The Sound Book: The Science of the Sonic Wonders of the World
Page 18
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.39
Captain Sampson was only a few tens of miles from the Indonesian volcano. The power of the eruption was so great that observers on the island of Rodriguez in the middle of the Indian Ocean 5,000 kilometers (3,000 miles) away heard the explosions. Chief of police for Rodriguez, James Wallis, noted, “Several times during the night . . . reports were heard coming from the eastward, like the distant roar of heavy guns.” This is a vast distance for audible sound to carry—about the same distance as separates London from Mecca in Saudi Arabia.40 I remember news reports of the dramatic eruption of Mount St. Helens in Washington State in 1980. Had the noise of that eruption been as powerful as the sound of Krakatoa, it would have been heard all the way across the northern US, reaching Newfoundland on the east coast of Canada.
The audible blasts and explosions from Krakatoa carried an astonishing distance, but other, inaudible sounds carried even farther. Volcanic eruptions produce lots of infrasound, sound at such a low frequency that it is below the range of human hearing. (The lowest audible frequency is roughly 20 hertz.) Weather barometers around the world picked up the infrasound from Krakatoa and showed that the low-frequency waves traveled around the globe seven times, a distance of about 200,000 miles, before becoming too small to be detected.
Today, scientists monitor volcanic infrasound to help forecast and differentiate eruption types, supplementing measurements of ground vibration made using seismometers. The infrasound emitted is altered by what is happening deep inside the ground, so it gives a unique insight into the internal workings of the volcano from a safe distance.
Volcanoes make other quieter but audible sounds, include bursting bubbles, magma fragments splatting onto rocks, and gas jets hissing and roaring through vents. To experience some of these without risking life and limb near an erupting volcano, head for an active geothermal area.
Iceland is like a geological textbook writ large, with sounds that powerfully portray the forces that shape the Earth. Iceland lies on the Mid-Atlantic Ridge between the North American and Eurasian tectonic plates. The diverging plates are shaping the landscape through earthquakes and volcanic activity; the countryside is strewn with cinder cones, knobbly lava fields, and rocky rifts. At Hverir in the north, the apricot-colored landscape looks as though it is suffering from a chronic bout of yellow acne. An evil sulfurous smell invades the nostrils, and visitors have to be careful where they tread, lest they end up knee-deep in scalding liquid.
Dotted here and there are waist-high cairns of rock, gravel, and earth that hiss alarmingly as steam rushes out, sounding as though they are in imminent danger of exploding. Groundwater percolates downward for over half a mile before being heated by magma and forced back to the surface as superheated steam at 200°C (400°F). The high-speed steam escaping through gaps in the mounds jostles the adjacent still air. The result is invisible spiraling of the air that causes the hissing. You might visualize these small vortices as tiny versions of the Great Red Spot on the surface of Jupiter or a spiraling tornado.
Elsewhere, tumultuous pools of battleship-gray mud bubble at a low simmer. They almost seem alive; some belch like a thick, gloppy lentil soup, while others rage and splatter like an unappetizing, thin gruel on a fast boil. Some have an almost regular rhythm, sounding like sped-up music. Hydrogen sulfide creates both the pervasive odor and the mud within the bubbling pots as sulfuric acid dissolves the rocks.41 The slurry in the mud pots is thrown up into the air by superheated steam and splashes into the water. Though no scientist has studied mud pot acoustics, I presume bubbles are what make the sound, as in a waterfall.
Struggling to find a scientific paper on the sound of mud pots, I contacted Tim Leighton from the University of Southampton. Tim looks like a middle-aged Harry Potter but is an expert in bubbles, not potions. He had not looked at mud pots, but he did mention the model geyser he had built at the age of twelve, complete with pressurized boiling water. Every three minutes, it erupted, throwing hot water an impressive 2–3 meters (6–10 feet) into the air. Tim lamented, “Unfortunately, I didn’t know then to write the work up and submit it to a journal. But I now have a duplicate in a lab directly below my office.”42
The word geyser comes from the hot spring Great Geysir in southwestern Iceland. Unfortunately, Great Geysir has not erupted naturally for many decades, but close by is Strokkur (the “churn”), which produces a 30-meter-high (100-foot) spout every few minutes. Amid the crowd standing behind a rope barrier is an excited multilingual chatter as the onlookers try to guess when the geyser will erupt. The first sign is a dome of water that billows up from an opening in the ground, quivering like a giant turquoise jellyfish before, suddenly, with a whoosh, hot water is sent flying high into the air. When the water hits the ground, it fizzes and hisses, like sea waves crashing onto rocks.
