Professor Trevor Cox on the acoustics of Stonehenge © Getty Images

Professor Trevor Cox on the acoustics of Stonehenge

Read the full transcript of our Science Focus Podcast interview with Professor Trevor Cox about his research into the acoustics of Stonehenge – listen to the full episode at the bottom of the page.

Jason Goodyer: 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 focussed on the astrological position of it or what potentially it means. It was a calendar. Was it used for rituals or whatever? So where did the idea to study the acoustic properties of stone and actually come from?

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Prof Trevor Cox: Well, there’s been a few people have looked into it in the past. There’s a whole field called RCU acoustics, which is the archaeological of, 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 site, architectural acoustic noise, 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 you construct this site. But if you think about use of this site to me, 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 speaking. Singing, playing instruments of some form. And as soon as you hand surfaces reflect sound, you have, you know, the music and the speech being enhanced. And that’s what we’re investigating.

JG: So I was wondering the whole concept of. Now, you’ve explained it to seem fairly an obvious thing to want to know.

But to me, it was sort of like, oh, it’s something I’ve never thought of. Are there any sort of precedents in other parts of the world? But are this monuments or is this kind that with with interesting acoustic properties?

TC: I guess the classic one in architectural acoustics would be amphitheatres, 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 with their nose might have affected people’s behaviour.

And you’ve got stuff across in Mexico looking at Mayhan pyramids. 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 stone circles impressed on one.

JG: Say, one sort of immediate change that struck me is in its current sort of very. I don’t know how many states there are in total, but there aren’t so many standing up now.

So how can we be sure how. How do you go about reconstructing, accurately reconstructing what this would have looked like thousands of years ago?

TC: 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 tap into the people, you know. So there’s been lots. As you can imagine, of archaeological studies at Stonehenge. And there’s been various, you know, papers to outline what they thought used to happen, because we think of Stonehenge as being ace one fixed thing. But it went through many stages even before it’s died, to bits start to go missing. So, in fact, you know, the first thing was a very large stone circle, maybe 100 metres across.

Back in sort 2900 B.C.. I mean, the water we’re looking at is much, you know, I say much younger, but 2500 B.C., 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 did it had done a reconstruction of this of the monument. 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 this latest archaeological evidence for how the stone circles may have been in the past. And it’s kind of odd, 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. This is gonna sting. And you look at Stereographic.

So you’re looking at now which bits lie on top of other things and work out the order of when things have happened. But there is someone, you know, quite a lot of uncertainty about how it was configured and in what way.

JG: Over the years. Yes. You mentioned that it was a kind of it was built up over many, many years and had different configurations. So we’ll go back back to that, because that’s something I think’s very interesting.

So just going through the sort of process of how you get this. So I believe used 3-D laser scanners at the site or that’s what where the data, the original data for the model came from.

TC: Yes.

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, you know, their nice software version of the of of 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’re 3D, print them. And then we worked out it would take six months to 3D printing them. And that was sort of kind of a bit impractical. You know, even to this trivial task of chopping up the computer aided design, a card model into all 157 stones is quite laborious task.

As I found out myself. So we ended up sometimes with 3D printing them when they were unique and when they were ones which are quite similar. We actually may just or archetype and made copies of it. And we what we did was made a silicone mould and and we cast it and still we could make a lot, much quicker as how we actually went about making it.

JG: How did you go about choosing the material that you made it from? Because I presume the the police, the surface texture of the material has a big impact on its acoustic properties.

TC: So the thing about acoustics go modelling is as soon as you start going to work at one to 12 scale, as we did, you have to work at 12 times 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, Mike, the middle of my speech range might be about a thousand hertz in the model. That’ll be 12000 hertz. So you’re trying to match acoustic properties across these two frequency ranges. He’ll say, why didn’t you make out stone? Well, you don’t just need to have it.

Have a material, which has a similar properties, 12 times the frequency. And actually, if you take something like 3D printed plastic printer hollow and felt backfill, the hollow concrete, you’ve got some really hard and impervious. And that’s what Stoney’s, which absorbs very little. We also sprayed it. We’ve can’t paint as well to fill all the little pores up, which also would have created absorption.

