sound field measurements, acoustic simulations, questionnaires and auralization procedures. The analysis reveals pronounced disparities in research foci depending on the type of religious historical building. For instance, studies on the soundscape in Christian churches tend to focus on objec-tive sound field attributes and frequently employ sound field simulations to analyse the acoustic parameters of diverse church spaces and materials. Conversely, research on the soundscape in Islamic mosques prioritizes speech intelligibility and acoustic comfort, while studies of the soundscape of Buddhist temples gravitate towards the impact of natural and religious sounds on individuals. This paper anticipates the future direction of soundscape research on religious historical buildings.


Introduction
The term "soundscape" was introduced by the Finnish geographer Granö in his 1929 work "Reine Geographie", where it was used to describe listener-centric acoustic environments [1].In the 1960s and 1970s, the Canadian composer and environmentalist Schafer pioneered the study of soundscapes as an artistic concept and advocated for the creation of harmonious sound environments through listening and other means.Schafer's groundbreaking 1977 book, "The Soundscape: Our Sonic Environment and the Tuning of the World", served as a cornerstone for the field [2].Schafer initiated "The World Soundscape Project" and defined a soundscape in the "Handbook for Acoustic Ecology" in 1978 as "the acoustic environment as perceived or experienced and/ or understood by a person or people, in context." Soundscape research in Europe was further promoted through collections, books, and academic presentations.
In recent decades, soundscape research has experienced a worldwide surge.The International Organization for Standardization (ISO) issued a series of international standards related to soundscapes, namely, ISO 12913-(1-3) [3][4][5].With contributions by scholars from diverse academic backgrounds, the scope of soundscape research has steadily broadened, yielding increasingly robust academic contributions.Review studies of soundscapes have covered the soundscapes of natural environments [6], psychoacoustic indicators in soundscapes [7], acoustic environments of heritage buildings [8], psychophysiological implications of soundscapes [9], social acoustic surveys of soundscape studies [10], and the impact of COVID-19 on soundscapes [11].In 2016, Professor Kang, chair of the European Soundscape Alliance, edited "Soundscape and the Building Environment" [12] and provided a comprehensive overview of soundscape research from ten different perspectives.As the first book to systematically address soundscapes in the fields of architecture and the environment, it is a seminal work with considerable theoretical significance.
Soundscapes in religious historical buildings are increasingly recognized as vital components of intangible cultural heritage [13,14].To date, however, few studies have comprehensively summarized research on soundscapes in religious historical buildings.Although some scholars have recently conducted a detailed analysis of the acoustics of occidental Christian churches [15], there is still a lack of collation and analysis of related research on soundscapes in the major world religions.
This paper reviews the research status of soundscapes in religious historical buildings over the past decade with regard to three aspects: religious categories, research objects and research methods.Religious categories are divided into Christianity, Islam, Buddhism and other religions; research objects include sounds inside and outside the building, the sound field and the evaluation of the acoustic environment; and research methods include measurements of the sound fields, acoustic simulations, questionnaires and auralization procedures.

