Stonehenge remains profoundly mysterious. We still aren’t certain who built it, or why they aligned its geometry with the summer solstice, or brought the smaller stones from 180 miles away, or what range of purposes it served. But every year scientists learn more about the great stone enigma on Salisbury Plain. Most recently, a team from the University of Salford, in Manchester, and English Heritage, the charitable trust that manages Stonehenge, made a breakthrough about the monument’s acoustical wonders.
Despite the lack of a roof, the research team has found, the original circle of 157 standing stones (only 63 complete stones remain today) once acted like a sound chamber. For people in the inner sanctum 4,000 years ago, the placement of stones would have amplified and enhanced human voices and music in a way that must have been spellbinding. If you were outside the circle, though, the sounds were muffled and indistinct. This finding has added credence to the growing consensus that rituals at Stonehenge were for a small elite.
The study was conceived by Trevor Cox, an acoustical engineer at the University of Salford. “Some acoustical research had already been done at Stonehenge, but it was all based on what’s there now,” Cox says. “I wanted to know how it sounded in 2200 B.C., when all the stones were in place.”
To find out, he borrowed a standard technique from architectural acoustics and built a scaled-down model. The tallest replica stones are approximately two feet high. Cox and his co-workers based the model on laser scans of Stonehenge that were provided by Historic England, the government agency responsible for preserving historic sites, as well as the latest archaeological thinking about the different construction phases and configuration of the original stones.
To create replicas, he 3-D-printed 27 of the stones. Then he made silicon molds of them and cast the other 130 stones. Some of the model stones were hollow plastic; cavities were filled with aggregate and plaster mix. The others were cast using a plaster-polymer-water mix. Gaps were filled with children’s modeling clay. All the replica stones were sealed with a cellulose car spray paint to prevent sound from being absorbed. Once the model was complete, he began experimenting with microphones and speakers, and measuring sound waves with a computer.
“We expected to lose a lot of sound vertically, because there’s no roof,” he says. “But what we found instead was thousands upon thousands of reflections as the sound waves bounced around horizontally.” These reflections would have produced “significant amplification—four decibels,” Cox says, as well as a powerful reverberation effect, meaning that the sounds would have boomed and lingered before fading away. “You can compare it to singing outside, and then singing in a tiled bathroom: Your voice sounds better in the bathroom.”
As modern people living in sound-reflective rooms and concrete cities, we are so accustomed to amplified, reverberating sounds that we barely notice them. In Neolithic Britain, however, people rarely heard them unless they entered a cave or a narrow rocky gorge. “It must have been magical to build Stonehenge, to make that massive community effort, to align it to the solstice, and then walk inside the circle and hear reverberating sounds,” says Cox.
He thinks it’s extremely unlikely that these acoustic properties were there by design, but once they were discovered, people surely would have exploited them. “Human ceremonies nearly always have speeches, singing or chanting,” he says. “We know there were musical instruments around—bone flutes, pipes, drums, horns—and they would have sounded amazing inside the circle. If you were important, you’d definitely want to be in there. If you were on the outside, not only was your view obscured, you couldn’t hear what was going on either.”
The next stage of research is to place scale replicas of people inside the henge, and find out how much sound they absorb. Cox has also been approached by a number of musicians who are eager to replicate the same precise reverberation in their recordings. “It’s an exciting thought for them,” he says. “Through a mathematical process called convolution, they can record their instruments to sound like they’re playing at ancient Stonehenge.”