I mean, if I showed this sort of scale modelling of Stonehenge is that as long as it’s not pay absorbing the stones, it’s actually the geometry, which really matters because most 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 have made them and lots of different ways.

JG: So you mentioned that it was we wanted to twelve scale model. Was there any particular reason that you chose that scale?

TC: Well, a few people have suggested it. Because I’m aping Spinal Tap. But no, it wasn’t.

I mean, for those, you know, the famous scene, they have a model of Stonehenge made, which is disappointingly small. And it is one to twelve scale because they mixed up inches and feet. But that was pure coincidence. We hope we had to fit it in a test chamber called a semi an echo 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? And you get model smaller, smaller. You get problems with things like AV absorption. It gets harder to get lost because that work properly.

So you tend to work in the biggest scale you can get away with. So the models about two and a half metres across, which just about fitted in our chamber. And that’s the reason we came up with one to 12 sgo.

JG: When you just mentioned that the the chamber, the U.S. Could you could you tell us a bit more about that? What’s what’s so special about that? And why was it particularly useful for this sort of research?

TC: Well, the Stonehenge, it’s so open field. So I suppose we could have taken the model and stuck it in an open field, but don’t 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. You know, they exist, John. It’s sort grey wages, which are acoustic foam, which absorb some value efficiently. So as saying any sound which went out to the stone circle and hit the walls as the chamber were absorbed, as would happen in Wilkshire, near sound goes out 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 any of the source sounds that easy, bouncing back off the wall into the modern interfering with.

Yeah, you have to you have to think a lot about these what we would call parasitic reflections. So and you the equipment in the room where we had very little equipment in the room, every time we measured, we’d walk into the model. But Mike friends, a loud speaker, walk out closest to doors which are between us, then do the test.

I mean, it was quite laborious. And then if you have any equipment like, you know, amplifiers there have to be covered in foams that are not reflecting stuff. And so you you have to work very hard to get rid of all these spurious reflections which have been present in Stonehenge.

JG: So, yeah. So it really is a nice thing to what I was going to ask next is about what the experimental satellite actually looked like. I mean, did you use banks of microphones? Where were they positioned round with the sound sources inside the circle, outside in different bits? Could you could you go into detail about that?

TC: So we have to work at twelve times the frequency.

So scientists work with stuff which is goes all into the ultrasonic range. So you can’t just pick up a a standard like phone like like we’re using to record this conversation with. You have to get. Well, you know, just smaller. So we use what’s called a core trench myco, old fashioned unit, but that’s what they’re still called. So that, you know, they’re about, what, four mm across.

They’re quite tiny little microphones that they’re not that easy. The last speaker 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 loudspeakers. And then in terms of what we tried to do is we tried to measure a lot of positions and we just used our sort of architectural acoustics now and thought, well, was going to make a difference.

So one thing about Stonehenge, which is really interesting, is this multiple rings of Cirque stones. And so it’s very easy if you’re in that beast 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 do things like, say, make sure there’s a line of sight between a microphone, loudspeaker, and then do a position which is equivalent. But the microphone, the last speaker hidden from each other. So you’re just getting the indirect reflection.

So we’ve kind of doubled up and not and we thought, well, maybe that’s focussing in the middle of that of the monument. So we tested the middle positions and 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 was banked, I guess about 30 or 40 different places we measured in the end. So a lot of measurements.

JG: So the sounds that you were using, were they beyond the range of human hearing?

TC: Well, they started in the human range. So it because we were testing, I don’t know, back down to 100 hertz, which was 12000 hertz in a model. So 1200 hertz are contour masses. That’s a physics professor. It’s a 1000 hertz. That’s definitely audible. But it would go up to 70000 hertz. So that’s sort of your dog might get interested or bats, you know, way beyond a havering.

So when you tested it, we pay what’s called a science sweep, which goes to. So has sweeping frequency and you could hear it start off with. And then it would disappear but still be going. Because our hearing wouldn’t work. And then you you you did Convolve. It was mathematical process got de convolution, which gets to what we really want, which is called the impulse response.

And that’s it couldn’t is if you go into a room and you clap your hands and pick the sound up on a microphone, is the response the room to the impulse. The impulse to some being a handclap in that case. And you can do some mathematical processing together out for a signed sweep, which is what we did.

