In a foam-covered room inside a university building in the UK, a replica of one of the most mysterious monuments ever built – Stonehenge – is in safekeeping.
These miniature monoliths are not available to the public, although they may well help to better understand the imposing lichen-covered stone structure that was cemented around 5,000 years ago.
Such a scale model is at the heart of ongoing research into the acoustic properties of Stonehenge and what its sound might tell us about its intent.
As expressed by Trevor Cox, professor and researcher in acoustics at the University of Salford in Manchester, “We know that the acoustics of places influence how you use them, so understanding the sound of a prehistoric site is an important part of archaeology.”
Some theories point to it being a burial site, or a healing space, or even serving as a celestial calendar, since the gaps in its outer stone ring are perfectly aligned with the summer and winter solstice.
Yet as the decades pass, this massive monument built on a grassy hill in the countryside of Wiltshire, England, remains a complete mystery.
“Gradually we’re finding out more and more about it, but some things we just don’t think we can ever figure out. We have no way of understanding why it was started to be built. And the reason they continued to work on it may well have changed over the hundreds of years it took to complete it,” expounded Susan Martindale, manager of English Heritage, the charitable trust that manages Stonehenge.
Thanks to Cox’s recent studies, a fascinating detail about one of the world’s most enigmatic sites is now known: it once acted as a giant echo chamber, amplifying the sounds produced inside the circle for those inside, but shielding the noise from those outside the circle.
This finding brings with it some questions about whether the monument was really built as a ritual site for a small, elite group. This new focus on the site has taken a decade of work.
Ten years ago, during research on “the sound wonders of the world,” Cox began to inquire, whether studying Stonehenge’s acoustic properties might help uncover some of its secrets.
“I realized that there was a technique in acoustics that had never been applied to prehistoric sites before, and that was acoustic modeling at scale,” said Cox, who put it into practice for the first time at Stonehenge.
The researcher set out to create a 1:12 scale replica that he could test inside the semi-anechoic chamber, a room that absorbs virtually all sound thanks to the geometric foam that covers all surfaces except the floor.
To create such a replica, Cox initially received a computer model from English Heritage, which facilitated his proper understanding of what Stonehenge looked like in its most complete configuration some 4,000 years ago.
“If you go to modern Stonehenge, it’s a magnificent site, but a lot of stones are missing or some are lying on the ground,” he said. “This configuration is particular. Actually, from about 2000 B.C. onward, it changed a lot for about a millennium.”
The process of creating 157 stones through 3D printing and molding techniques took about six months to fully complete. Over the course of that time, Cox noted that his dining room floor was covered with pieces of the project in a laborious effort to achieve the qualities of real stones to scale.
As soon as the stones were painted gray and arranged in the correct layout according to the computer model, the challenges of the testing process began.
“Everything is one-twelfth the size in real life, and that means we have to test at 12 times the frequency,” he says. “You have to get all the speakers and microphones that work in those frequency ranges, and they’re not commonly available.”
Cox and his team placed the speakers around the stones and played the different frequencies they were interested in measuring, to complete each test. The microphones in the room collected data on how the stones affected the sound.
Through mathematical processing, Cox was able to create a computer model that simulates the acoustic properties of Stonehenge and can distort voices or music to give an idea of what they would sound like inside the circle.
The results were surprising: despite the fact that Stonehenge has no ceiling or floor, sound bounces between the gaps in the stones and remains within the space. In acoustics, lingering sound is known as reverberation.
“We know that music is enhanced by reverberation, so we imagine that, if music were played, it would sound a little more powerful and impactful inside the circle,” he said.
The effect of the stones on voice directionality is one of the most notable findings of Cox’s research.
In an open, natural setting, such as the grassy hill on which Stonehenge is built, a speaker speaking with his back to a listener would only be understood by about one-third.
The reflections from the stones at Stonehenge would have amplified the voice by four decibels, raising the number of understandable sentences to 100%.
Such results have shown that Stonehenge would have allowed people inside the circle to be heard quite well, while those outside would have been excluded from any ceremony taking place.
This research by Cox adds to a growing body of evidence that Stonehenge may have been used for rituals reserved for a select few, and one study even points to the possibility that a hedge may have been grown to shield the view of those not participating.
“The research definitely provides more information about how Stonehenge might have been used. Even if you walk away, there are always stone reflections to reinforce your voice, so it really doesn’t matter if you can’t see the person speaking. It would be quite good for oral communication,” says the researcher.
Cox compares the acoustic properties of Stonehenge to the difference between standing in an empty movie theater and standing in a cathedral. Walking in and out of such buildings for someone today may not generate a very noticeable effect, but Cox points out that the late Neolithic people who built Stonehenge and who were not used to the acoustics of large walls and enclosed spaces would probably have found the effect fascinating.
After publication of the initial findings in 2020, Cox and colleagues addressed new questions, such as the ways in which people within the Stonehenge circle might change the acoustics. The team recently completed a new set of measurements by placing up to 100 small wooden figurines around the model.
“We know that the people inside would have changed the acoustics because we absorb sound,” he said. “We want to quantify how it might have changed as more people entered the circle, because presumably there were people inside the circle during the ceremonies.”
Acoustic research, he believes, is another tool to find more clues and build a clearer picture of the site’s qualities.
“If we think about human ceremonies, they usually involve some kind of sound, whether it’s music, speech or chanting. And we know that, if they really wanted to be heard, people should have been inside the circle,” he concludes. “The problem with acoustic archaeology is that the sound disappears, so we can never be sure what was done there.”
Published by Emirates Herald, news and information agency.