The Sound Book: The Science of the Sonic Wonders of the World (25 page)

The ice instrument sounded like a member of the xylophone family, but I could immediately hear that its bars were not made from wood or metal. They clinked like an empty wine bottle being struck with a soft mallet. The pure, clear note suited the material perfectly. But these last two adjectives—
pure
and
clear
—might just be evidence of how aural judgments are affected by what we see. What other sound could a transparent bar make, apart from a clear one?
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Scientists have found that we can reliably discriminate between materials only when those materials have very different physical properties, like wood and metals.
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Listeners latch on to how long the ringing lasts. The internal friction within a grainy wood is higher than within metal, so the wood stops vibrating sooner. This is why a rosewood xylophone makes a “bonk” and a metal glockenspiel tends to ring.

The clink of the ice xylophone was a long way from the cracking, booming, and zinging heard by the harvesters as they cut into the frozen lake to make Terje's instruments. Wait quietly by a frozen lake as the sun comes up and the ice might shift and crack, or wait as the sun sets and the ice will start to crackle and sing as it cools. These are sounds of geology in motion, an auralization of forces that shape our planet. Scientists have been measuring the noise from these seismic activities using hydrophones to estimate the thickness of ice sheets in the Arctic.
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To find out more about the incredible range of natural sounds from ice—the cracks, fizzes, bangs, and twangs—I met up with artist Peter Cusack in a noisy cafe in Manchester. A member of the sound intelligentsia, Peter speaks softly and describes what he hears with great precision. Peter told me about the ten days he had spent recording at Lake Baikal in Siberia. Nicknamed the “Pearl of Siberia,” the lake holds about 20 percent of the world's fresh surface water, which is more than in all the North American Great Lakes combined. The thick ice sheet gradually melts in the spring, first by splitting into separate flows. Thin, icicle-shaped pieces break off from the edges of the sheet and drift about on the surrounding water, nudged by wind and waves. Millions of these ice shards jostle, creating what Peter described as a “tinkling, shimmering, hissing sound.”
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On the opposite side of the world, at the Ross Sea in Antarctica, sound recordist Chris Watson captured a similar transformation from glacial ice to seawater using hydrophones, either underwater or wedged into glaciers. The Ross Sea is a deep bay of the Southern Ocean where early Antarctic explorers such as Scott, Shackleton, and Amundsen were based. Chris described huge blocks of ice, some the size of houses, calving from the glacier and landing on the still-frozen sea. The calving sound was explosive, like a percussive bang from a pistol. The ice also rubbed and scraped together to create “a remarkable squeaking . . . sound[ing] like 1950s or early 60s electronic music.”
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Aboveground, the ice was largely silent and appeared inert, but Chris's hydrophones revealed how mobile it was beneath the surface. Later in the transformation, “Slush Puppie” ice produced grinding and crushing noises. “One of the most powerful sounds I have heard, because you realize what you are hearing,” Chris explained. The Southern Ocean was moving this vast mass of ice, causing it to break up, from tens of miles away.

Walk on a thickly frozen lake, and thunderous reverberations can ricochet through the ice as it rearranges itself. On thinner ice, throwing rocks onto the surface can create alien chirps. On a winter's day, mountain biking in the Llandegla Forest in northern Wales, not long after hearing the concert of ice music, I came across a frozen reservoir with a thin layer of ice, about 5 centimeters (2 inches) thick. Skimming stones on the surface produced repeated twangs, like a laser gun from a sci-fi movie. The sounds appeared alien because each twang had a quick downward drop in pitch, a glissando that is rarely heard in everyday life.