Geysers are rare because they need an unusual set of conditions. Belowground, the natural plumbing must have watertight walls, with a supply of water to replenish the pipework and geothermal heat. Superheated water fills the plumbing from the bottom, while colder groundwater enters the pipework nearer the surface. The weight of the cold water at the top allows the heated water lower down to exceed the normal boiling point of water without bubbling. The dome bulges from the top of Strokkur when the plumbing is completely full. Inevitably, a few steam bubbles form that push a small amount of water out the top of the geyser. This release of water lowers the pressure deep inside, causing an explosive formation of more steam from the superheated water. The steam then forces the column of water out the top of the geyser and high into the air.43
Some of the most amazing natural sounds, like those generated by Strokkur, originate in places far from habitation. A few years before venturing into the Mojave Desert, I encountered tuneful sand at Whitehaven Beach on Whitsunday Island, Australia, but that was a soprano compared to Kelso’s bass voice. The hot, blindingly white sands of the Australian beach squeaked at a much higher frequency, typically around 600–1,000 hertz. I came across the sound effect unexpectedly while on vacation, and I had fun shuffling up and down the beach trying to get the best squeal. Charles Darwin described hearing something similar in Brazil: “A horse walking over dry coarse sand, causes a peculiar chirping noise.”44 This high-pitched sound is more common than the booming of sand dunes, and in Australia you can even go to a place called Squeaky Beach.
The fact that a squeaky beach or a droning dune produces distinct notes you can sing along with implies a coordinated motion of the sand grains. If the grains moved haphazardly, the sound would be more like the random rustling of leaves in a deciduous tree. The opening lines of Thomas Hardy’s pastoral novel Under the Greenwood Tree reveals how complex the sound of wind through trees can be:
To dwellers in a wood almost every species of tree has its voice as well as its feature. At the passing of the breeze the fir-trees sob and moan no less distinctly than they rock; the holly whistles as it battles with itself; the ash hisses amid its quiverings; the beech rustles while its flat boughs rise and fall. And winter, which modifies the note of such trees as shed their leaves, does not destroy its individuality.45
Scientists such as Olivier Fégeant from Sweden have been researching the different ways these sounds are made.46 For a deciduous species in leaf, such as the beech described by Hardy, the leaves and branches bang into each other as the tree sways in the wind, causing the leaves to vibrate and rustle. A birch tree is a good impersonator of waves crashing on a shore.47 When the wind gets stronger, more impacts create a lo
uder sound, but the dominant frequency remains surprisingly unaltered.
Fégeant is interested in whether tree rustles might hide the swish of wind turbine blades. Most wind turbines are usually very quiet, but in remote locations there are very few other sounds to mask even the tiniest murmur made by the blades. Fégeant concluded that aspen was the best deciduous choice out of the trees he tested, creating sound 8–13 decibels louder than birch or oak. With an increase in 10 decibels being roughly a doubling in perceived loudness, an aspen will sound twice as loud as the other trees. However, there is an obvious drawback to using deciduous trees, because in winter they have no leaves and thus cannot rustle.
Evergreens offer the possibility of year-round noise. At the foot of the Kelso Dunes I heard the wind whistling through the wispy foliage of the tamarisk tree. A distinct note waxed and waned, but it was not a clear sound like a musical instrument produces. It was more like the sound that a child learning to whistle would make: a tone could be heard, but it was breathy and inconsistent. The soughing is made by the movement of air around the needles, like the whistling of wind through telegraph wires. (The production of these Aeolian tones is discussed in Chapter 8.) Each needle creates a tone whose pitch depends on the wind speed and needle diameter. Thousands of these tiny sound sources conspire to create Hardy’s moaning and sobbing sound. For Fégeant’s measurements on spruce and pine with a moderate breeze of 6.3 meters (21 feet) per second, the soughing was at 1,600 hertz, a frequency toward the top end of the flute’s range. Double the wind speed to a strong breeze, and the breathy tone increases in frequency by about an octave, to around 3,000 hertz, a note within the piccolo’s range.
Hardy’s description of trees moaning can be very apt, because as the wind speed naturally decreases, the sound droops in frequency like a sad person talking. For me, the tamarisk whistles were too high a frequency to remind me of moans. They were more like the sounds made by the casuarina tree (she-oak) in Australia. The drooping foliage consisting of spindly branchlets is known to create an eerie whistling, an ideal haunted-house effect for a movie. Mel Ward, a naturalist who spent many months living on islands around the Great Barrier Reef, wrote of being “lulled by the music of the sea and the sighing trees.”48 Unfortunately, nowadays the casuarina tree is largely missing from tourist destinations, and “air-conditioning, music and other amenities mask or obliterate the sounds of the outdoors.” The tamarisk brought back memories of beach holidays for me. I now realize that the whooshing sound I associate with childhood visits to the seaside was made by wind whistling through the gorse bushes on the sea cliffs.
