The Sound Book: The Science of the Sonic Wonders of the World (12 page)

Some water boatmen use the resonance of the air within the bubble they carry around for breathing to amplify their calls. They do this by closely matching the frequency of their body vibrations to the bubble's resonant frequency. As the air bubble shrinks, the resonant frequency rises, and the water boatman needs to stridulate faster.
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Snapping shrimp also use bubbles to help make their sound—sometimes for communication, but at other times to kill their prey. The method of making the sound is remarkable because it does not come from the claws tapping each other. In 2000, Michel Versluis, from the University of Twente in the Netherlands, and collaborators used high-speed video to reveal the secret. The shrimp closes its claws very rapidly, with the tips moving at 70 kilometers (about 45 miles) per hour, creating a jet of fast-moving water. Following Bernoulli's principle, the pressure drops in the rapidly moving water, low enough for the water to start boiling at sea temperature. A bubble of water vapor forms, which immediately collapses and creates a shock wave that stuns or kills prey.
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(Light is also made, in a process, nicknamed “shrimpoluminescence.”)

Large colonies of snapping shrimp create a noise like the crackling from a roaring fire. Chris Watson reckons this must be the most common animal sound on our planet, yet “it is a sound that not that many people get to hear.”
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The shrimp also pose problems for natural-history recordists: “I was trying to record the voice and song of the blue whale off the north coast of Iceland, the largest and loudest animal that has ever lived,” Chris told me, “and alongside that, at times I couldn't hear the blue whales at a distance because of the snap, crackle, and pop of these animals, which are a couple of centimeters long.”
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This problem is familiar to the military; the study of snapping shrimps began in World War II because the noise was interfering with efforts to hear enemy submarines.
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It seems odd that tiny, vulnerable animals draw attention to themselves by making so much noise. The Victorian missionary and explorer David Livingstone wrote on a trip to Africa, “The stridulous piercing notes of the cicadae are perfectly deafening; a drab-colored cricket joins the chorus with a sharp sound, which has as little modulation as the drone of a Scottish bagpipe. I could not conceive how so small a thing could raise such a sound; it seemed to make the ground over it thrill.”
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Perhaps he heard the African cicada? This is the loudest insect, reaching 101 decibels 1 meter (about 3 feet) away—as noisy as a pneumatic drill.
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But cicadas are not the only incredibly loud chorusing animals. David Livingstone reported, “When cicadae, crickets, and frogs unite, their music may be heard at the distance of a quarter of a mile.”
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Frogs are meant to go “croak,” but someone failed to pass along this information to the amphibians in Hong Kong Park. Built on a former army barracks site in Central District, the park gives respite from one the most densely populated cities in the world. When I visited in 2009, the frogs in the park made squelching chatter like a poor impersonation of Donald Duck. Frogs mostly call with their mouths shut, their vocal sac swelling up beneath the mouth like a giant bubblegum bubble. Frogs do not breathe out when calling for a mate; they circulate the air from lungs to mouth to vocal sac, with the sound escaping via vibrations of their head, vocal sac, and other body parts.
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Like humans, frogs have a pair of vocal folds that open and close as the air rushes by, breaking up the constant flow of air into pressure pulses that form sound. Humans amplify their voice using the resonances of the air in their vocal tract (the mouth, nose, and air cavity at the top of the throat). But in frogs, the amplifying resonance comes from the skin of the vocal sac. If a human talks after breathing in helium, the change to a lighter gas in the vocal tract shifts the resonances up in frequency, producing a funny squeaky voice. Get a frog to inhale helium, as some scientists have done, and the call is largely unaltered—evidence that the resonance of the air in the frog's vocal sac is not what amplifies the calls.
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Rather than imperiling the community, the communal racket creates an evolutionary defense. While bigger frog choirs attract a few more predators, they also attract a lot more females. Each individual frog is less likely to die, and more likely to find a mate.
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When I walked too close to the frogs in Hong Kong Park, the croaking suddenly stopped, with the wave of silence signaling a threat through the froggery.

