Instant Genius - Professor Trevor Cox: Was Stonehenge an ancient acoustic chamber?

Episode Date: September 14, 2020

For decades, Stonehenge, the mysterious prehistoric circle of stones built on the Salisbury Plain in Wiltshire, has left scientists scratching their heads. Who exactly built it and what was it used for? In the latest attempt to get to the bottom of this mystery, a team of engineers based at the University of Salford have 3D-printed a scale model of the ancient monument in order to investigate the effect its unique structure would’ve had on conversations, rituals, and even music. We spoke to Professor Trevor Cox, the acoustic engineer heading up the study, to find out more. Let us know what you think of the episode with a review or a comment wherever you listen to your podcasts. Subscribe to the Science Focus Podcast on these services: Acast, iTunes, Stitcher, RSS, Overcast Read the full transcription [this will open in a new window] This podcast was supported by brilliant.org, helping people build quantitative skills in maths, science, and computer science with fun and challenging interactive explorations. Listen to more episodes of the Science Focus Podcast: Trevor Cox: To become Prime Minister, change your voice Natalie Starkey: What asteroids can tell us about our Solar System Mike Garrett: Is there anybody out there? Colin Stuart: The most mysterious objects in the Universe Dr Lucy Rogers: What makes a robot a robot? Pete Etchells: Are video games good for us? Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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So at low frequencies, sound struggles to get between the gaps of these very large uprights because the gap's a bit too small and it gets constrained and so you get this base boost. We're at high frequency, it's easier for it to disappear down the gap because the sound of the sound wave is smaller than the gap. So first, you know, first bounce, half the energy is gone because about half the area is air. Effectfully, it's not stone. And so it decays away much faster. You're listening to the Science Focus podcast from the BBC Science Focus magazine team. With the UK's best-selling science and technology monthly, available in print and in several digital formats throughout the world. Find out more at sciencefocus.com or look out for us in your app store.

Welcome to the Science Focus podcast. I'm Sarah O'Ruipi, online assistant at BBC Science Focus magazine. For decades, Stonehenge, the mysterious prehistoric circle of stones built on the Salisbury Plain in Wiltshire, has left scientists scratching their heads. Who exactly built it and what was it used for? In the latest attempt to get to the bottom of this mystery, a team of engineers based at the University of Salford have 3D printed a scale model of the ancient monument. They investigated the effect its unique structure would have had on conversations, rituals and even music. Jason Goodyear, commissioning editor of BBC Science Focus magazine, spoke to Professor Trevor Cox, the acoustic engineer, heading up the study to find out more. So your current project on Stonehenge is all about the sonic and acoustic properties of Stonehenge. So I think that's really interesting because usually the research on Stonehenge is focused on the,

astrological position of it or what potentially it means, was it a calendar, was it used for rituals or whatever. So where did the idea to study the acoustic properties of Stonyland's actually come from? Well, there's been a few people have looked into it in the past. There's a whole field called Archaeoacoustics, which is the, you know, the acoustics of archaeological sites. And I guess what motivated me was people were publishing about what the acoustics were like in the current site, but of course it's a ruin. And I used my sort of architectural acoustic knowledge to think, well, it would have been very different back in the day. And that was the kind of nugget of sort of thought to investigate this. And I think it's also, you know, it's very easy to think about it very visually

and also the, you know, the problems of how do you construct this site. But if you think about use of this site, I mean, just think of a ritual that we have nowadays, but it doesn't involve sound. I mean, it would be very surprising if they weren't in there speaking, singing, playing instruments of some form. And as soon as you have surfaces that reflect sound, you have the music and the speech being enhanced, and that's what we're investigating. Sure, so I was wondering, the whole concept of this,

and now you've explained it, it does seem fairly, yeah, an obvious thing to want to know. But to me it was sort of like, oh, it's something I'd never thought of. Are there any sort of precedence in other parts of the world about other monuments or of this kind that within, interesting acoustic properties. I guess the classic one in architectural acoustics would be amphitheaters, which have been studied and are supposed to have these amazing, mythical,

acoustic properties. So they've been very thoroughly studied. And there was a study many years ago looking at burial mounds and looking at how the acoustics within those might have affected people's behavior. And you've got stuff across in Mexico looking at Mayan pyramids. So there's been little bits here and there, but no one has looked at stone circles before. and no one has tried to apply this method of acoustic scale modelling to stone circles and prehistonic monuments.

