The Sound Book: The Science of the Sonic Wonders of the World (24 page)

After the subjects had scored how much they liked each sound, Watts came to the conclusion that the worst noises had a booming quality, reminiscent of water flowing down into drains or utilitarian culverts. The most pleasing sounds sploshed and splashed, having a natural randomness as the water fell onto an uneven surface formed from small boulders. In similar tests, Galbrun found that the gentle babbling of a slow-moving natural stream was the most relaxing of all water sounds tested.
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The source of the waterfall sounds surprised me at first. A television crew recently filmed what happens in my university's anechoic chamber. A high-speed camera captured a single drop falling into a fish tank of water. A slow-motion video from the top looks pretty. The drop causes a narrow column of water to rise up from the surface, creating ripples. But to understand what is heard, you need to look from the side, just underneath the surface of the water. While the ripples are visually impressive, a single tiny air bubble generates most of the sound. As the bottom of the drop penetrates the surface of the water, a bulging meniscus is created from which a tiny air bubble suddenly breaks away. This bubble of trapped air is only a few millimeters in diameter and so is easy to overlook and difficult to film. It may be small, but the air inside the bubble vibrates, resonates, and creates a plink that travels through the water and into the air.

Water falling onto rocks sounds very different because no underwater bubbles can be made (unless a layer of water has built up on the stone). Again, it is easiest to think about what happens when one drop smashes onto a rock and is splattered across the stone. As the falling drop smears itself into a thin layer of water on the stone, it disturbs the air around it, creating the sound.

A couple of months after the television crew filmed a single bubble, I got to learn more about aquatic sounds from artist Lee Patterson. We met up in the English Lake District, where Lee described how, in local ponds and water courses in northern England, he had discovered underwater sounds as rich as tropical rainforests. We chatted about the piece he was going to compose from his recordings in the Lake District.
The Laughing Water Dashes Through
was going to be a work prompted by the devastating floods that had hit the nearby market town of Cockermouth a few years previously. Lee explained how the work would explore the “different forms of energy embodied by water flow, and the sound that happens as a by-product of the water flow.”
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He was recording in a small, enclosed, flooded quarry on the day I visited. With blazingly hot sunshine and the birds singing around us, it was an idyllic spot (provided we stood with our backs to an ugly concrete shed). Lee had simple homemade hydrophones, constructed from a sliver of shiny piezoelectric material, which makes electricity when it is deformed by underwater sound waves, embedded in the tops of brightly colored plastic screw caps from pop bottles. He cast these into the water, turned up the amplifier, and passed me the headphones.

I heard a malevolent munching and crunching. It was as if an animal was trying to nibble away at my eardrum. The sound came from tadpoles scraping the hydrophones, in the vain hope that there was algae on the bottle caps. The tadpoles were swimming among oxygenating pondweed, and with a careful repositioning of the hydrophones, strange mechanical chirps could be heard, like bacon being deep-fried. These were caused by a fast stream of small bubbles rising up from the pondweed, looking like champagne bubbles rising up in a glass. It turns out that photosynthesizing plants were making the bubble streams.
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A few days later, I spoke to Helen Czerski from the University of Southampton, who studies how sound is made as bubbles are created. Her research shows that as bubbles form at a small nozzle, sound is created because the bubble initially has a teardrop shape while attached to the nozzle, but when it breaks away into the body of the water it forms a sphere. This shape change causes the bubble to vibrate, resonating the air inside and creating sound. Helen was skeptical that this is what happens with pondweed because natural bubbles from photosynthesis form more slowly and therefore probably lack the impulsive kick as they break off. She thought it was more likely that I was hearing the bubbles bumping into each other or into the hydrophones.

The waterfall at Dettifoss in Iceland can be explained by scaling up the effects of a single oscillating bubble, to consider the vast number of bubbles in the white cascade. Each ball of trapped air is a different size and plinks a note at its own particular frequency. In the cascade, the combination of millions of random plinks creates a vast bubble orchestra, which fizzes and roars.

Each waterfall has its own voice. If it tends to have lots of larger bubbles, it will have a bassy rumble. Smaller bubbles result in more hissing, like the Yosemite Falls described by Muir. Surrounding rocks can further alter the sound. Svartifoss in southern Iceland is only about 20 meters (65 feet) high. It has water cascading from a horseshoe of overhanging cliffs made up of hexagonal basalt columns. The name, meaning “black fall,” comes from the color of the rocks, and on the day I visited, the color was strongly emphasized by the overcast, drizzly weather. However, it is worth an hour's walk even in the rain, because not only do the surrounding rocks make stunning holiday snapshots, but they also amplify the water as it slaps and hisses against them.

Another impressive Icelandic waterfall is Seljalandsfoss, where you can go behind the curtain and be surrounded by noise, as the fizzing from the water hitting the pool is reflected from the cliff behind you. The flow of water is not constant, which means the noise splutters. Close your eyes, and you can imagine a small freight train rumbling past overhead.

While waterfalls are common, the sound of a tidal bore, a single tall wave that sweeps inland up a narrowing estuary, is much rarer. The bore of Rio Araguari in Brazil is named
pororoca
in the language of the native Tupi, which translates as “mighty noise.”
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Closer to my home is the Severn bore, near Gloucester in England. Early on a misty September morning, part of a brief Indian summer, forecasters predicted a four-star bore on the heels of a large tide driven by the autumnal equinox. As I wandered along the banks of the river, I saw a few surfers midstream, clutching their boards ready to catch the wave; this must be a good place to watch, I thought. I first stood close to the water's edge but then realized that the silt all around me was from the previous night's tide, so I retreated higher up the bank. You have to be careful when dealing with tidal forces. In China, eighty-six people were swept away by a tidal bore on October 3, 1993.
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Then I waited, waited some more, and then waited even longer. Twenty minutes behind schedule, a rumble started downstream. The bore came into view and broke on the opposite bank, forming a continuously breaking wave right across the river. It resembled a big ocean breaker, but instead of the soothing rhythm of waves crashing on the shore one after another, the bore produced the continuous sound of a breaking wave.

Right behind the Bay of Fundy in Nova Scotia, the Severn Estuary has the second-highest tidal range in the world—as much as 14 meters (45 feet) for spring tides. A map of the River Severn shows its sinuous funnel shape. What the map does not show is that the depth of the river decreases rapidly as you go inland. When the huge tide enters the estuary at the sea, the water is forced up the narrowing channel, which gets shallower and shallower. The excess water can go only one way, upward, thus forming the surge wave.

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.”
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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.”
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Hubert Chanson has studied the acoustics of the bore near Mont Saint-Michel in northern France.
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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.”
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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.
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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.”
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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.