Literature collection and statistics
In this paper, relevant terms pertaining to the soundscape in religious historical buildings were input into Web of Science (a globally recognized database reflecting the standard of scientific research) using the following search string: (religious buildings OR religion OR Christian OR church OR cathedral OR Islam OR mosque OR Buddhism OR temple OR worship) AND (soundscape OR sound environment OR sound field OR acoustics).The search method was a subject search.The search yielded a total of 472 papers, of which 338 were dated from January 2011 to December 2022.Subsequently, 264 papers were excluded based on relevance and other criteria.The inclusion criteria were as follows: (1) studies of religious buildings that did not focus on the soundscape (e.g., research on the images, history, culture, and belief of religious buildings); (2) soundscape studies at nontraditional religious sites (e.g., medical facilities); and (3) other unrelated research.After these exclusions, 74 journal articals were selected for analysis.Because the conference papers were not peer reviewed, they were not the core content of this study.However, considering the lack of journal papers in some research areas, 10 representative conference papers were selected as a supplement to this review.
From January 2011 to December 2022, research on soundscapes in religious historical buildings generally exhibited a progressively increasing trend, as depicted in Fig. 1a.Subjects related to the soundscape included the study of sound, the sound field and the human perception of sound.Currently, most related research focuses on the sound field.Figure 1b and c display statistics related to the first author of each paper and the publication journals.Figure 2 categorizes the papers according to their professional direction.The highest proportion, approximately 32%, comprises acoustical papers.Engineering papers follow at 13%, and architectural technology papers constitute approximately 12%.There are also papers from disciplines such as archaeology, religious studies, and art theory that demonstrate interdisciplinary interest in the soundscapes of historical religious buildings.
Citation statistics for the selected papers were compiled using Web of Science data.The average annual citations were chosen as the ranking criterion because earlier publication dates increase the likelihood of citation.Table 1 lists the top ten most cited papers.The paper with the highest annual citation rate was cited 7.8 times per year on average.
The visual analysis of keywords in these papers was performed with the author's country as the main cluster item.As depicted in Fig. 3a, the countries most actively engaged in researching soundscapes in religious historical buildings were Spain (22 papers) and Italy (9 papers), followed by China (7 papers), Poland (5 papers) and Turkey (4 papers).In addition, scholars from the U.S., France, Korea, and Japan made contributions to this area of study.Figure 3b presents a time map corresponding to the development of keywords in each cluster.The X-axis represents time nodes, while the Y-axis denotes clustered keywords.Scholars in Switzerland and Australia displayed a predilection for examining bells as their research object, while over the past five years, researchers in China and Turkey have considered the functional role of the soundscape in various spaces in religious buildings.
Before 2011, authors from Spanish, Italian, Portuguese, and Polish acoustic groups made numerous significant contributions to the study of religious soundscape.For example, Spanish and Italian authors devised semiempirical models to elucidate the distribution of sound energy in distinct types of churches located in southern Spain and Italy [15].Moreover, additional groundbreaking studies referenced in their research examined crucial facets of the sound environment within intricate Christian worship settings.These included the analysis of coupled spaces, subjective preferences, and the differential limitations of certain acoustic parameters within churches [15].This work paved the way for further research.

Soundscape research in different religious categories
Christianity, Islam, and Buddhism are the three major religions worldwide and boast vast populations of adherents.Therefore, the most abundant research is on the soundscapes of the religious historical buildings of these three religions.Figure 4 shows the distribution of various religious soundscapes in the selected papers according to clustering analysis.The largest proportion of studies is devoted to the soundscape in Christian churches (73%), followed by Islamic mosques (12.2%) and Buddhist temples (10.8%).Research on the soundscapes of other

Soundscapes in Christian churches
In the past decade, significant advances have been made in the study of soundscapes in traditional Christian churches in Europe.Numerous studies have investigated church bells and analysed their characteristics Research has investigated the relationship between the sounds of church bells and people's sleep at night using field investigation methods, and measures have been proposed to control noise [25,26].The history of the development of bells in the Roman Empire from the 1520 s to the end of the seventeenth century has been traced, and contemporary people's transformation and perception of the religious soundscape have been reconstructed [27].Moreover, a methodology combining experimental and numerical techniques from materials science and music acoustics has been used to restore the bells from the church of S. Pedro de Coruche [28].From an auditory health perspective, studies have been conducted to determine whether the sound of church bells causes hearing damage to bell ringers.The results show that church bell ringers do not present symptoms of occupational hearing loss, unlike musicians and construction workers [29].
In addition to research on bells, religious music constitutes a significant portion of the soundscape in traditional churches.Studies have examined two categories of vocal music in the Greek Orthodox Church: the Byzantine chant (BC) and ecclesiastical speech (ES).Through the analysis of acoustic parameters, it was demonstrated that BC and ES differ from each other as well as from common Greek speech and opera style [30].Other studies have indicated that the most appropriate sound source positioning for music in churches is aligned with organ performance, an essential position in Baroque polychoral compositions [31].
From the perspective of traditional acoustics, research has shown that in the surrounding environment of the church, vegetation plays a role in reducing noise in the church [32].Various studies have concentrated on the influence of different architectural forms on the acoustic fields in churches, including Romanesque domed churches, Gothic basilicas [33] and Baroque domed churches [34].Earlier churches with domes were designed as meeting and preaching spaces.They boasted suitable acoustics with midfrequency values of reverberation time (T 30 ) less than 1.0 s and speech transmission index (STI) values of 0.60.Following the proclamation of Christianity as an official religion, larger spaces were built to accommodate the growing numbers of faithful people.Scholars have examined the acoustics of Christian churches from varying perspectives.For instance, Jones's "Sound of Worship: A Handbook of Acoustics and Sound System Design for the Church" systematically expounds on the acoustic design of churches [35].Similarly, Girón and others have described and analysed the principal contributions to the acoustics of occidental Christian churches from the second half of the last century to 2017 with a primary focus on ancient historical churches [15].Important milestones in church acoustics include the Reformation and Counter-Reformation movements in the sixteenth century and the Second Vatican Council in the twentieth century [15].In research on different subjects in Catholic churches after the Second Vatican Council, intelligibility has become a subject of primary importance, and many studies consider this aspect in their objectives [19,[36][37][38].
Because Christianity is one of the most influential religions globally, scholars worldwide have accorded great importance to the study of church soundscapes.While most related research has concentrated on the sound or sound field of traditional churches, investigations of people's feelings about the sound environment in these buildings have been relatively limited, indicating a potential area for further analysis.