So one sort of immediate challenge that struck me is, in its current, it's sort of very, I don't know how many stones there are in total, but there aren't so many standing up now. So how can we be, how do you go about reconstructing, accurately reconstructing, what this would have looked like thousands of years ago. Well, the first thing is an acoustics professor is not to do it myself, but to talk to the archaeologists, because I'm not the expert, and you need to, you know, really need to tap into the people who know. So there's been lots, as you can imagine, of archaeological studies at Stonehenge, and there's been various, you know, papers that outline what they thought used to happen. Because we think of Stonehenge as being, you know, one fixed thing, but it went through many stages,

even before it started to, a bit started to go missing. So, in fact, you know, the first thing was a very large stone circle, maybe 100 metres across back in sort of 2,900 BC. I mean, the model we're looking at is much, you know, I say much younger, but 2,200 BC, it's still pretty old, when there was 157 stones. And you're right, there's quite a lot which are either lying on the floor or actually completely missing. So historic England had done a reconstruction.

of the monument of different states, when it was actually putting together its new visitor centre a few years back. And we used that as a basis, which is based on some of the sort of latest archaeological evidence for how the stone circles may have been in the past. And it's kind of the, you know, I'm not an archaeological expert, but it's that kind of thing where you find a hole, you know, you presume that hole had something in it, you know, these kind of thing. And you look at stereography, so you're looking at, you know, which bits lie on top of other things and work out the order of when things have happened. But there is some, you know, quite a lot of uncertainty about how it was configured and in what way over the years.

Yeah, so you mentioned there that it was a kind of, it was built up over many, many years and it had different configurations. So we'll go back to that in a bit because that's something I think is really interesting. But so just going through the sort of process of how you went through this. So I believe you used 3D laser scanners at the site or that's, where the data, original data for the model came from. Yeah, so Historic England had done this very remarkable laser scanning. So they had detailed knowledge of all the stones that they had used in the reconstruction. I mean, fortunately, I started off with their nice software version of the stonehenge geometry.

I didn't have to reconstruct it myself. So I was given a model where all the stones were in where they should have been in various configurations. But then you have the problem of physically making them. And, you know, it's 157 stones, and we sort of started off thinking, well, we'll 3D print them. And then we worked out it would take six months to 3D print them. And that was sort of kind of a bit impractical. You know, even the sort of trivial task of chopping up the computer-aided design, the CAD model, into all the 157 stones is quite a laborious task, as I found out myself. So we ended up, sometimes we 3D printed them when they were unique.

And when there were ones which are quite similar, we actually made a sort of archetype and made copies of it. And what we did was made a silicon mold and then we cast it. And that's so we could make a lot much quicker as how we actually went about making it. How did you go about choosing the material that you made it from? Because I presume the, at least the surface texture of a material has a big impact on its acoustic properties. So the thing about acoustic scale modeling is as soon as you say I'm going to work at 1 to 12 scale as we did, you have to work at 12 times. You have to work at 12 the frequency. So you're not trying to get exactly the same material. What you want is the same acoustic properties, but 12 times the frequency. So in a, you know, the middle of my speech range

might be about 1,000 hertz. In the model, that would be 12,000 hertz. So you're trying to match acoustic properties across these two frequency ranges. So a lot of people say, why didn't you make out of stone? Well, you don't. You just need to have it, have a material which has a similar property at 12 times the frequency. And actually, if you take something like 3D printed plastic, print a hollow of it and fill back fill the hollow with concrete you've got something really hard and impervious and that's what stone is which absorbs very little we also sprayed it with car paint as well to fill all the little paws up which also would have created absorption i mean one of the advantage of this sort of scale modeling a stonehenge is that as long as it's not very absorbing the stones it's actually the

geometry which really matters because most of the loss of sound energy is from between the stones or into the air So it's actually getting the geometry right, which is most important. The stones have to be hard and non-absorbing, but you could have made them a lots of different ways. So you mentioned there that it was a one to 12 scale model. Was there any particular reason that you chose that scale? Well, a few people have suggested it might be because I'm aping spinal tap, but no, it wasn't that.

