The Unique Acoustics of Traditional Chinese Interlocked Timber-Arched Covered Bridges

The traditional Chinese structure of the interlocked timber-arched covered bridge serves multiple purposes within Chinese culture—both as a physical connection between communities, but also as a function space for various rituals and performances. Because of these structures multiple uses, the acoustics are different from other bridges or event spaces. In this episode, we talk with Dongxu Zhang (Guangzhou University) and Jian Kang (University College London) about their work to better understand the unique acoustics of these structures.

Associated paper:
- Dongxu Zhang, Guanyu Ren, Fei Cheng, Dong Xiao, Mei Zhang, and Jian Kang. "Sound field characteristics and influencing factors of traditional Chinese interlocked timber-arched covered bridges." J. Acoust. Soc. Am. 158, 1156-1176 (2025). https://doi.org/10.1121/10.0038959

Read more from The Journal of the Acoustical Society of America (JASA).
Learn more about Acoustical Society of America Publications.

Music Credit: Min 2019 by minwbu from Pixabay. 

Transcript

ASA Publications (00:24)

Today we're delving into the world of architectural acoustics. With me are Dongxu Zhang, Guanyu Ren, and Jian Kang, whose article, “Sound field characteristics and influencing factors of traditional Chinese interlocked timber-arched covered bridges,” recently appeared in JASA. Thank you for taking the time to speak with me today. How are you?

Jian Kang (00:44)

We are fine, thank you. Thank you for this opportunity talk about this bridge’s sounds. Thank you.

ASA Publications (00:51)

Yeah, this is a super cool subject. So first, tell us a bit about your research backgrounds.

Jian Kang (00:56)

My name is Jian Kang, and I’m professor of acoustics and soundscape at the Institute of Environmental Design and Engineering, Bartlett Faculty of Built Environment, University College London. And my field is architecture and environmental acoustics. We do research on how sounds propagate inside architectural spaces and also how sounds propagate in urban areas. That's our research area.

Dongxu Zhang (01:32)

[Chinese]

Jian Kang (01:50)

Yeah, also with me is Professor Dongxu Zhang from Guangzhou University. And he did a PhD in architectural acoustics, and his specific field is about sound field in traditional Chinese architecture.

Also with us is Guanyu Ren, and he is a masters student working with Professor Zhang, and he has been working in the sounds in the bridges for a while.

ASA Publications (02:21)

Awesome, great. Well, welcome to all three of you. So what are Chinese interlocked timber-arch covered bridges and what makes them different from other covered bridges?

Jian Kang (02:33)

Yeah, the Chinese interlocked timber arched covered bridges, quite long name, is quite typical Chinese wooden architecture form, and it has a history of about 900 years. Different from other kind of bridge houses, they are built purely with wood and also using a structure called mortise and tenon joints. That means its all connected by wooden components without using any metal components, such as nails. Yeah, that's a special feature. It has a function of connecting two sides of a river, of course, and also actually connecting two communities. And more importantly, it's a place for different activities, including cultural activity and also religious activity. So it's a quite important architecture type, and same time quite important typical kind of cultural place.

ASA Publications (03:41)

Interesting. So it doesn't act just as a bridge, it's also a community space, essentially. 

Jian Kang (03:46)

Yeah, that's right.

ASA Publications (03:49)

So why were you interested in studying the sound fields of these types of bridges?

Jian Kang (03:54)

The reason to study this is because this kind of architectural forms has quite a special acoustic feature because it's quite long and thin and also it's not totally covered, it's got some openings. And from acoustic viewpoint, it's quite interesting and different from other kind of building types. And of course also because the type of activity is not only for speech, but also for some performance and religious activity. So we would like to see what the sound field is like and also how this sound field satisfies or unsatisfies the activities performed there. So that's the motivation we want to study these kind of spaces in terms of acoustics.

ASA Publications (04:42)

Yeah, yeah, very interesting. So what do we know about the sound fields of Chinese interlocked timber arched covered bridges?

