Wednesday, September 1, 2010

Echoes of the past: The sites and sounds of prehistory

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Did our ancient ancestors build to please the ears as well as the eyes? Trevor Cox pitches into the controversial claims of acoustic archaeologists. And in our web-only article Acoustic archaeology: The secret sounds of StonehengeSpeakerMovie Camera, he explains how the acoustic footprint of the world's most famous prehistoric monument was measured
"The wind, playing upon the edifice, produced a booming tune, like the note of some gigantic one-stringed harp. No other sound came from it... Overhead something made the black sky blacker, which had the semblance of a vast architrave uniting the pillars horizontally. They entered carefully beneath and between; the surfaces echoed their soft rustle; but they seemed to be still out of doors..."

This atmospheric description of a "temple of the winds" comes from the dramatic climax of Thomas Hardy's novel Tess of the d'Urbervilles. The setting is Stonehenge, arguably the most famous prehistoric monument of all. Its imposing ring of standing stones is visible for miles on Salisbury plain in southern England. On the day of the summer solstice its outlying "Heelstone" is exactly in line with rays of the rising sun. A more perfect example of the visual impact of an ancient monument would be hard to find.


Assessing the claims of "acoustic archaeology" rapidly encounters a fundamental problem: sound is ephemeral. Pottery fragments, coins, bones and bits of buildings can survive for centuries, waiting to be analysed, interpreted- and reinterpreted. The sounds of the past, by contrast, have long since died away. Where historical records make mention of acoustic intent in designing structures, the claims are often based on faulty science (see "Sound design?"). Going back into prehistory, we do not even have the luxury of knowing what our ancestors were thinking- or often a clear idea of the original layout and acoustic properties of the structures we are interpreting.

There is, however, a plausible argument that sound must have been important to our ancestors, perhaps more so than it is to us now. "Today we guzzle sounds and make a lot of noise," says UK archaeologist Paul Devereux, an advocate of the claims of acoustic archaeology. "We are visually very sophisticated, but acoustically very primitive." Our ancestors, by contrast, would have been "acoustically more calm and attentive in a much quieter world", he says. Without artificial light, listening intently would have been imperative to ward off night-time predators. In a time before writing, moreover, information was principally communicated orally. It seems reasonable that prehistoric humans would have paid more attention to their acoustic landscapes than we do today. "Senses as a whole were more fused," says Julian Thomas, an archaeologist at the University of Manchester, UK. "There wasn't the separation of vision from the other senses as there has been over the last few centuries. Nowadays we tend to prioritise vision."

Sounding stones

We also know that our ancestors appreciated their ability to exploit their environment to make sound early on. The discovery of three flutes in 2009 in a cave in south-west Germany, the best preserved of them made from a vulture's wing bone and containing five finger holes, pushes the origins of music back to the middle Palaeolithic era, 40,000 years ago.



Raised voices

Systematic analyses of such sites are few and far between. Rupert Till, a musicologist from the University of Huddersfield, UK, says acoustic archaeology is where conventional archaeology was a century or more ago. "The subject is in its infancy," he says. "It's like the days of the Victorian gentleman wandering around digging holes in the ground."

It is one thing to show that our ancestors were aware of their acoustic environment. It is quite another to prove that they intentionally designed their surroundings with acoustics in mind. As soon as humans began constructing their own dwellings and other structures, this question of "intentionality" looms large.

One focus of this debate lies with enclosed spaces such as burial mounds, underground temples and burial chambers dug out of rock and earth. In the 1990s, Devereux and his colleagues measured the acoustics of six sites in the UK and Ireland dating from around 3500 BC to 400 BC, and found that all of them have resonant frequencies between 95 and 120 hertz, within the range of a male voice. Chant in a drone at the right frequency and you can map out the shape of the acoustic resonance, hearing the sound loud in one place and hardly at all in another- a dramatic and impressive sound effect (Journal of the Acoustical Society of America, vol 99, p 649). "Certainly this would have been a dense sensory experience," says Thomas- one accentuated by tight squeezes, poor light and the stench of rotting bodies.

Devereux thinks this is no coincidence: the spaces were tuned to maximise the acoustic impact of ritual chanting. The burial mounds consist of a stone chamber and entrance passage covered in earth. In theory, at least, the builders could have moved stones around and tuned the chamber before piling on the earth, perhaps copying previous burial mounds with particularly good acoustics. Similar claims of acoustic tuning for religious purposes have been made of pre-Inca temples in Peru (New Scientist, 6 September 2008, p 37).

It's a nice theory, but not everyone is convinced. Matthew Wright, an acoustics researcher at the University of Southampton, UK, is scathing in his commentary. "If you are going to conclude that particular burial mounds were designed for chanting, then you have to also conclude that my bathroom was made for singing," he says.

If you're going to conclude that burial mounds were designed for chanting, then my bathroom was made for singing.
Till thinks that this argument overlooks how unusual stone buildings were in the past, and therefore underestimates the amount of thought that would have gone into building them. Both he and Devereux accept that acoustic intent in the design of burial mounds is far from proved, and it will be difficult to do so conclusively. Till suggests that proof might come in identifying repeat features in burial mounds that change over time, indicating an empirical development process.