Soundscapes in Islamic mosques
Over the past ten years, research on soundscapes in traditional Islamic architecture has been conducted primarily by scholars from Central and Western Asian countries.In Muslim society, the main activities in mosques are praying and listening to speech; therefore, the listener's understanding of speech is of paramount importance [39].In terms of the acoustic parameters of traditional mosques, research has included the measurement and simulation of mosque sound fields.These studies have identified the types of mosques that are suitable for acoustic design, which has helped save materials, energy and time for future acoustic renovation or major building replacement [40].Through the acoustic measurement of existing mosques and reconstruction of the architectural configuration of different spaces throughout history, the sounds of past mosques have been restored.This technique has become an important basis for archaeological acoustics [41].Additionally, comparisons of acoustics and speech intelligibility between mosques as a function of the size, volume, occupancy and other parameters of the main prayer hall have been used to develop a set of general acoustical guidelines for mosque design [42].One study analysed the results of the room acoustic measurementsesults of a mosque to evaluate speech intelligibility and explain the sense of sacredness generated by sound within this sacred structure [43].Further studies have analysed sound comfort and suggested a correlation between the acoustic design of mosques and the comfort level of the worshippers [44].Some researchers have even argued that the main hall of the mosque may not be suitable for holding worship activities based on the consideration of acoustic comfort [45].
By underscoring the intimate relationship between traditional Muslim sonic performances and conventional Muslim architectural spaces, the book "Music, Sound, and Architecture in Islam" identifies Islam as an ideal site for investigating the relationship between sound and architecture, which in turn proves to be an innovative and significant angle from which to explore Muslim cultures [46].This further highlights the role of acoustics in contributing to the overall spiritual experience in religious buildings and indicates a potential area for continued research and exploration.