Each time the stone struck the frozen surface, a short-lived vibration traveled through the ice before radiating into the air as a twang. In air, different sound frequencies travel with the same speed, so they all arrive at the same time. But ice is different. The high frequencies move fastest and thus arrive first, followed by the slower, lower frequencies arriving at the end of the glissando. The same effect happens in long wires. When sound designer Ben Burtt was creating effects for the
Star Wars
films, he based the laser gun on a recording of a hammer hitting a high-tension wire that was holding up an antenna tower.
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According to Swedish acoustician and skater Gunnar Lundmark, the chirping sound of ice can be used to test the thickness and safety of frozen lakes. As a skate moves across the surface, it creates tiny vibrations in the ice, which create a tone whose dominant frequency depends on the thickness of the frozen layer. You cannot hear the note from your own skate, because it squirts out sideways, but you can hear the sound from a friend's skate about 20 meters (65 feet) away. Lundmark did a series of measurements to test this out: “My assistant, my little lightweight son . . . hit the ice with an ax[e] and I . . . recorded the sound with a microphone and a mini-disc at a safe place.”
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He concludes that if the tone was at 440 hertz (in musical terms, the A used by orchestras to tune up), then the ice in most cases is safe, but if the tone is a bit higher in frequency—say, 660 hertz (or an E, five white notes up on a piano keyboard)—then the ice thickness is only about 5 centimeters (2 inches) and is dangerously thin. To take advantage of the singing ice, however, a skater needs to identify the frequency or equivalent musical note, which is something only people with absolute pitch can do. Unmusical skaters will have to discern ice thickness some other way.

With ice, the size of an icicle and the frequency of the noise it creates are inexorably linked. The same is true for air bubbles in water. Is there a similar mathematical relationship between grain size and frequency for singing sand dunes? One would expect so because such a relationship exists for most sound sources: violins are smaller than double basses. But whether the sand grain size is important to the frequency of the booming dunes has been hotly debated, and thus far the data have been inconclusive. However, recent laboratory tests by Simon Dagois-Bohy and colleagues at Paris Diderot University in France may have tipped the balance of scientific evidence, showing that grain size dictates the dune's frequency. Dagois-Bohy took sand from a dune near Al-Ashkharah in Oman and showed that when the sand was sieved to select a particular grain size, the boom altered. Before sieving, the grains ranged from 150 to 310 microns in size, producing a hum over a broad frequency range from 90 to 150 hertz. When the sand was sifted to select a narrower range of grains, from 200 to 250 microns, a clear single note at 90 hertz was heard.
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T
he early-twentieth-century adventurer Aimé Tschiffely once slept on a booming dune on the Peruvian coast during his 16,000-kilometer (10,000-mile) horseback ride from Argentina to Washington, DC. A report tells how the “natives” explained to him that, “the sand hill . . . was haunted and that every night the dead Indians of the ‘gentilar' danced to the beating of drums. In fact, they told him so many blood-curdling stories about the hill that he began to consider himself lucky to be alive.”
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Unsurprisingly, a rich vein of folklore develops around unexplained natural sounds. Writing about rock art in North America, Campbell Grant notes the frequent drawing of thunderbirds and says, “Thunderstorms were believed to be caused by an enormous bird that made thunder by flapping its wings and lightning by opening and closing its eyes.”
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Thunder has two distinct acoustic phases: the crash and the roll. There is an old thunder sound effect, originally recorded for the film
Frankenstein
in 1931, that perfectly encapsulates these two stages. SpongeBob SquarePants, Scooby-Doo, and Charlie Brown are among the cartoon characters that have been scared by this particular recording. Indeed, it has been used so widely that for many years, if you saw a haunted house in a storm, this is the thunder you would have heard.
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The noise is actually quite tame, and my strongest memories of actual thunderstorms are much more scary. I can remember leaping out of bed petrified by a crack of thunder so loud that I thought my house had been struck. Hollywood sound designer Tim Gedemer explained to me that if he wants to reproduce a big thunderclap for a film—one that rips across and lights up the whole sky, one that “hits you in the gut”—it is impossible to use just a recording of thunder from nature. You might start with a real recording, but then he would add sounds that are not from thunderstorms to get a “visceral experience.”
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As a child, I was taught to count the time between the flash of lightning and the rumble of thunder to estimate how far away an electrical storm was. The calculation exploits the fact that sound travels much slower than light. Because sound moves at about 340 meters (1,115 feet) per second, then a 3-second delay between lightning and thunder indicates that a storm is about 1 kilometer away (5 seconds would indicate a distance of 1 mile). So I have never doubted that lightning causes thunder, but surprisingly, up until the nineteenth century this causal relationship was in doubt. Aristotle, the Greek philosopher and pioneer of applying scientific methods to natural phenomena, believed that thunder was caused by the ejection of flammable vapors from clouds. Benjamin Franklin (one of the founding fathers of the United States), Roman philosopher Lucretius, and René Descartes, the French father of modern philosophy, all believed the rumble came from clouds bumping into one another. One of the reasons lightning was not proved to be the cause of thunder earlier was the difficulty of studying the phenomenon. It is impossible to predict exactly where and when lightning may be produced; scientific measurements are thus often made a long way from where the action is.