Sound designer Julian Treasure believes that most people find birdsong reassuring because over hundreds of thousands of years we have learned that when the birds are singing, everything is OK. It is when the birds go quiet that you need to worry, because it could be a sign that a predator is about. A plausible argument, but not one, I think, that has ever been tested scientifically.
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This idea has led Julian to use birdsong in some of his sound designs, including as a crime deterrent in Lancaster, California. Loudspeakers looking like small green bollards dot the flower beds along the main shopping street and are meant to play a mix of twinkly electronic music, water movement sounds, and songbirds.
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Unfortunately, when I visited Lancaster one Sunday afternoon, the day after I met the Hollywood sound designers, the loudspeakers were just pumping out middle-of-the-road country-and-western music. Not a particularly soothing choice perhaps, but there is precedent for using music as a crime deterrent. In Australia, the “Manilow method” uses easy-listening tracks to disperse teenagers, by making places uncool to hang out in. There is plenty of anecdotal evidence that this tactic works, even though Barry Manilow asked, “Have they thought that the hoodlums might like my music? What if some of them began to sing along with ‘Can't Smile without You'?”
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T
here are regular news stories about animal noise disturbances; it is difficult to believe, for example, that people complaining about their neighbor's rooster find this natural sound restorative. While loud animal calls are thrilling to listen to, these sounds overload our auditory system, prevent us from hearing other signs of danger, and can even trigger an early warning system that puts us on alert.

Familiarity plays an important role in our response to all sounds, including animal calls. Andrew Whitehouse, from Aberdeen University in Scotland, has been researching the relationship between birds and people, specifically the effect of birdsong. Early on, the media picked up on his research, prompting people to write to him with their personal stories, which produced a gold mine of data for an anthropologist. Take the following story sent to Andrew, from someone who had moved from the UK to Australia:

The Australian birdsong is really quite disruptive. We have heard of people emigrating BACK to the UK because of the “ugly” birdsong here. In a nutshell I would describe the sub-conscious effect of “birdsong” here as being to raise people's tension. It is a series of screeches or other worldly sounds.
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There are many stories, like this one, from people who emigrated and were surprised at how much the change in birdsong affected them. Even people who had previously largely ignored natural sounds felt alien because of the bird calls.

Unfamiliarity can also be a source of delight, as I found when I visited the dry rainforests in Queensland, Australia, a few years ago. The eastern whipbird is named after the sound of its call. The male starts with a sustained whistled note for a couple of seconds before an explosive crescendo on a glissando, which abruptly ends like a whip being cracked, leaving the sound to reverberate through the trees.
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The starting note is at a high frequency, somewhere in the middle of a piccolo's range, and the glissando sweeps across a wide span of almost 8,000 hertz in just 0.17 second, like a piccolo player starting on the lowest note and sweeping through the whole of the instrument's range and beyond.
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Since the whip cry must be a difficult vocal skill, female whipbirds may well use the quality of the performance as a sign of how fit a male is. Sometimes the song turns into a duet as a female responds with a couple of quick syllables: “chew-chew.” The calls are more common when mates are being chosen—a strong indication that the duets play an important part in forming and maintaining partnerships.
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Of course, one can hear unexpected bird sounds even in one's home country. Bitterns are reclusive wading birds that, for most of the last century, hovered on the brink of extinction. They are a type of heron that make the most extraordinary bass sounds, which can carry for many miles over their reed bed habitat. Many scientific papers detail how to count and identify individual bitterns from their calls, because they are very difficult to see but easy to hear. Their call is immensely powerful: at 101 decibels at 1 meter (about a yard) away, it has a volume similar to that of a trumpet.
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And at about 155 hertz, a typical frequency produced by a tuba, the bittern's call is often likened to a distant foghorn.

As sound moves through the air, tiny amounts of energy are lost through absorption every time the air molecules vibrate back and forth, and this absorption limits how far the wave can travel. By definition, lower-frequency sounds vibrate fewer times than high-frequency ones, and thus they lose less energy over long distances and travel much farther than high frequencies. So the bittern's bass booms are effective across the reed beds.