Soundscapes in Buddhist temples
In the past ten years, scholars from East Asian countries, including China, Japan, and South Korea, have conducted research on soundscapes in historical Buddhist temples.Sound studies of Buddhist temples have shown that bells with a historical atmosphere positively impact people's subjective sound evaluations just as natural sounds do.Thus, the soundscape should align with the local natural environment and the historical and cultural background [47].By recording the various sounds heard in Han Chinese Buddhist temples and analysing their acoustic parameters, research has shown that the physical acoustic and psychoacoustic parameters of these sounds correspond to the roles they play in the temple [48].
Comparative studies have also been conducted on the sounds of Buddhist temples and Catholic churches.These studies reveal that cathedral precincts play a more important role in social functions related to mainly visual components than temple precincts do, whereas functions for religious activities related to sound elements are emphasized more in temple precincts [23].In terms of the sound field of Buddhist temples, scholars have discussed the acoustic characteristics of Japanese Buddhist temples in relation to sound source location and direction [49].By combining sound field measurements with acoustic simulations, scholars have analysed the effects of spatial elements and sound source characteristics in the main hall of the Buddhist temple and found that both fabric sound absorbers and the position of Buddha statues had a pronounced effects on the sound field [50].Additionally, the influence of the courtyard scale and layout factors on the sound field of Buddhist temple courtyards has been analysed [51].In terms of soundscape evaluation, there is a significant correlation between the acoustic environment evaluation and the sound level measured in temples [18].Among the physical acoustic and psychoacoustic parameters of the sounds in Han Buddhist temples, only sharpness is closely correlated with sound preference [48].Researchers have also analysed the relationship between soundscape evaluations of Buddhist temples and mental health and have identified the influence of the degree of religious belief-related factors as a mediating variable [52].However, research on soundscapes in Buddhist historical temples, especially traditional temples of Tibetan Buddhism and Theravada Buddhism, remains sparse, suggesting many potential areas for further study.
Although previous papers on soundscapes in religious historical buildings have mainly focused on the analysis of the three main world religions, some scholars have also explored soundscapes of other religions.For instance, a study explored the religious soundscape of Hindu traditions in Kerala and examined the role of sonic amplification in the sacrifices of Nambudiri Brahmins [53].Using acoustic and perceptual measures, the voice characteristics of priests in Indian religion were investigated [54].A study of the soundscape of Chinese Taoism, a tradition with nearly two thousand years of history, found that bird sounds significantly improved the acoustic comfort of Taoist priests [55].These studies indicate that the soundscape in other religious buildings is an essential area for future research.
The above analysis has shown that the study of soundscapes varies significantly across different types of religions.Figure 5 presents a comprehensive overview of soundscape studies of religious historical buildings classified by discipline (the darker the colour, the richer the research findings).In the field of natural science, the most in-depth research has been conducted on soundscapes in Christianity.In the field of social science, there are relatively more studies on Islamic soundscapes.However, in the field of humanities, there are fewer studies on various types of religious soundscapes.

Soundscape research on different objects
Research on soundscapes in religious historical buildings has focused primarily on the following three objects: sound field, sound, and acoustic environment evaluation.The main research objects of various articles differ, as shown in the attached table.Sound field research, which accounts for approximately 69% of all papers, analyses the acoustic characteristics of the internal or external spaces of religious buildings.These characteristics include the reverberation time and the sound absorption or reflection coefficient of the inner surface materials of buildings.Sound research, which constitutes approximately 14.8% of studies, involves the analysis of sound parameters inside and outside buildings with certain religious characteristics.Both of these research fields focus on objective acoustic parameters.Soundscape evaluation, which accounts for approximately 16.2% of research, involves respondents' subjective feelings about the sound environment of religious historical buildings.

Sound field
To date, scholars have conducted measurement and simulation-related work on the sound field of religious historical buildings.These studies considered a variety of factors, such as building surfaces made of hard stone [22,36], timber [43,56], ceramic materials [57], and even soft materials [58][59][60].Studies have also explored spaces of different sizes and shapes [31,33,[61][62][63][64][65][66][67][68][69][70][71] and the impact of different positions of indoor furniture on the sound field [72].Several studies have shown that the large places of worship in cathedrals are highly reverberant spaces [36,73] with low speech intelligibility and that they are unsuitable for listening to spoken or musical sound signals [74].The position of the sound source has no significant effect on the level of clarity [14].In one case of Orthodox churches, although the reverberation time was longer than ideal for intelligibility purposes, the short source-receiver distance contributed to maintaining clarity within acceptable limits [75].The results of a study of the Cathédrale Notre-Dame de Paris showed that compared to 2015 data, the reverberation time in worship spaces decreased significantly (20%) after the 2019 fire [76].These works not only contribute to the understanding of the characteristics of the sound field but also improve the accuracy of sound field simulation.In addition, some researchers have used virtual acoustics to analyse the church space sound field [77.78].For example, one study established virtual models to demonstrate that different structures and changes in furniture and paint had significant effects on the sound field of churches [72].Another study used scaled-down physical models of a reverberant chamber to investigate how sound-absorbing materials such as curtains should be modelled in geometrical acoustic software when hung freely in ancient churches [58].In a comprehensive investigation of room acoustics coupling, Hagia Sophia's rich formal and material characteristics were examined [79].Other studies have shown that fabric sound absorbers and Buddha statues installed in the main hall have an obvious influences on the sound field of Buddhist temples [50].An increase in the height of a temple courtyard wall increases sound reflection and, in turn, significantly increases the sound pressure level of each courtyard and making the sound fields more uniform [51].