JG: So just one quick point, I might might be wrong here, but this human hearing’s about 20 to 20000 hertz.

TC: Yeah, so. So, yeah. So typical human hearing goes from 20 hertz to 20000 hertz when you think about room design. We really think about a hundred to maybe 4000, 5000, 6000 hertz. That’s the key. That’s kind of like the the keyboard range of a piano. And actually, when you get to my age, but things above 10, 12000 hertz, we don’t exist much more. We can hear them if they’re very loud. But I haven’t got much hearing left because of, unfortunately, old age.

JG: So did you. You mentioned design suite. So did you use that very pure sine wave signal or did you try to have more messy, complicated signals as well?

TC: Now, the same suite, because what you want to do is measure every single frequency. So it stepped through every frequency. So so you get the data, every frequency and then you can match just to get. That’s 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 signaller, not much noise. It’s easier to have a loud speaker radiating something continuously. That’s a really common technique is going to ubiquitous now in room acoustics.

JG: So once you’ve got all your all your recordings and all your data, how did you go about processing? And what sort things were you looking for?

TC: Well, once you 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 respond. So when we designed a concert hall, there’s a set of parameters that we derive from this impulse response from some calculations which we know correlate really well with people’s hearing. And then we kind of work with those parameters.

So one of those is reverberation time. So that’s really obviously to go into cathedral. You go to Cathedral speak and the sound rattles around for a long time for dying away to nothing. The time it takes a sound doorway to nothing is called the reverberation time. It’s the oldest parameter in architectural acoustic design. So that’s the first thing we we calculated.

And we’ve got reverberation, time, mid frequency, about nought point six, nought point seven seconds. And you can start thinking, oh well, what’s that like? I mean, the nearest space I mean I mean, isn’t is quite unique. You know, there is spaces quite like this, but maybe a cinema cinemas have got a little bit reverberation, but a quite large but they’re actually quite dead because they have quite low absorbent round.

So definitely there 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 fat you get in Stonehenge.

JG: Yes, the romance and tell me that there’s been sort of several different reconfigurations is the kind of architecture, if you like, Stonehenge and for the years. So how did you go about approaching that?

TC: Well, we didn’t actually set up all the different configurations, partly really because of time is very laborious to set up an discernible a model that all has to be sealed up and things like that. It’s quite a quite tedious. So what we actually did was we took elements out. So we go, you’ve got all these mazing Twilla Athans with their sort of caps on top. They’re 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 of the monument to see what they what purpose they’re serving. And the sort of thing we found was the bluestones didn’t go to them all out. It didn’t make much difference to acoustics.

So if you go to Stonehenge, the obvious things are these big uprights were lintels on tops. That’s triphones. But actually, there’s was a lot of standing stones which are just like normal ones. You’d see in stone circle, maybe one and a half, two metres 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 two single circles. So we know they played around with them, but no one would have been able to hear the difference. And that kind of sense to me. Okay, acoustics is a really interesting 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.

JG: So I’m sort of going slightly back. We were mentioning the frequency sweep. So could you just sort of give us a sort of acoustic one or one on how how is what makes them the different frequencies travel and reflect and different differently?

You know, high frequencies as opposed to lower frequencies.

TC: If you listen back to our recordings in the space so you can do what’s called normalisation and you can add some speech to inherit, it sounds like what you’ll hear is it’s much bass here. So the voice, you can hear the same facts, the strings, 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, a 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, that 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 guess or scattering effect. If this sound wave is very much smaller, which is what hums are high frequencies. You just go soft. Direct reflection. Angle of incidence equals angle of reflection.

Some you might learn the law of reflection back in school. So at lowering says sounds struggles to get between the gaps of these very large uprights because this gaps a bit too small and it gets constrained. And so you get this bass boost. High frequency is 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 bams half the energy is gone because about half the area is air. Effectively, it’s not stone. And so it decays away much faster.

JG: So let’s sort of move on a bit to the to the to the brass tacks, to the findings. So what what sort of key things that you found?

Let’s say I’m the one twelfth-size Jason and I go and work walking to model and perhaps I sing a song or something. What happens? Well, how do I experience that? And what happens to the sound?