Wind can make annoying sounds when it whistles through man-made structures. Completed in 2006, the 171-meter-tall (561-foot) Beetham Tower in Manchester, England, periodically features in local headlines because it howls in the wind. Once, the hum got so loud that it disrupted the recording of Coronation Street, the world’s longest-running soap opera. (The set for the program is located only 400 meters, or 440 yards, from the tower.)49
Rising vertically out of the top story of the skyscraper is a sculptural louver of glass planes supported on a metal scaffold. This structure made the tower the tallest residential building in Europe when it was finished, but it also causes the whistling. In very strong winds, the air blowing across the edge of the glass panes creates turbulence and noise. The turbulence is due to haphazard changes in air pressure—a smaller-scale version of the turbulence that causes aircraft to shudder and drop. Since sound is fundamentally tiny variations in air pressure, the turbulence creates noise (the same thing happens when a flutist blows across the instrument’s mouthpiece). A temporary solution to the tower’s hum, implemented in 2007, was to attach foam to the edge of the glass to mask the sharp edges and so prevent them from creating turbulence. Later that year, aluminum nosing was added, which stopped the noise for moderate wind speeds, but the building still defiantly hums in the worst storms.50
Turbulence is often created as wind rushes past structures such as bridges, railings, or buildings, but usually the noise is too quiet to be audible. Beetham Tower has awakened local residents and prompted dozens of complaints to the local government. To make noise that loud requires amplification by resonance. For a flute, it is the resonance of the air within the instrument that increases a note’s volume. For Beetham Tower, the resonances arise from the air trapped between the many parallel rows of deep glass panes.51
At low wind speeds, the turbulence noise generated by the edges of the glass is below the natural resonant frequency of the structure, and the tower is quiet. This fact hints at one answer to the problem: changing the size and spacing of the glass panes so that the resonance is at the wrong frequency to be excited by high winds. Such a solution solved the whistling of New York’s CitySpire Center. The drone from the tower was so bad that the building’s managing agents were fined for the noise, although for only the paltry sum of $220.52 The drone was about an octave above middle C and likened to a Second World War air-raid siren. It was generated by louvers that made a dome at the top of the building. Removing half of the louvers lowered the resonant frequency and solved the problem.
My late-night trip to hear Beetham Tower started unexpectedly. Idly browsing the web before bed, I noticed tweets from people complaining of being kept awake by the hum. One message from an acoustic engineer reported a level of 78 decibels about 100 meters (110 yards) from the base of the tower, which is equivalent to the volume you would experience while standing close to a tenor saxophone playing at a moderate volume level.53 When I went into my garden, I could hear a faint humming. Was that the tower, the drone of a nearby road, or maybe a distant helicopter? I pulled on some clothes over my pajamas, grabbed my sound recorder, jumped into the car, and drove into town. Ignoring the cold winter air, I opened up the sunroof and drove around the city with a microphone sticking up toward the night sky recording the hum.
I immediately confirmed that the hum was the same sound I had heard in my garden, which meant that it was carrying at least 4 kilometers (2½ miles) across the city. Ironically, it was difficult to get a good recording because it was so windy. Gusting air was creating turbulence across my microphone; the same physics causing the building to sing was ruining my recording. I put a foam windshield over the top of the microphone to alleviate this problem, but in such high winds it was almost useless.
The hum came and went as the wind gusted. It was an eerie, long note from a bass musical instrument, a distinct tonal sound at 240 hertz (roughly B below middle C). Because it was distinctive, the note was easy to pick out among the traffic noise, and this is probably why residents found it so disturbing. Our hearing finds it hard to ignore such tones—sounds one can sing along with—because they might contain useful information. After all, the vowels in speech (a, e, i, o, u) are often pronounced in a singsong way with distinct frequencies. Knowing that tones stand out to us also explains the simple short-term solution used by the television sound recordists for Coronation Street. By the addition of very quiet broadband noise to the sound track—the rumble of a distant busy road would have been a good choice—the hum was hidden by a noise less likely to grab a listener’s attention.
But just as the burping on the sand dunes is only the starting point of the sound, the noise created as wind whistles past the edges of the glass in Beetham Tower is only the initial impulse. Both the sand and the wind noise require amplification. For the dunes, what causes the amplification is still debated. One theory focuses on a layer of dry, loose sand, roughly 1½ meters (5 feet) deep, that sits on top of harder-packed material lower down.
Nathalie Vriend explained to me that the layer hypothesis came from her doctoral supervisor Melany Hunt, of the California Institute of Technology. To test Hunt’s theory, Nathalie has carried out field measurements on various dunes in the southwestern US. To reveal the underlying structure, she turned to geophysics, using ground-penetrating radar and seismic surveys. She also described using a probe 1 centimeter (about half an inc
h) in diameter to take samples from the dune. Getting the probe to go through the top layer of loose sand was straightforward, but about 1.5 meters (5 feet) down it hit a layer as hard as concrete: “We had our biggest, most muscular guy hitting that probe with the hammer and he couldn’t get it in any further.”54 A sample from the top of this very hard layer showed moist sand grains welded together with calcium carbonate, forming a largely impenetrable barrier to sound.
The top layer of loose sand acts as a waveguide for sound, similar to the way an optical fiber channels light. The avalanching sand produces a range of frequencies. The waveguide then picks out and amplifies a particular note. Similarly, the wind whistling past the louver of Beetham Tower creates sounds at many frequencies. The resonance between the glass panes then selectively amplifies particular notes, creating the audible hum.