Nevertheless, he thinks that some of the evidence brought for ancient acoustic effects is judged too harshly. If artefacts or monuments look good, then we believe with little hard evidence that they were designed to look good. Where interesting sound effects are heard, though, we dismiss them as flukes or demand evidence for acoustic purpose- perhaps because, in our noise-filled modern environments, we are far more attuned to visual cues than acoustic ones.

Acoustic consultant David Lubman agrees. In 1998 he suggested that the curious echo reflected from the steps of the El Castillo pyramid at the Mayan site of Chichen Itza, Mexico, was no accident. The steps had been designed, he said, to generate a sound that resembles the chirping of the quetzal, the sacred bird of the Mayans.

Many archaeologists dismissed his idea. "Some of this is understandable," says Lubman. "Many outsiders promote unscientific, ignorant, and even superstitious ideas." But acousticians have a crucial role, he says, in getting archaeologists to recognise the visual bias they bring to their studies.

Ancient vibes

Which brings us back to Stonehenge. With no roof on the monument, any sound made within it might be expected to scatter vertically and be lost to the atmosphere, either directly or after reflecting from the ground. Yet the stones of the monument are oddly two-faced: while the sides facing outwards are roughly hewn, the interior surfaces have been chipped away at laboriously to provide a surprisingly smooth, slightly convex form that is ideal for high-frequency reflection.

Stonehenge's stones are oddly two-faced, laboriously chipped away on the inside to produce a surface ideal for reflecting sound.
Early one misty spring morning in 2009, Till and my colleague Bruno Fazenda at the University of Salford, UK, measured Stonehenge's acoustic signature by bursting balloons to map out the reflections within it. Because many of the stones of Stonehenge are now missing or have fallen down, he also made measurements at a reconstruction of the monument at Maryhill, Washington state. Although this mock-up is constructed from concrete and its surface finishes do not replicate exactly the original stones, it offers perhaps the best chance of hearing and measuring Stonehenge in one of its ancient configurations, that of 4000 years ago.

What Till and Fazenda measured was impressive. "It's actually like walking into an enclosed space with a lively acoustic," he says. "It is a really good space for speech because reflections from the stones mean you can be heard everywhere within it, even if you're hidden behind a stone." Clap your hands in the space and the sound reverberates around the monument as it reflects from stone to stone. The reverberation takes about 1.2 seconds to die down- typical for an opera house or school hall but astoundingly long for a space with no ceiling.

Accident or design? Till suggests that our ancestors might have learned that such smoothly shaped surfaces gave off stronger echoes by observing the properties of other standing stones. Fazenda is more sceptical, suggesting it is more likely that Stonehenge was built for religious purposes, and that our ancestors exploited the lively acoustic they had created to support speech.
Here as elsewhere, perhaps we shall never know for certain whether our ancestors built with acoustics in mind. "There may never be 100 per cent proof of acoustic intentionality," Till says. "That uncertainty just adds to the interest, mystery and aura of the ancient sites." Catch Stonehenge in a quiet moment, and it is hard to disagree.

When this article was first posted, we failed to mention that Rupert Till worked on the Stonehenge measurements with Bruno Fazenda.

Sound design?

Written documents can provide the most convincing evidence of deliberate acoustic design- but they should not always be taken at face value. The Roman engineer Vitruvius wrote a guide for building projects, De Architectura, towards the end of the first century BC. An entire chapter is devoted to resonating bronze vases which were meant to improve the acoustics of Greek and Roman theatres. Vitruvius suggests placing 13 vessels in niches half-way up the auditorium to amplify the actors' voices:

Unfortunately, much of what is written by Vitruvius does not stand up to scientific scrutiny. The vases were probably intended to be Helmholtz resonators, providing tones to support the sound in the way that blowing over the top of a bottle produces a note. But the vases would probably not radiate enough sound to make a difference, much as you don't hear beer bottles singing as you walk past them in a bar.
Various scientists have examined vases found in about 200 churches and mosques built between the 11th and 16th centuries in many parts of Europe. Measurements have shown that the vases do very little for the acoustics of the churches: there are usually not enough of them, or they resonate at frequencies different from those needed to support the human voice. Why they were put there remains a mystery (Journal of the Acoustical Society of America, vol 112, p 2333).

By contrast, other aspects of Greek and Roman theatre design would definitely have helped the sound: using a semi-circular plan and raked seating puts the audience close to the stage and maximises the sound intensity; the use of reflections from the stage floor, and to a lesser extent from walls behind the stage, reinforce and amplify the sound. In 2006, Jian Kang and Kalliopi Chourmouziadou at the University of Sheffield, UK, reconstructed the evolution of theatre acoustics over 2000 years using computer models, showing how steeper seating rakes and harder building materials gradually improved things over time - evidence for an empirical trial-and-error design process in action (Applied Acoustics, vol 69, p 514).

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