The Sound Book: The Science of the Sonic Wonders of the World (9 page)

An old staff room at my university had an amazing ability to color sound. It was a plain, narrow, rectangular room with chairs lined up on either side; it was like a waiting room at a train station. The first few times I went into the room I noticed a strange distortion as other people spoke. Moving my head back and forth dramatically changed the timbre of my colleagues' voices. With my head in one position, their speech sounded very bassy and powerful, but elsewhere their voices went all tinny, distorted, and horrible. Colleagues probably wondered if I had been drinking, as I gently swayed back and forth listening to our lunchtime conversations, scientific curiosity trumping self-consciousness.

As I moved my head from side to side, voices in the room changed as if someone was rapidly altering the settings on a hi-fi's graphic equalizer. This coloration was caused by a change in the balance of the sound, with some frequencies being boosted while others were suppressed. It might seem odd to talk about the
color
of a sound, but many of the words we use to describe sounds are appropriated from elsewhere:
bright
,
warm
,
dead
,
live
. The link between color and sound goes back many centuries, with Sir Isaac Newton spotting the similarity between the distance his prism spread out light colors and the lengths of strings needed to sound out a musical scale.
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Even today, acoustic engineers carry out measurements using “white” and “pink” noise. When paints are mixed together they form a particular color because the various pigments alter the frequency balance of the reflected light. Blue paint reflects light of a higher frequency than red paint. Similarly, acoustic engineers use colors to describe the dominant frequencies in sounds. White noise contains all frequencies in equal quantities and hisses rather like a poorly tuned radio. Pink noise contains more low frequencies, so it rumbles with a more thunderous quality.

Stairwells with two large, flat, parallel walls are a great place to hear coloration. Just clap your hands and you should hear a shrill, high-pitched note. This is a flutter echo, which is caused by sound bouncing back and forth between the walls, passing your ears over and over again at regular intervals. The frequency of the tone depends on how long it takes for the sound to go from your ear to the walls and back again.
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If the stairwell is narrow, this round-trip is quick, the reflections from the wall arrive quickly, one after another, and a high-pitched note is heard. For wider stairwells, there is a longer delay between the reflections you hear, and a lower frequency results.

The most extreme flutter echo I have experienced was in
Spiegelei
, a temporary work of art at Tatton Park, Cheshire, England, by artist Jem Finer. This was a spherical camera obscura, a metal sphere about 1 meter (3 feet) in diameter on top of what looked like a large garden shed. Stick your head into the middle of the sphere and you could see images of the park projected upside down onto the inside—the visual distortions being inspired by the artist's memories of taking drugs in the park as a teenager. The exhibition catalogue described the sound inside as “distorted and deranged”—fitting for a work that was playing on the absurdities of gravity.
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It was fascinating to see how many people experimented with the acoustic once they poked their head inside the sphere. Like a stairwell, the sphere provided sound reflections arriving at regimented intervals. As the curved walls of the sphere focused the sound, the reflections were particularly strong and the coloration especially marked.

You are unlikely to find a perfect sphere in a natural cave. Nevertheless, distinct coloration is heard in caverns. Would prehistoric man have exploited the coloration caused by tight squeezes in caves or the longer-lasting reverberation offered by large caverns? It would be extraordinary if our ancestors had overlooked these effects, especially when you consider how poor the lighting was and how unusual such acoustic effects would have been in an era before buildings. Indeed, starting in the 1980s acoustic archaeologists have been building up evidence that rock art is found in places where the sound is especially noteworthy. One of the pioneers of this work is Iegor Reznikoff:

A remarkable discovery in the study of ornate caves is the relationship between painted red dots in narrow galleries, where one has to crawl, and the maxima of resonance of these galleries. While progressing in the dark gallery, crawling and making vocal sounds, suddenly the whole gallery resonates: you put the light of your torch on, and a red dot is there on the wall of the gallery.
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Sound also appears to have influenced what our ancestors painted. Acoustic archaeologist Steven Waller tried to put the work on a more robust scientific footing by statistically analyzing what appears in each acoustic zone. In a paper in
Nature
he wrote, “In the deep caves of Font-de-Gaume and Lascaux, the images of horses, bulls, bison and deer are found in regions with high levels of sound reflection, whereas feline art is found in regions of the caves with poor acoustics.”
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It seems that our ancient ancestors were exploiting cave acoustics as they told stories around their drawings, with tales of loud hoofed animals being amplified by reflections, whereas quiet cats called for no sonic reinforcement.