Jian Kang (04:50)

I think before our study, there was a very limited study, we were aware of, about sound fields for this type of building type. And of course there are a lot of, because that's quite important architectural form in Chinese architecture,  so many studies have been carried out about architectural side or cultural side, such as how this type is developed in history and how it was constructed and what's the artistic or architectural value of it. But from acoustic viewpoint, very little study has been done. That's why we want to study this from acoustic viewpoint.

ASA Publications (05:34)

Yeah, yeah. What bridges did you use for your study and how did you take measurements of their sound fields?

Jian Kang (05:42)

Maybe, because the actual measurement was done by Professor Zhang's team, so maybe Professor Zhang you can say something about this in Chinese, and I can translate this into English.

Dongxu Zhang (05:55)

在实地研究中，我们选择了两个省内四座具有代表性的木拱廊桥进行声学测量，包括登龙桥，双门桥、福田桥和兰溪桥，它们在大小、长度、开门方式等方面各具特色。

由于现场条件限制，桥梁所处位置偏僻且桥梁上缺乏可供声学设备使用的电源，所以使用气球爆破作为脉冲声源进行混响时间测量。

根据祭祀活动中演唱者的典型位置和廊桥的长度，我们在桥梁上设置了多个测点，测量的声源放置在廊桥开间中央的佛龛内或桥梁入口的中央，以尽可能真实地反映廊桥内部的声学体验。声学接收点则布置人群在祭祀活动中所处位置，一般设在在靠近声源的开间中央或两柱之间的中点。声源和接收点均设置在距地面1.5米处，两者之间的距离超过三米。 

为尽量减少测量误差，每个接收点的混响时间测量至少重复三次，并取其平均值用于后续分析。

Jian Kang (07:17)

Thanks Professor Zhang for the detailed description. So in our study, we selected four typical bridges in two provinces, and they’re named Denglong Bridge, Shuangman Bridge, Futian Bridge, Lanxi Bridge, and they differ in terms of size, and the length, and also opening type. And because the on-site condition is quite limited because it's quite a remote location, and there are not power for acoustic equipment, so we use balloon for the source because this is typical method to measure reverberation time. You generate impulse and then you measure how this space responds to this impulse, called impulse response. And the way to introduce this impulse can be done by acoustic equipment, by loudspeaker, but also you can burst a balloon to generate impulse, which is an acceptable and usual method to measure reverberation time. And in this case, because the site condition, we use burst balloon as a sound source. And then in terms of receiver, we put a lot of receivers along the bridges, just to simulate how users hear the sound. So the source was placed in where the singers were, or are, and then we put a lot of receivers. And for each receiver, to ensure the measurement is accurate, we made several balloon bursts, and then we can make average, so we get quite accurate and representative results along the bridge, so that was matching the method

ASA Publications (09:12)

That's really cool. It's fun and kind of halfway low tech. The receivers are obviously not low tech, but the balloon is. So what did you learn from these measurements? 

Jian Kang (09:23)

Yeah, so, because reverberation is the most important acoustic index for architectural acoustics. Actually, the whole architectural acoustics started from the introduction of reverberation time by Sabine, who was a professor in Harvard University. So he established discipline of architectural acoustics about 100 years ago. So his most important contribution or index is about reverberation time. It's still being used a lot in modern times. So that's why we measured the reverberation time.

And we found for these four bridges, the reverberation time and middle frequency, which is where the voice or singing sounds are, is about 0.37 to 0.5 seconds. And they are not particularly long, but it's quite helpful. And in terms of frequency distribution, from middle frequency to high frequency the reverberation time becomes shorter, which is quite common in different kind of room acoustics or buildings. And we also found for all four bridges, the end wall has quite important impact on the sound field. The wall at two ends, because it reflects sound back from two ends. And the third finding or interesting phenomenon is the reverberation time at the bridge entrance and also where the statues are, which is in the middle of bridge, they are shorter, but between these two points, the reverberation time is slightly longer. So that means along the bridge, the RT is not evenly distributed. There are some variations along the length. So people will hear different sound effect when you move from entrance of bridge to middle of bridge. So those are some interesting findings we found in these bridges.

ASA Publications (11:22)

So there's better seating and worse seating or better standing areas and worse standing areas for depending on what you want to hear or how you want it to sound.