Sound
The soundscape in religious historical buildings originates from sounds with religious characteristics, including bells, religious music, sounds of worship, and chanting.These sounds, as part of historical and cultural heritage, are as ancient as the religious buildings themselves and serve to cleanse the minds of believers [48].In the study of sounds in traditional religious places, some scholars have evaluated church sounds and the chants of monks to understand how religious sounds can be clearly and powerfully perceived [17].Other studies have synthetically evaluated the acoustics of churches by defining a dual-composite index related to music and language [37].
From the perspective of archaeological restoration and historical preservation, many researchers have studied religious sounds as a special cultural heritage.For instance, the sounds of the Cistercian churches in Portugal are regarded as cultural heritage to safeguard the memory that is part of the cultural identity of the country [80].Other researchers have reproduced the bells of Coruche (a damaged musical cultural relic) using archaeological means and have emphasized the importance of original sounds as cultural heritage [28].With the support of new technology, scholars recovered one of the lost or forgotten intangible cultural heritage values of the cathedral of Santiago, the value of sound, which helped to establish its status as a factor in cultural dynamization [21].
It has been observed that the three main religions place different emphases on sound in religious historical buildings, which is likely due to varying religious doctrines and activities.Christianity worships God as the creator of heaven and earth without deliberately highlighting the status of humanity, so the focus of sound research is often bells and Christian music [27,30,31].The main religious activities in Islam are reading and teaching the Quran, so the study of sound tends to focus on speech intelligibility in the religious venues [42,43].Buddhist doctrine emphasizes self-cultivation, with monks seeking their "true self" through daily chanting of the Buddhist scriptures.Most temples are located on scenic mountains, and research on sounds in Buddhist temples often focuses on natural sounds such as birds, wind, and rain or man-made sounds such as monks chanting, bells, and other ritual sounds [47,48].

Evaluation of the acoustic environment
In the last decade, many studies on the subjective evaluation of soundscapes in religious historical buildings have focused on analysing the effects of acoustic comfort and sound on personal physical and mental health [18,52].For example, one study showed that the gurgling sound of water from a pond in front of a mosque could overcome background noise and put hearts at ease, thereby minimizing disruptions from screaming children or passing motorcycles [81].The effect of activities in the sacred domains of the mosque on aural behaviours has also been tested [82].An evaluation of the comfort of the acoustic environment in Chinese Buddhist temples indicated that when the sound level in a temple exceeded 60 dBA, respondents were more likely to feel uncomfortable.Furthermore, the correlation between the sound level and the evaluation of acoustic comfort increased significantly under these conditions [18].
In terms of the impact of sounds from religious buildings on human emotions or physical and mental health, one study aimed to investigate whether the emotions evoked by music were enhanced by the acoustics of the space in places of worship in Western countries.The outcome partially confirmed that some perceptual dimensions of music were perceived differently depending on the spatial acoustics [83].Another study analysed the physiological influence of church bells on bell ringers.Although bells produced high-intensity sounds, the hearing loss to the ringer was very low.It is possible that bell ringers are only exposed to sound for only a short time at high sound pressure levels [29].Other studies have demonstrated that religious sounds may have a significant effect on mental health and that Christian music may reduce anxiety [84].Furthermore, the Quran's recitation may serve as an effective remedy for physical and mental ailments for followers of Islam [85], and there is variance in sound preferences among individuals in Buddhist temples with different mental health levels [52].

Soundscape research methods
The research methods used in the 74 papers on soundscapes in religious historical buildings were counted, and the results showed that sound field measurement methods were used in 54 papers, acoustic simulation methods in 42 papers, and questionnaire or auralization procedures methods in 19 papers (some of the papers adopted two or three types of research methods).Sound field measurements and computer acoustic simulations were primarily used to analyse objective acoustic parameters, while questionnaire and auralization procedure methods were utilized to analyse people's subjective perceptions of sound or sound environments in religious historical buildings.