TC: The first thing that happens to the sound is normally you get 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 was 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 long. It did look like distance potential 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’re 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 there’s 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 improve the quality of the music. Because it does. It does nowadays.

JG: So is it your feeling that these acoustic properties are there by design?

TC: 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, it is. There’s no sign that it dead.

So I think it’s much more likely they were designing for other reasons. And then, you know, it had an acoustic and then they will exploit it. So I think whether they designed it or not for sound, there would be a bit daft not to exploit the sound.

So if I wanted to have a conversation with someone, it would be very hard for me to stand in the middle, to stand outside the stone circle. That would be a much harder conversation. And 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 be gathered inside the circle.

It’s more likely than they would be outside just because it would just be harder to communicate. So despite it wouldn’t surprise me if it influenced how people used to space. But as in deliberate design, I think that’s unlikely.

JG: So another thing that I wrote a while ago was this idea that the mention I think it’s the blues things that you mentioned earlier, which had certain sort of like if you struck them, they’d make like gong like sounds like a gamble or something like that.

Is that something that’s credible or is that a bit of speculation?

TC: So we certainly know that rock gongs have been used for a long time called lithophones. And we certainly know in in caves there’s evidence that there were people striking cave formations before Stonehenge was built. So it’s certainly less of a technology maybe and not quite the right word, but certainly a musical instrument has been around for for thousands of years.

So, yes, potentially they could strike the bluestones. 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 where lithophones, which makes it very clear they’re being used, is when you see lots of hammer marks on them.

But the other problem you got, lithophones, is dating them. So even if they were hammer marks on Stonehenge bluestones, when were they made? And so there’s that kind of problem that we reasonably can date. Some of the lithophones which were in caves because they’ve been cased, are being blocked. And therefore, we know that the percussion marks are very old because the cave was only found later on and revealed in modern times.

So there’s just a sort of buried left tangent where I just think that’s where these things being found.

JG: And the how sophisticated are they? Are they tuned, for example, or did they just make a single tones?

TC: I don’t I don’t know. Well, no modern lithophone has been tuned.

I mean, the famous ones or the well-known ones are probably theirs. There’s rock gongs which and places like. So in Gatta, you’ll find them around various places which are literally just rocks lying around their neck, which, you know, which they hit.

And then you find ones in Indian temples which have obviously been made to be hit and make particular notes that actually they have been shaped. So you do get ones which have been shaped. But generally, there’s not really old ones just to see. Seem to be. They are what they are. They make a gong sound and then optically tune to anything.

JG: There’s nothing that I’ve actually never, unfortunately never visited Stonehenge, and a lot of people have told 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 spent? People assign all sorts of reasons to this, I found, but not entirely why?

Can you hear anyone who’s recorded it? And definitely Western witness there.

TC: So there’s a there’s an old like there’s an old crow and Thomas Hardy, Tess of the d’Urbervilles about about the stone as Stonehenge humming. And that has been picked up as being. Oh, right. So maybe in the 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 he’s at home in the wind. I mean, I supposed it could do it. And we know it happens with modern buildings. So because of the one there met mine is Mimi is to be from Tower in Manchester, which is a Louvre on top, which does a spectacularly hum when there’s high winds and gets really, really loud.

But whether these stones, which were more in shapes, would have created the day facts and they were all a little bit guilty penalty 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 wind wind tunnels. But that’s that’s a project for another day.

JG: I think that’s that leads on nicely to my final question. Have you got any any plans to investigate this model further? Or in fact, the other work on the acoustic properties of Stonehenge, still one of the disappointments.

TC: I did the actual four measurements as we tried to measure the effect of occupation. And what happens when people inside the circle, because we’re quite absorbing our clothing, is quite absorbent. And you’d expect it with dead in the space and make the acoustics less good. And we literally a week before doing the measurement store, this would be a good idea.

Let’s make some one to try to scale model people we had no time to test and we literally, literally made them overnight. A couple of nights and we didn’t make them absorbing enough. So when we came to test them after the fact, the big you know, the effect was too small.

So one of things I’d like to do is make some proper one to twelve scale model people with the right kind of absorption and see what effect they have on the acoustics. Because we talk about this occasion, it could disappear. She had a lot of people in that space and that’s something I’d like to test.


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