A suffocating thick fog hung over Ham Wall wetland reserve near Glastonbury, England, when I went to hear bitterns one spring morning. We had a very early, five o'clock, start because, like most other birds, bitterns are most vocal at sunrise. My guide was John Drever, who had stuffed his car trunk with strange-shaped microphones, recorders, and boom arms. Wearing a flat cap to protect him from the bitter cold, John looked more like a cat burglar than the friendly musician and acoustic ecologist he really is. Once we had parked at the reserve, we staggered down the path to the nesting sites, barely able to see in the dark fog. We eventually stumbled across a bench in a bird blind, sat down, and listened.

I first heard the call off to the left, sounding like a distant industrial process starting up—quite unlike any other bird I had previously encountered. In
The Hound of the Baskervilles
, the villain, Jack Stapleton, tries to fool Sherlock Holmes by suggesting that a “deep roar” and a “melancholy, throbbing murmur” was a bittern calling and not the hound of hell. Unluckily for Stapleton, a bittern sounds nothing like a dog.
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The bittern I heard reminded me of someone blowing across a large beer bottle in the pub, or the jug from an old-fashioned jug band. Then a moment later another bittern to the right joined in at a slightly higher frequency. We moved to another blind, where I was close enough to a bittern to hear the buildup to the call. The bird gulped in air four times and then let out seven clear booms a couple of seconds apart. Exactly what the bittern does to make the call is unknown, for this is a secretive and well-camouflaged animal. Rare video footage shows the extraordinary prelude to the booms, as the bittern's throat swells up and the body convulses while the air is gulped in, looking like a cat preparing to cough up a fur ball. But then the bird remains almost still as the sound is made. Since the number of bitterns is growing, maybe more observations of the booming will be possible and will solve the mystery. In 1997 there were only eleven booming males in the UK; in 2012 there were at least a hundred, because of restoration of the reed beds.

Scientists have been measuring when the booms are made to try and understand the purpose of the call and whether it relates to breeding success. The fact that the males call before mating implies that the females assess the fitness of competing males by the strength of their booms. Booms are produced during nesting as well, suggesting that they are also used to defend feeding territories.

An hour and a half after we arrived, the light grew brighter and the bitterns stopped booming. Frozen to the bone, we headed back to the car. I suddenly became aware of the bedlam from the songbirds twittering around me. I had been concentrating so hard on low-frequency booms that I had tuned out the high-frequency warbling. In this environment, the bittern call could easily be mistaken for a loud human-made noise. For a sound to be restorative, it needs to be unambiguously natural and avoid setting our alert system on edge. If we are familiar with the source or have an expert guide to explain it to us, we can categorize the sound as natural and a nonthreat and take comfort in it.

A
couple of months before the trip to hear bitterns, at a TEDx conference in Salford, England, biologist Heather Whitney talked about how plants evolved to attract pollinators, such as orchids that look and smell like female wasps to con the males into trying to mate with them and thus spread pollen.
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It was a great talk, but it was the new acoustic research Heather told me about later in the café that really got me excited. One of her colleagues had found plants that evolved leaves specially shaped to attract their pollinators: echolocating bats.

Beyond the human hearing range is an extraordinary ultrasonic world. Bats exist in a plane of hearing where nearly all sounds exceed 20,000 hertz, or 20 kilohertz (1 kilohertz = 1,000 hertz), the upper threshold of our auditory perception. Three months after the TEDx event, I joined a group of about twenty people for a bat walk at twilight in the moorland village of Greenmount, England. The meeting point was the parking lot of the local pub. It was not difficult to identify the guide. With pictures of flying mammals on her T-shirt and phone, Clare Sefton personified bubbly bat enthusiasm. A research scientist in a different subject, she goes to academic conferences on bats just for fun and is an amateur veterinarian for bats. Before walking down the Kirklees Valley, she showed us a couple of patients she was nursing back to health. One came from the largest species in the UK, a noctule bat, with reddish brown fur, very cute, like a big mouse with wings. It kept opening its mouth and flashing its teeth; as Clare said, it was “having a good look at us,” shouting its echolocation signal. The other bat was a tiny common pipistrelle, which, despite having a body only about 4 centimeters (1½ inches) long, manages to eat 3,000 insects in one night.