Measurement of the sound field
Sound field measurements serve as a foundational method of acoustic research.This method measures acoustic parameters and different spatial elements, such as size, length, volume, style, material, and openness in the sound field, to evaluate the spatial acoustic environment.Adherence to the international standard ISO 3382-1 (2006,2009,2010) is mandatory during the measurement process.With regard to Christian churches, one study chose 20 Orthodox churches that differed in terms of size, architectural style and other characteristics.The acoustic measurements showed that the clarity of music parameters (C 80 ) decreased as the volume of the room increased, except for one church made of wood [86].Another study measured and analysed the acoustic properties of eight Roman Catholic churches in Poland and found that in addition to the increasing cubic capacity, the acoustic parameters worsened compared with the preferred values [38].For the measurement of the sound field of mosques, a study selected and measured 20 mosques (nine semiclosed and 11 closed) and found a clear tendency towards increased reverberation in these buildings with an increase in the room's height and volume [87].Concerning the measurement of the sound field in the main hall in Han Buddhist temples, a noticeable linear decline was found in the reverberation time, moving from low frequency to high frequency [50].
According to the sound field measurements of religious historical buildings, the reverberation time in Christian churches at 500 Hz predominantly ranges from 6-8 s, which is too long for preaching [19,21,22,60,[88][89][90].This time can exceed 10 s in larger cathedrals [24], far surpassing the reverberation times in Buddhist main halls [49,50].The likely explanation is that Christian believers associate taller building spires with proximity to God [33].Many churches, as venues for daily religious activities, are constructed on a larger scale (exceeding 5000 cubic metres in volume).Additionally, church interiors typically feature stone materials with low sound absorption coefficients and are generally empty.In stark contrast, Buddhist temples are composed of multiple buildings scattered around a series of courtyards.Each individual building, including the main hall, is usually smaller than a church.Buddhist halls, which are made of wooden enclosures with poor airtightness, also employ methods to reduce reverberation time, such as hanging fabric sound absorbers indoors [50].Consequently, the reverberation time in temples is significantly less than in churches (see Appendix).

Acoustic simulation
In recent decades, many scholars have adopted an approach that combines sound field measurements with acoustic simulations.This involves the use of computer simulation software such as EASE, CARA, CATT, ODEON, and RAYNOISE to construct virtual models of sound fields and analyse the acoustic impact of different building materials.The primary aim is to identify the characteristics and influencing factors of sound fields in both the indoor and outdoor spaces of religious historical buildings.The simulated spaces include various structural forms, such as towering churches [19,24,60,70,91,92], domed mosques [40][41][42]93], and traditional Chinese Buddhist temples [18,50].The most commonly used parameters in sound field simulation are reverberation time (RT) and early decay time (EDT), followed by the sound pressure level (SPL), sound clarity, and loudness.
In a representative study, scholars simulated 25 buildings by varying the size ratio of the length, width, and height.They considered the trade-off among the dimensional ratios of the room, volume and source position and presented formulas to predict acoustical parameters of box-shaped churches [20].These authors concluded that the degree of crowding and type of surface material in religious places significantly affect the objective parameters of sound, particularly at low frequencies [94].Other studies have demonstrated that the influence of clappers with different mechanical properties on church bells can be identified through parametric calculations [28].Furthermore, researchers created a digital history reconstruction system through the application of acoustic technology simulation to restore the sounds of various religious ceremonies [88] and liturgies [95].