I mean, for those who know the famous scene, they have a model of Stonehenge made, which is disappointingly small. And it is one to 12 scale because they mixed up inches and feet, but that was pure coincidence. We had to fit it in a test chamber called a semi-anacroic chamber. So this is a room which has got very absorbent walls on it. And it was literally, what's the biggest model we could fit in this room? As you get models small and smaller, you get problems with things like air absorption,

it gets harness, get loud speakers that work properly. So you tend to work in the biggest scale you couldn't get away with. So the model's about two and a half metres across, which just about fitted it in our chamber. And that's the reason we came up with 1 to 12 scale. So you just mentioned there that the chamber that you use. Could you tell us a bit more about that? What's so special about that and why was it particularly useful for this sort of research? Well, around Stonehenge, it's sort of open field. So I suppose we could have taken the model and stuck it in an open field. But if you try to measure there, what's going to happen when it might be a windy day, it might be raining, there might be traffic noise.

You don't want to work in those conditions because you're so dependent on factors out of your control. So we bring it inside to this room, which has got a hard floor, but all the walls are covered in these wedges. There's these giant sort of grey wedges, which are acoustic foam, which absorbs sound very efficiently. So any sound which went out of the stone circle and hit the walls of the chamber were absorbed, as it would happen in Wiltshire. Sound goes out of the stones and then just disappears into the countryside. So it's a really good way of getting controlled environment. you know, it's a very low noise level, very well isolated room without having all the problems of working outdoors.

So you don't have any of the source sounds that you're using, bouncing back off the wall into the model and interfering with recordings. You have to think a lot about these, what we would call parasitic reflection. So any of the equipment in the room, where we had very little equipment in the room, every time we measured, we'd walk into the model, put the microphones and loudspeakers out, walk out, close the two doors which are between us, then do the test. I mean, it was quite laborious. And then if you have any equipment like amplifiers, they have to be covered in foam so they're not reflecting stuff. And so you have to work very hard to get rid of all these spurious reflections

which wouldn't have been present in Stonehenge. So yeah, that sort of leads on nicely to what I was going to ask next, is about what the experimental setup actually looked like. I mean, did you use banks of microphones, where were they positioned, where were they positioned, where were the sound sources, inside the circle, outside, and different bits. Could you go into a bit of detail about that? So we have to work at 12 times of frequency. So you're starting to work with stuff which goes all into the ultrasonic range. So you can't just pick up a standard microphone like we're using to record this conversation with. You have to get, well, they're just smaller. So we use what's called a quarter inch mic, old-fashioned units, but that's what they're still

called. So they're, you know, they're about, what, 4 mill across. So they're quite tiny little microphones. They're the easy bit. The loudspeaker is the hard thing to use because you have to find these high frequency sound sources which aren't generally made. So we had to make special sort of loud speakers. And then, yeah, in terms of what we try to do is we try to measure a lot of positions and we just used our sort of architectural acoustics now. So what's going to make a difference. So one thing about Stonehenge, which is really interesting, is there's multiple rings of stones. And so it's very easy if you were in there to be hidden behind a stone. And we know if you can't see someone talking, the acoustic is very different to if you can

see them. So we would do things like, say, make sure there's a line of sight between the microphone and landspeaker and then do a position which is equivalent, but the microphone and loudspeaker hidden from each other. So you're just getting the indirect reflection. So we sort of kind doled it up in that and we thought, well, maybe there's focusing in the middle of the monument, so we tested some middle positions and then we, you know, so we just got considered lots of different things we were interested in and that determined where we placed it. I don't know how many places we measured. I was about, I guess, about 30 or 40 different places we measured in the end. So it's a lot of measurements. So the sounds that you were using, were they