Jian Kang (11:41)

Yeah, that's right. Yeah, that's a feature of this long space compared to regular spaces. That's why we want to study this kind of spaces.

ASA Publications (11:51)

Yeah, right, right. So you went on to simulate the sound fields of the bridges. What was your goal with these simulations?

Jian Kang (11:59)

Because there are many such stages in these two provinces, and we only measured four bridges. So on the one hand, through this measurement we learned the feature of this kind of bridges, but on the other hand we want to know generally speaking what's the general tendency of those kind of bridges. What happens if bridge become longer and cross-section become bigger, and also when the boundary condition becomes different, ceiling condition becomes different. So what are the impact of those factors? If we are going to do measurement for all the bridges, then it's very time consuming and also some bridges are not well preserved, so you can't have idealized situation. So that's why we think using computer simulation you can control all the conditions, and then you can learn what is the effect of each condition or each factor, for example the size, length or those boundary conditions, so we can get some generalized conclusion.

ASA Publications (13:10)

That it makes a lot of sense. So how did you go about building a standard model for these bridges?

Jian Kang (13:17)

Again, Professor Zhang did more detailed work. Maybe you can say something, I can then translate into English.

Dongxu Zhang (13:27)

本文的标准木拱廊桥声学模型是基于我们在福建和浙江的实地调研以及既有研究建立的。模型主要模拟能够举办各类祭祀和庆典活动的大型廊桥。其几何尺寸和布局等等参考了所调研的 99 座廊桥，从而能够较好地反映木拱廊桥的普遍特征。

我们使用odeon声学软件模拟标准木拱廊桥声场，模型内的声学参数（例如吸声系数与散射系数）是参考以前对多个类似材料的中国传统建筑的实测与模拟，然后我们对四座廊桥的声学模型进行了反复调整与校正，直至四个廊桥的模拟结果的混响时间都能够较好地对应实际测量数据，从而确定了适用于木拱廊桥声场在odeon模拟软件中设置的声学参数，建立了木拱廊桥的标准声学模型。

Jian Kang (14:37)

Yeah, thanks, Zhang. So, the computer model is based on large scale survey in Fujian and Zhejiang province. And this survey is mainly architectural survey about how big and how long and what's cross-section material of those bridges. And then we built bridge based on on those slightly larger bridges, which are for various religious and celebration activities. And the size and boundary conditions are based on 99 bridges. And so this can reflect generalized condition of this type of bridges for relatively large size of activities.

And after you build the architectural model in terms of size and dimension, then we need to assign material features which are very important for acoustic simulation, which we use a software called Odeon Acoustic Simulation Software. And to assign the acoustic parameter, we used the research we did for other kinds of Chinese architecture, such as temples and other kinds of architectural acoustics material. We already knew some of the material features, so initially input those. And then we do the simulation and the compare with our measurements for these four bridges, so then calibrate the material input until, for these four bridges we can get very good agreement between measurement and simulation. From now on we are very confident our model is not only representative but also very accurate in terms of material assignment. And based on that we can do all kinds of simulations, for example, increase the length and decrease the cross-section and the change of materials, higher the ceiling, lower the ceiling, to see what happens if a material or a size design changes and this can represent this 99 or more bridges. So that's the whole process.

ASA Publications (16:52)

Okay, that's very cool. It sounds useful, you know, too. So then you use this to do some further analysis of the factors that might impact a bridge's sound field. So what did you find in these analyses?

Jian Kang (17:03)

Yeah, through all these simulations we found the three major factors which influence sound field is firstly size of the space, and also the enclosedness of the bridge and what kind of material or kind system it uses, and also occupancy condition, so whether it's fully occupied or half occupied. For example, we found if you increase the bridge length, or add these end walls for the bridge on two sides, then you can greatly increase the reverberation time. And when the panel on the bridge angle becomes increase by 15 degrees, then the reverberation time will decrease by about 10 percent. So that means that's also a very important influencing factor, which is how you enclose the bridge. And between occupied and unoccupied conditions, because people are good sound absorbers and also good sound diffusers, but we also want to know when more people or less people, what’s the difference in sound field. Between occupied and unoccupied conditions, the reverberation time varies by about 20%, which is quite significant. Also, although people haven't done sound field for these type of bridges, but there were some previous work for long spaces, for example underground stations. Actually, that was one of my previous work. I had a book called Acoustic of Long Spaces. So we compared this bridge sound field and there were also other kind of long spaces such as underground station. And we found there are some similar acoustic features. And that's also quite interesting. But there's also differences because the construction method and also the size, everything are different, but there are some basic similarities between different long spaces. So this also contributes to the discipline in general, not just for this type of architectural types.