Questionnaire and auralization procedure
Questionnaire (and interviews) and auralization methods have been conducted with various categories of people to understand their subjective psychological and physiological responses to sound environments and to clarify the relationship between sound or the sound environment and individuals' feelings.These studies usually fall under the domains of psychology and sociology.As an important part of acoustic science, the study of psychoacoustics is currently receiving increasing attention [48].
The number of questionnaires has usually ranged from 30 to 600.For example, in a previous study, 521 questionnaires were distributed to believers and tourists in Buddhist temples.This study found that the overall evaluation of the acoustic environment in Buddhist temples and the preference for three sounds in Buddhist temples (i.e., bells, wind chimes and chanting sounds) were significantly correlated with the mental health of the respondents [52].Another investigation was conducted in four typical Han Chinese Buddhist temples to identify the subjective and objective factors that influence soundscape evaluation [18].Considering the recent epidemic situation, the researchers compared bell-ringing practices in 2020 with a survey from 2018 in New South Wales, Australia, and concluded that bell-ringing practices in 2020 were heavily affected by regulations due to the COVID-19 pandemic, which impacted both the soundscape and those who valued sound [96,97].Furthermore, some studies have used in-depth interviews with individuals at religious sites and surrounding areas to explore the psychological and physiological reactions of believers and lay people and have proposed strategies for renovating the soundscape of churches [98], mosques [82], and Taoist temples [55].
Auralization procedures are performed through listening tests on auralized stimuli under laboratory conditions.The use of listening tests supported by virtual reality immersion in auralized religious historical buildings provides an appropriate scenario for quantifying acoustic experience and preferences within these sacred spaces.Postma and Katz presented a methodical calibration procedure for geometrical acoustic models using room acoustic prediction programmes based on geometrical acoustics to create realistic virtual audio realities or auralizations.A test case study was conducted for a typical seventeenth-century configuration of a former church [16].In another study, listening tests were employed to investigate the subjective perception of the acoustic environment of seven cathedrals in the southern half of Spain.The analysis showed both independence of the recognition of reverberation and dependence on auralized signals based on instrumental music and female and male voices [89].Another study used a subjective listening test to measure and simulate binaural auralizations and compared them according to eight acoustic perceptual attributes.A methodical calibration procedure was employed in combination with attention to control factors, which led to ecologically and perceptually valid auralizations.The test sites were three traditional churches [99].

Conclusions
Given that scholars worldwide are expanding their research on soundscapes in religious historical buildings, this study selected 74 relevant papers from the past ten years for systematic review and meta-analysis and sought to identify potential future research directions.These studies could be classified as soundscapes in Christian churches, Islamic mosques and Buddhist temples.Soundscape studies of Christian churches are the earliest and most numerous, while relatively little research has been conducted on the soundscapes of Islamic mosques, Buddhist temples or other religious buildings.Two main reasons are evident: on the one hand, modern acoustic research originated in the West; on the other hand, there is greater typological complexity in churches, and hence in their sound fields, than in mosques and Buddhist temples, which may present greater uniformity and a lower level of typological complexity.In terms of research objects and methods, many studies have focused on objective sound fields and sounds and have employed methods such as sound field measurements and computer simulations.Based on questionnaire surveys and auralization procedures, the influence of the soundscape in religious historical buildings on individuals' feelings has been determined.
In light of the above analysis, future research could compare soundscapes between different types of religious historical buildings or soundscapes in other religious historical buildings beyond the three major religions.Studies could also analyse the effect of religious architectural soundscapes on human physiological indices or adopt the methods from sociology, aesthetics and other disciplines for cross-disciplinary research.Despite the progress made, there is much to be done in this field.Collective effort from scholars worldwide is necessary to establish a cohesive research system, conduct a detailed analysis of the cultural and social value reflected in religious soundscapes, and identify the potential benefits to human health.
This review has certain limitations.It primarily selected architectural soundscape research on the world's three major religions and lacked an analysis of soundscapes of folk or local religions or of religious historical buildings centred on mythology (such as the Parthenon Temple).Only English-language literature was considered and the papers were manually selected, which may have led to some omissions.Nevertheless, by summarizing the current research status, readers can gain a broad understanding of recent decades of research on soundscapes in religious historical buildings.This paper also forecasts future research trends with the goal of promoting a more comprehensive understanding of soundscape studies.Sound field/Spain The tapestry hangs at Cologne Cathedral Acoustic simulation; modelling The effect of textiles on church acoustics proved to be more evident at medium frequencies than at low frequencies, as expected considering sound absorption coefficients.The reverberation times and centre time were affected most, while early-to-late energy ratios showed smaller variations, probably as a result of placing the material in a position that did not affect the stronger reflections.

Measuring acoustic parameters
The results confirm that both the distribution of the early reflected energy and the acoustic characteristics of reverberation, spaciousness, strength, and clarity support the favourable transmission of spoken or sung messages and music and exhibit remarkable dependence on sound-source location and its areas of influence.