Close to the striking point there is an explosion that is among the loudest sounds created by nature. The subsequent rumble typically peaks at a bass frequency of about 100 hertz and can last for tens of seconds. The electric current of the lightning creates an immensely hot channel of ionized air, with temperatures that can exceed 30,000°C (54,000°F). This heat creates immense pressure, ten to a thousand times the size of normal atmospheric pressure, which creates a shock wave and sound.
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Lightning follows a jagged, tortuous path to the earth. If lightning happened in a straight line, thunder would crack but not rumble. Each kink on the crooked path—the kinks occur every 3 meters (10 feet) or so—creates a noise. Together, the noises from the kinks combine into the characteristic thunder sound. The rumble lasts a long time because the lightning path is many miles long, and it takes time for the sound to arrive from all the distributed kinks.
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Shock waves might also be the cause of mysterious booms heard around the world. They have colorful names:
Seneca guns
near Lake Seneca in the Catskill Mountains of New York,
mistpouffers
(“fog belches”) along the coast of Belgium, and
brontidi
(“like thunder”) in the Italian Apennines.
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In early 2012, residents of the small town of Clintonville, Wisconsin, thought they were hearing distant thunder when their houses shook and they were awakened during the night. One witness, Jolene, told the
Boston Globe
: “My husband thought it was cool, but I don't think so. This is not a joke . . . I don't know what it is, but I just want it to stop.”
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These sounds were caused by a swarm of small earthquakes, as confirmed by seismic monitoring.
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In 1938, Charles Davison interviewed witnesses to similar moderate earthquakes, and the sounds from the quakes were variously described as the boom of a distant cannon or distant blasting, loads of falling stones, the blow of a sea wave on shore, the roll of a muffled distant drum, and an immense covey of partridges on the wing.
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Like UFO sightings, many of the booms can be explained by nonsupernatural reasons. In April 2012, a terrifying noise in central England was attributed to sonic booms created by a pair of Typhoon fighter jets. A helicopter pilot accidentally sent out a distress signal indicating his aircraft had been hijacked, forcing the Typhoons to break the sound barrier to quickly intercept the helicopter. When a plane moves through the air at low speed, sound waves ripple and spread out from in front and behind the plane at the speed of sound. The ripples are similar to the gentle bow and stern waves created by slow-moving boats. When the plane accelerates to the speed of sound, about 1,200 kilometers (750 miles) per hour, or faster, then the sound waves can no longer move fast enough to get out of the way. These waves combine to form a shock wave, which trails behind the aircraft in a V shape like the wake from a fast-moving boat. A plane creates a continuous sonic boom, but the wake passes over people on the ground only once. As one earwitness to the Typhoon jets reported, “It was a really loud bang and the room shook and all the wine glasses on the rack shook . . . It was weird, but didn't last long.”
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(Sometimes a double bang is heard—one caused by the wake from the nose, the other by the wake from the tail.)

A sonic boom is a weakling, however, compared to the most powerful natural sound ever experienced by humans: the 1883 eruption of Krakatoa, a volcanic island in Indonesia. As one eyewitness, Captain Sampson of the British vessel
Norham Castle
, wrote:

I am writing this in pitch darkness. We are under a continual rain of pumice-stone and dust. So violent are the explosions that the ear-drums of over half my crew have been shattered. My last thoughts are with my dear wife. I am convinced that the day of judgement has come.
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