beyond the range of human hearing? Well, they started in the human range. So because we were testing, I don't know, about down to 100 hertz, which was 12,000 hertz in a model. Sorry, 1,200 hertz, I can't do my math. That's good for a physics professor, isn't it? 1,200 hertz, that's definitely audible. But it would go up to 70,000 hertz, so that's sort of, your dog might get interested or bats, you know, way beyond our hearing. So actually, when you tested it, we pay what's called a sign sweep, which goes,

ooh, it's sort of a sweeping frequency. And you could hear it to start off with, and then it would disappear, but still be going, because our hearing wouldn't work. And then you deconvolve it in a mathematical process called deconvolution which gets to what we really want, which is called the impulse response. And the equivalent is if you go into a room

and you had to clap your hands and pick the sound up on the microphone, it's the response to the room to the impulse, the impulsive sound being a hand clap in that case. And you can do some mathematical processing to get that out of a sign sweep, which is what we did. So just one quick point.

So I might be wrong here, but is human hearing about 20 to 20,000 hertz? Yeah, so yeah, so typical human hearing goes from 20 hertz to 20,000 hertz. When we think about room design, we really think about 100 to maybe 4,000, 5,000, 6,000 hertz. That's the key, that's kind of like the keyboard range of a piano. And actually, when you get to my age, things above 10,000, 1,000, probably don't exist much more. I can hear them if they're very loud, but I haven't got much hearing left because of, unfortunately old age. So did you, so you mentioned a sign sweep there?

So did you use like a very pure sign wave signal? Or did you try sort of more messier, complicated signals as well? No, the sign sweep, because what you want to do is measure every single frequency. So it stepped through every frequency. So you get the data at every frequency. And then you can, it's a mass just to get back to the impulse response. You don't use impulses, so you can do it because in general it's quite hard to make a very short sound, which is very loud.

And so in terms of trying to get good signal-to-noise ratio, lots of signal and not much noise, it's easier to have a loudspeaker radiating something continuously. So it's a really common technique. It's kind of ubiquitous now in room acoustics. So once you've got all your recordings and all your data, how did you go about processing that then? And what sort of things were you looking for? Well, once you've got the impulse response, which is the sort of fingerprints, you know, the acoustic sonic fingerprint of a space, then there's various ways we process it to look at how people might

responds. So when we design, say, concert hall, there's a set of parameters that we derive from this impulse response through some calculations, which we know correlate really well with people's hearing, and then we sort of kind of work with those parameters. So one of those is reverberation time. So that's, you know, really obviously if you go into Cathedral, you go into Cathedral speak and the sound rattles around for a long time before dying away to nothing. The time it takes the sound to die away to nothing is called the reverberation time. It's the oldest parameter in architectural acoustic design. So that's the first thing. we calculated. And we got reverberation time, mid-frequency, about 0.6, 0.7 seconds.

And you can start thinking, oh, well, what's that like? And it's, I mean, the nearest space, I mean, it's quite unique, you know, there isn't spaces quite like this. But maybe a cinema. Cinemas have got a little bit of reverberation, but they're quite large, but they're actually quite dead because they have quite a lot of absorbent round. So definitely in this space, you can hear your voice being supported by reflections. Musical notes would be slightly, enhanced, but it's quite subtle is the kind of effect you get in Stonehenge. Yeah, so we mentioned earlier that there's been sort of several different reconfigurations of the kind of architecture, if you like, Stonehenge over the years. So how did you go

about approaching that? Well, we didn't actually set up all the different configurations, partly really because of time. It's very laborious to set up and disassemble a model because it all has to be sealed up and things like that. It's quite a quite, quite, tedious. So what we actually did was we took elements out. So we go, oh, you've got these amazing trillathons with their sort of caps on top, the lintels. Well, take them off. What difference does it make if they weren't there? And we did that for all the different parts of the monument to see what they, what purpose they were serving. And the sort of thing we found was the blue stones, didn't, if you know, took them all out, it didn't make much difference to acoustics. So if you go to