The sheer volume of evidence that prehistoric rock art was influenced by cave acoustics is quite persuasive. But David Lubman, a retired aerospace engineer who has been applying acoustic science to archaeological sites, warns that correlation does not necessarily mean causation.

I met David in a Vietnamese restaurant in Los Angeles to discuss his work in archaeoacoustics. His wife, Brenda, accompanied us and took the wise precaution of bringing her own car to allow an early escape, because once you get David talking about his favorite subject, it is very difficult to stop him.

“High praise for Dauvois [another researcher] and Reznikoff, and their discovery of that correlation,” said David. “I think [this] was a turning point for me.”
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He went on to explain that a proper scientific sound source would have been better than using Reznikoff's voice to test the caves, and that the whole methodology is vulnerable to experimenter bias. David's hypothesis is that the painters chose nonporous rocks for their art because they would be easiest to paint. By chance, nonporous rocks also provide the strongest reflections. Sound waves cannot penetrate an impervious surface; the sounds just bounce off it. In contrast, porous rock has microscopic holes, air channels that the sound waves can enter. In acoustics, air is modeled as a viscous fluid, like molasses, except much runnier. And like molasses, air does not like being forced into the small channels. As sound enters these tiny holes in the porous rock, the vibrating air molecules carrying the sound wave lose energy to heat. Consequently, porous stone provides weaker reflections than do nonporous rocks.

When you come out to a place like this, where it is very quiet and you can hear the echo and imagine what the ancient people were thinking, there is something hypnotic about it, and it really reaches into certain areas of the brain and the soul, and you hear these ancient voices.
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This is Steven Waller describing the experience of visiting outdoor ancient rock art. Waller believes that many people miss a trick when visiting prehistoric sites. Not only should we test the sound close to the paintings by clapping, yelling, or singing, but we also need to step back to search for acoustic effects. Stand back from examples in Australia, for instance, and the effect is “almost spooky,” he says. “Where they've drawn a person, and you yell at it, it's like the person is speaking to you.”
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A similar effect is heard at Indian Hill, near San Diego, where sound repeatedly echoes from a cave entrance “as if the rock is calling out . . . and spirits were speaking back, right from the place where they chose to decorate.”
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To get these effects, the sound reflecting from the wall or cave must be heard separately from the sound going directly from your throat to your ear. And this happens only if you stand back from the surface so that the reflections are delayed. “Unfortunately, most people walk right up to a painting and study it from inches away, talking in hushed voices,” says Waller. “They never step back and see, or hear, the forest for the trees.”
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I've found rock art difficult to explore as a sonic tourist because many sites have restrictive access to preserve the paintings and some sites have been altered. I was hoping to see if there was an echo at L'Abri du Cap Blanc in France, which is a stunning frieze of prehistoric sculptures carved into a rock shelter. But I was dismayed to have my sonic explorations thwarted by the building that had been constructed to protect the frieze from the elements.
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One thing that can endanger sonic wonders is well-meaning conservation that considers only the visual to be important.