Key findings
Iannace, 2016 [57] Sound field/Italy The Cathedral of Benevento Measurement of the acoustic characteristics; study sound-absorbing materials; evaluate the impact of sound insulation systems on acoustic characteristics with room acoustics software The introduction of material for acoustic correction reduced the length of the sound tail and improved both the understanding of speech and listening to musical performances.
Kosała, 2016 [70] Sound field/Poland Hungary Church Using computational models to evaluate acoustic quality The analysis showed that the audience improves the acoustic properties of the church, as indicated by a 23%higher average value of the global index GAP (Global Acoustic Properties) index = 0.8 and the distribution of GAP index values in the space defined by test points, which is more uniform compared with the church without an audience.
Martellotta, 2016 [63] Sound       Sound field/Turkey Hagia Sophia of Istanbul Acoustic measurement; acoustic modelling; simulation and analysis Among the many variables, the source-receiver distance and positioning within different subspaces appear to be the underlying determinant of multislope sound decay pattern.The Grand Mosque of Yogyakarta Acoustic measurements Background noise can be overcome with the gurgling sound of water in the pond in front of the mosque that can put hearts at ease so that noises from screaming children and passing motorcycles can be minimized.Sukaj et al., 2021 [39] Sound field/Albania Ottoman mosques

Acoustic measurements
After comparing the acoustics of these two mosques, some solutions to improve the acoustics are suggested.The cathedral precincts play a more important role in social functions related to mainly visual components than the temple precincts do, whereas the functions for religious activities related to sound elements are emphasized in the temple precincts.The results did not reveal any vocal deviations in purohits using perceptual and traditional acoustic measures.In contrast, the Indian Hindu purohits had higher intensities and increased cepstral values compared with normal controls.

Microphones and loudspeakers
In contrast to the private sacrifices of previous generations, "amplified sacrifice" is now carried out as a public Hindu festival with thousands of attendees and a full suite of marketing and media coverage.In this way, the local, sonic amplification of performance tracks with a regional, cultural "amplification" of Vedic ritual and Nambudiri identity.

Fig. 2
Fig.2The proportion of professional field research on soundscapes in religious historical buildings

Fig. 3
Fig. 3 Visual analysis of religious soundscapes in the past decade

Fig. 4
Fig. 4 Cluster analysis of religious categories

Fig. 5
Fig. 5 Overview of religious soundscape research Zhang et al., 2016 [18]  Perception of sound/China Four typical Han Chinese Buddhist temples Soundscape evaluation questionnaire; investigate subjective and objective factors influencing soundscape evaluations There was little difference among the mean values of the day-long sound level measurements of the four temples and a significant correlation in the sound levels between each temple.The sound preferences are significantly correlated with sharpness value of the sounds in temples.Zhang et al., 2018 [48] Perception of sound/China Han Buddhist temples Questionnaire The physical acoustic and psychoacoustic parameters of various sounds correspond to the roles they play in the temple.Buddhism-related man-made sounds dominate the sound environment in temples.Zhang et al., 2020 [50] Sound field/China Main Hall of Chinese Buddhist temple Sound field measurement and acoustic simulation This study investigated the effects of spatial elements and sound source characteristics in a Main Hall and revealed that both fabric sound absorbers and Buddha statues had a pronounced effect on the sound field.Zhang et al., 2020 [51] Sound field/China Han Buddhist temples Sound field model; measurements In traditional Buddhist temples, spatial elements, such as the height and sound absorption coefficient of temple courtyard walls, the position of courtyard partition walls, and the position and height of bell towers, could significantly affect the sound pressure level (SPL), reverberation time (RT), and musical clarity (C 80 ) of each courtyard.

Authors/Years of publication Titles of publications Annual average number of citations/total citations Keywords Main contents
The top ten papers with the most annual citations

Table 2
Research papers on soundscape in Christian historical buildings

Table 3
Research papers on soundscape in Islamic historical buildings

Table 3
(continued)The results of the analysis show that the overall acoustic and speech quality of Masjid Jamek is better when compared to the overall acoustic and speech quality of Masjid UPM.These results are then used to develop a set of design recommendations to ensure adequate speech intelligibility quality a mosque.

Table 4
Research papers on soundscape in Buddhist or other religious historical buildings

Table 4
(continued)With the increasing proportion of the natural elements in the visual landscape in the temple, the acoustic comfort of Taoist priests and lay people increased significantly with the addition of bird sounds.However, with the increasing proportion of Taoist scenes, Taoist music only significantly improved the acoustic comfort and heart rate of lay people.