ASA Publications (19:32)

Interesting. Yeah, right. Because you can see where the overlap is between your subway station and your Chinese interlocked timber bridge and, you know, see what's common and what's different and deduce kind of what factors may or may not influence, right? Yeah.

Jian Kang (19:46)

Yeah, this work not only contributes to kind acoustic theory but also kind cultural and the heritage side.

ASA Publications (19:58)

Right, right. So, getting into kind of other spaces, how do these bridge sound fields compare to those of performance or religious spaces? And actually, all this talking about the shapes and such, I was thinking about those, what is it? Shoebox concert halls? Like how does this compare to shoebox concert halls and so on?

Jian Kang (20:18)

Yeah, generally speaking, this bridge space has very short reverberation time. As I said, it's 0.37 to 0.5 for the measured results. So, for ideal conditions of concert hall, modern concert hall, you will need a reverberation time of about two seconds. But the interesting thing is we also studied other kind of Chinese performance spaces. For Chinese performance, like Beijing Opera or other similar performances, actually the ideal reverberation time is about one second. So, although they are all performance spaces, firstly, their actual condition is different, but also the ideal requirements are also different depending on your activity and also type of music, for example. So it's a very basic kind of question whether a place is designed for activity or activity kind of performs according to the place, but that's more deep exploration and we can maybe study across different places and I hope this bridge acoustics or situation or case study can contribute to this general overall discussion.

ASA Publications (21:39)

Yeah, right, which came first, the chicken or the egg, the performance space or the performance itself, right? 

Jian Kang (21:44)

Yeah.

ASA Publications (21:45)

Yeah. How can new or renovated Chinese interlocked timber-arch covered bridges be optimized for acoustics? Are you wanting to get it to have a one-second reverberation time, or would you still be keeping it within the normal expectations for this structure?

Jian Kang (22:02)

Yeah, I think this will need more studies about what is people's preference. If people are very happy about this, maybe we don't need to do much. It's just a kind improvement of the architectural condition rather than acoustic condition. But if we find through subjective evaluation, for example, if you have a computer simulation, we can ask people to listen. What if it becomes longer? If people prefer that, maybe in the future we can do some innovation according to that kind of finding. And the possible way to do that without destroying the current heritage is maybe, make the ceiling slightly different, use different material but that look same, and also slightly change the enclosed condition and also maybe change the end wall. So overall you can still keep the original features, but there are some kind improvements, if needed. So there are some potentials and room for improvement.

ASA Publications (23:07)

That makes a lot of sense. What are the next steps for this research?

Jian Kang (23:10)

Yeah, I think, as I said, we want to do some subjective evaluation about what is the current quality and the expected quality and the wished quality, and then towards innovation of this kind of bridges spaces. And on the other side, I think we also mentioned before, it's quite useful to compare these kind of spaces with other kind of performance spaces, also compare with other kind of long spaces, and to put this research into a wider context, and so this study can contribute to different aspects of discipline of acoustics and also discipline of heritage and beyond.

ASA Publications (23:55)

Yeah, it can help with a variety of viewpoints and understandings of, you know, both the artistry and the heritage, as you mentioned, and the acoustics and soundscape aspects of it, yeah.

Well, thank you again for taking the time to speak with me today. It's really fun hearing about Chinese interlocked timber-arch covered bridges, which aren't something we encounter in the US, obviously. I wish you the best of luck in your research, and have a great day.

Jian Kang (24:25)

Thank you for giving us the opportunity. That's a great opportunity to talk about our research and also let more people know about interlocked bridges.