Waller carried out statistical analysis of Horseshoe Canyon in Utah and Hieroglyphic Canyon in Arizona. The latter is in the Superstition Mountains on the edge of Phoenix, and when I was in the US for the Great Stalacpipe Organ, I took the opportunity to visit it. I set off at sunrise to avoid the worst heat of the day (it peaked at 41°C, 106°F, that day) and admired the stately saguaro cacti dotted over the hillside as I hiked up the 2.4-kilometer (1.5-mile) trail to the Native American rock engravings. The petroglyphs are in a canyon, etched into rocks just above where a stream normally flows (it had dried up when I was there in June). Thousand-year-old geometric shapes—lines of sheep and deer drawn by the ancient Hohokam people—intermingle with more recent graffiti etched by vandals.
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Not long after I arrived, I was joined by a large, friendly family whose parents had somehow managed to get their children out of bed very early. Unable to make any acoustic measurements, I sat back and listened to the family playing and exploring the space. As the children yelled, a distinct echo reflecting from the U-shaped mountains could be heard. As they ran about the canyon close to the engravings, their footsteps and high-pitched voices were colored by the reflections from the semi-enclosed rocks. But these effects were not just restricted to the area around the engravings; plenty of unadorned spots had a similar acoustic.

The heat was debilitating, even in the shade. I could not help thinking that however interesting the acoustics are in the canyon, the presence of water must have made this place significant to the Hohokam. The only archaeological study into the canyon that I've been able to find says the spring was a natural place for the artwork because that is where sheep would have gathered to drink.
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4

At Horseshoe Canyon in Utah, the Great Gallery contains particularly fine, often ghostly figures, some of which are even life-sized. These were described by Polly Schaafsma as “dark, tapering, immobile anthropomorphic form[s], painted in dark red pigment . . . hovering in rows against a sandstone backdrop within arched alcoves and rock-shelter.”
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Along the canyon, the four sites where the echoes are strongest are those where the paintings are found; Waller's statistical analysis shows that the probability that this co-occurrence arose by chance is one in 10,000.
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Places with no echoes and good rocks to paint on are not decorated.

Ninety percent of drawings in the Horseshoe Canyon include hoofed animals such as bison or buffalo. Waller has suggested that the percussive echoes mimic the sound of the animals moving and stampeding. Slow-motion footage of horses reveals that two of their feet land on the ground almost, but not quite, at the same time, giving a double “clop.” Stand a few tens of yards from a large, flat surface and clap with a steady rhythm, and you can mimic this sound. But you could also produce the rhythm without the echo. When a hoofed animal walks or gallops, there is a distinct jaunty rhythm to the hooves hitting the ground—something I remember simulating with two halves of a coconut when I was a child.

Such theories in archaeoacoustics are necessarily speculative. Some mainstream archaeologists initially doubted David Lubman and his ideas about echoes from Mayan pyramids. As he explained to me, “I thought the archaeologists would be jubilant that somebody has discovered something that they had understandably overlooked, but instead they are angry with me.”
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The pyramid of Kukulkan, a Mayan feathered serpent deity, at Chichén Itzá in Mexico was built between the eleventh and thirteenth centuries. It is the height of a six-story building, with a square base that is about the size of half a soccer field.
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On every side it has a staircase running up the middle that contains ninety-one steps, and at the top there is a square temple. Visit the site, and guides will delight in clapping and producing a chirping sound. Stand in the right place, about 10 meters (30 feet) from the bottom of one of the staircases, and reflections from the stairs create a squawking echo with a distinctive descending pitch. David Lubman claims that this echo mimics the call of the scared and venerated quetzal bird.

Imagine an ancient Mayan priest presiding over a ceremony and, with great theatricality, summoning the sound of the quetzal bird by clapping his hands. Did this happen? And is there an even greater story to tell, about how the Mayans built their pyramids with specific acoustics in mind? Is this perhaps another example of their legendary technological talents, now lost?

I will return to the physics of the sound effect in Chapter 4, but for now it is important to know that many staircases can be made to chirp. The Mayan pyramids are not particularly unusual. Rupert Till, a musicologist from the University of Huddersfield, demonstrated this fact while waiting for his
X Factor
audition at the Old Trafford soccer stadium, the home of Manchester United. Till's studies of ancient acoustics made him curious about whether the steps between the terraces in the stadium would behave like a Mayan pyramid. Sure enough, when he clapped his hands, a distinctive chirp could be heard.
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Now, no sane person would suggest that the stadium steps were deliberately designed to chirp, so why should anyone assume that the echo from the Mayan pyramid is anything but an acoustic accident, or that it was used during ceremonies?