Stonehenge, the obvious things are these big uprights with lintels on top. So there's the, they're the trillathons. actually there was a lot of standing stones, which are just like normal ones you'd see in a stone circle, maybe one and a half, two meters tall. So actually, you know, relatively large. And there were about 80, 90 of those. There was a lot of those around. You take all of those out. It doesn't really change the acoustics very much. And so kind of tells us, you know, we know these were rearranged. At some point, they were in a double circle potentially and then in two single circles and so we know they played around with them but no one would have been able to hear the difference and that kind of says to me that okay acoustics is really interesting this space but it

probably didn't drive what they were trying to do in terms of designing it or how they decided to lay it out so um you sort of going slightly back we were mentioning the uh the frequency sweep so could you just sort of give us a sort of like an acoustic 101 on how how's what makes them the different travel and reflect and diffract differently, you know, higher frequencies as opposed to lower frequencies. If you listen back to our recordings in the space, so you can do what's called oralization, and you can add some speech to it and hear what it sounds like. What you'll hear is it's much basier. So the voice, you can hear the effect of the stones, but you can hear there's much more bass in people's voices. It's a bit like going into your bathroom and singing.

And that's because how sound interacts with these stones varies with frequency. So you've got various effects going on. For example, the ground is a bit more absorbing at high frequency. So that sound tends to die away quicker. But you've also got the fact that depending on the size of the wave, what we'd call a wavelength, how it interacts with the stones is very different. So if you have a sound wave, which is roughly the same size of the stone, you get a sort of scattering effect.

If the sound wave is very much smaller, which is what happens at high frequencies, you just get a sort of direct reflection, angle of incidence, equals angular reflection, something you might have learned the law of reflection back in school. So at low frequencies, sound struggles to get between the gaps of these very large uprights because the gap's a bit too small and it gets constrained. And so you get this base boost where at high frequency, it's easier for it to disappear down the gap because the sound of the sound wave is smaller than the gap. So first bounce, half the energy is gone because about half the area is air.

Effectively, it's not stone. And so it decays away much faster. Okay, so let's sort of move on a bit to the brass tacks, to the findings. So what are the sort of key things that you found? Like, you know, say I'm the 112th size, Jason, and I go and walk into your model and perhaps I sing the song or something. What happens? How do I experience that?

And what happens to the sound? So the first thing that happens to the sound is normally you'd you get this amplification due to the reflections. And it's, you know, it's about, it depends on which what you're looking at is about four decibels in the model on average. And you can imagine a case where, I don't know, you're trying to talk and your speech is only just audible. I mean, you're talking along a large distance potentially in this place. Maybe, I don't know, 30 metres is the furthest you could be apart. Maybe there's a bit of noise from the crowd. And that four decibels can be just enough to lift your voice to make a lot more the speech

audible. It's particularly true if you were doing facing away. So if you were actually not facing the person you're talking to, say the crowd was big enough, you couldn't face all of them at once, then actually these reflections are really useful in sort of an evening out the fact that, you know, the voice is more powerful in some directions than in others. So the amplification is the first thing. And that will make music sound better as well. Anything louder tends to sound a bit better. But then you also get this reverberation, which in, terms of music, we know that reverberation improves modern music. We don't know exactly what music they may or may not be making, but we know in general music sounds better with some reverberation.

Even pop music has lots of reverberation on it, even it's recorded in just a studio. So we can imagine it would have improved the quality of the music because it does nowadays. So is perhaps a tough question, but is it your feeling that, um, these acoustic properties are there by design? I think that's fairly unlikely, for several reasons, one of which is because we know the reconfigurations didn't change the acoustics in a particularly audible manner. So even if they thought they were changing it for the better,

there's no sign that it did. So I think it's much more likely they were designing for other reasons, and then it had an acoustic, and then they will exploit it. So I think whether they designed it, or not for sound, they would be a bit daft not to exploit the sound that's there. So if I wanted to have a conversation with someone, it would be very hard for me to stand in the middle and you to stand outside the stone circle. That would be a much harder conversation than if you stood inside the stone circle. So it kind of implies to me that rituals, so people who had to

