The Sound Book: The Science of the Sonic Wonders of the World

by Trevor Cox

  While the first wave is the star of the show, if you rush off too soon you miss the sound of the “whelps,” the secondary undulations that follow the bore. Floodwater surges behind the bore for a good thirty minutes after the main wave, the force of which is apparent as it pulls along whole trees and other debris. Large undulations form in the water. These waves break here and there, creating a crashing sound to accompany the gurgling and rumbling of the huge mass of water being moved—an audio mixture of waves on a beach and water running down a municipal drain.

  In terms of bore heights, the River Severn comes in fifth, with larger ones, like the pororoca in Brazil, having an even more dramatic sound. The Qiantang River bore was described by the Chinese poet Yuan as, “10,000 horses break out of an encirclement, crushing the heavenly drum, while 56 huge legendary turtles turn over, collapsing a snow mountain.”14 In 1888, W. Usborne Moore, a commander in the Royal Navy, described it in a more understated way: “On a calm, still night it can be distinctly heard, when 14 or 15 miles distant, an hour and twenty minutes before arriving. The noise increases very gradually, until it passes the observer on the bank of the river with a roar but little inferior to that of the rapids below Niagara.”15

  Hubert Chanson has studied the acoustics of the bore near Mont Saint-Michel in northern France.16 The rumble of the main wave is caused by bubbles in the bore roller, along with higher frequencies from the waves crashing onto rocks and bridge piers. Low frequencies between 74 and 131 hertz dominate, equivalent to a low octave on a piano.

  If a writer needed adjectives to describe the sound of a tidal bore, she could do worse than consult “The Cataract of Lodore” by the Romantic poet Robert Southey. Written in the early nineteenth century, the poem depicts the Lodore Falls, a waterfall in the English Lake District, using onomatopoeia. Stretching over more than a hundred lines, it probably exhausts the lexicon of descriptors for moving water: “And whizzing and hissing . . . And moaning and groaning . . . And thundering and floundering.” But water sound is more than just waterfalls and rare tidal bores; there is immense pleasure to be taken from the quiet and subtle, like a babbling brook. The remarkable thing is that in both a roaring tidal bore and a lazy winding creek, the tiny air bubbles make sound at the frequencies where our hearing works best. The physics seem just right for Southey’s Romantic poetry. But maybe this is more than coincidence. Perhaps our hearing has evolved specifically to discern the frequencies produced by running water. After all, if our hearing worked in a different frequency range, we would be deaf to water, a substance vital for survival.

  The frequency of the plink when a drop falls into water can be calculated from the radius of the air bubble formed. There is also a mathematical relationship between size and frequency with frozen water. During our visit to Iceland, my wife and I were on the south coast where the calving Breiðamerkurjökull glacier forms icebergs that float away on the Jökulsárlón lagoon. The haphazardly shaped blocks, looking too blue to be natural, break up and drift out to sea, or become stranded on the volcanic black beaches. Tourists make brief stops here, snapping pictures or taking a boat tour to get close to the ice, before carrying on their journey around the main ring road. We decided to camp by the lagoon. During the night, without the noise of cars and boats, we were serenaded by a tinkling sound. Small chunks of ice on the shoreline gently rocked on the lapping waves, clinking together and making rhythmic music like sleigh bells.

  The frequency of the sound depended on the size of the icicles, something that Terje Isungset, a Norwegian drummer and composer, demonstrates with his ice xylophone. Many years after my trip to Iceland, at the Royal Northern College of Music in Manchester, England, I went to hear what Isungset described as “the only instruments you can drink after you’ve finished playing.”17 He is an archetypal Norwegian Viking, tall with rough tousled hair, and plays while wrapped in a parka. The performance was full of atmospheric and ambient sounds, evoking memories of trips to Norway.

  Like a Scandinavian summer, the concert hall was cold. Even with that precaution, the musical instruments do not last long. Dressed in a large winter coat and gloves, an assistant brings out the ice trumpet or the bars of the xylophone. Once the performance has ended, the attendant quickly wraps the instruments and whisks them away to the freezer.

  The ice trumpet flares outward dramatically. It is treated at the mouthpiece to prevent Terje’s lips from sticking to the instrument. It has a primitive sound, like a hunting horn, and reminds me of the conch shells I once heard in Madrid. From an acoustic perspective, the material of a wind instrument is not so important if it is hard, as discussed in Chapter 4. Shell, horn, and ice may look very different, but as far as a sound wave traveling in the bore is concerned, they are similarly impervious materials. It is the shape of the outward-flaring bore and what the musician does with his lips that are most significant. Scientific measurements have shown that conch shells have exponential flares, like a French horn, creating a distinctive timbre and helping to amplify and project the sound.18 I imagine the ice trumpet works on the same principle.

  The xylophone had five bars resting on an ice trough, with their various sizes determining the different note frequencies. The bars had been cut from a frozen Norwegian lake with a chain saw, expertly carved, and then transported all the way to England. In contrast to the trumpet, the material of the xylophone is critical because the ice is actively vibrating. Once the bar is trembling, the air molecules next to the bar pick up the vibration, creating sound waves that move through the air to the listener. The air in the trough also resonates, amplifying the oscillations of the air and making the sound louder.

  Terje cannot use just any old ice. He must find ice with the right microscopic structure. As Terje explains, “You can have 100 pieces of ice; they will all sound different. Perhaps three will sound fantastic.”19 The microscopic structure of a bar depends on how many impurities there were in the water when it froze, and the conditions under which the ice formed, especially the ambient temperature, which affects the speed of freezing. A slow freezing process is best because it allows the crystalline structure to form in a regular pattern with fewer flaws, enabling the ice to ring rather than emit a disappointing thud.

  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?20

  Scientists have found that we can reliably discriminate between materials only when those materials have very different physical properties, like wood and metals.21 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.22

  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 wate
r, 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.”23

  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.”24 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.25

  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.”26 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.27

  The 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.”28 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.”29

  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.30 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.”31

  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.32

  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.33

  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.34

  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.”35 These sounds were caused by a swarm of small earthquakes, as confirmed by seismic monitoring.36 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.37

  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.”38 (Sometimes a double bang is heard—one caused by the wake from the nose, the other by the wake from the tail.)