understand what was being said, those people would be gathered inside the circle. It's more likely than they would be outside just because it would just be harder to communicate. So it wouldn't surprised me if it influenced how people used the space. But as in deliberate design, I think that's unlikely. Yeah. So another thing that came, that I wrote about a while ago, was this idea that the, you mentioned, I think it's the blue stones that you mentioned earlier, where it had certain sort of, like if you struck them, they'd make like gong light sounds like a gamelan or something like that. But is that something that's credible, or is that a bit of speculation? So we certainly know that rock gongs have been used for a long time. They're called lithophones. And we

certainly know in caves, there's evidence that there were people striking cave formations before Stonehenge was built. So it's certainly a sort of a technology, maybe not quite the right word, but certainly a musical instrument that has been around for thousands of years. So, yes, potentially they could have struck the blue stones. They seem to be of a sort of a type that rings, because some stones ring and some stones don't. I don't know if there's a lot of evidence in terms of percussive marks or anything. So one thing you do get with lithophones, which makes it very clear they've been used, is when you see lots of hammer marks on them. But the other problem you get with lithophones is dating them. So even if they were hammer marks on the Stonehenge

bluestones, when were they made? And so there's that kind of problem. The reason we can date some of the lithophones that were in caves, it's because there have been caves that have been blocked, and therefore we know the percussion marks are very old, because the cave was only found later on and revealed in modern times. So these just a sort of bit of a lithophone tangent, I just think that's interesting.

Where have these things been found? Are they, how sophisticated are? Are they tuned, for example, or they just make a single tone or something? I don't know, well, I know if a modern lithophone has been tuned. I mean, the famous ones or the well-known ones are probably, there's rock gongs, which in places like Serengeti, you'll find them around various places, which are literally just rocks lying around near, you know, which they hit.

And then you find ones in Indian temples, which have obviously been made to be hit and make a particular note that actually they have been shaped. So you do get ones which have been shaped. But generally, there's. There's not really old ones just to seem to be they are what they are. They make a gong sound and they're not particularly tuned to anything. Okay, let's see. And another thing that I've actually never, unfortunately, I've never visited Stonehenge,

but a lot of people have tell me it makes a strange humming tone as the wind blows through it. I wondered what your thoughts were on that. Is that just a complete accident or is it's a bit of people assign all sorts of reasons to this I've heard, but not entirely what I think. think about it myself. Well, I've yet to hear anyone who's recorded it and definitely witnessed it. So there's an old...

There's an old quote from Thomas Hardy in Tesla Dervervilles about the Stonehenge humming. And that has been picked up as being, oh, right, so maybe back in those days it used to hum because of the wind. It was still a ruin back in those days, although it's changed a little bit. There was still a lot of stones missing. And as far as I know,

No one in Wiltshire goes along to this place and hears it hum in the wind. I mean, I suppose it could do it. And we know it happens with modern buildings. So the one near me is the Beatham Tower in Manchester, which is a louvre on top, which does a spectacularly hum when there's high winds. It gets really, really loud. But whether these stones, which were amorphous in shapes, would have created the effects, and they're all little bit hickledy, bickledy and a bit different.

I think it's unlikely. I mean, I'd love to test it. I'd love to make a model and stick it in one of our wind tunnels, but that's a project for another day. Okay, that leads on nicely to my final question, then. Have you got any plans to investigate this model further, or in fact, other work on the acoustic properties of Stonehenge at all? one of the disappointments when I did the actual full measurements

is we tried to measure the effect of occupation and what happens when people are inside the circle because we're quite absorbent, our clothing is quite absorbent and you'd expect it were dead in the space and make the acoustics less good. And we literally, a week before doing the measurements, thought, this would be a good idea,

let's make some one to 12 scale model people. We had no time to test them. I literally made them overnight on a couple of nights and we didn't make them absorbing enough. So when we came to test them after the effect, the effect was too small. So one of the things I'd like to do is make some proper one to 12 scale model people with the right kind of absorption

and see what effect they have on the acoustics. Because we talk about this amplification, it could disappear if you had a lot of people in that space, and that's something I'd like to test. That was Professor Trevor Cox, an acoustic engineer based at the University of Salford talking to Jason Goodyear about his latest research investigating the acoustic properties of Stonehenge.

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