The Sound Book: The Science of the Sonic Wonders of the World (14 page)

Sudden loud events like naval sonar are not the only sounds that affect marine wildlife. There is chronic shipping noise. In the northeastern Pacific Ocean, shipping noise increased by approximately 19 decibels from 1950 to 2007.
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This ever-present din could damage aquatic life. It overlaps the frequencies that whales use to communicate, changing vocal patterns: they sing longer, call louder, or move elsewhere. Often whales simply stop communicating, which is a reasonable reaction to short-lived natural sounds, like storms, but not to perpetual shipping noise. Unfortunately, while shipping creates a background cacophony, the noise does not project forward of a ship's bow, which can lead to collisions because whales do not hear boats approaching.

A piece of ingenious scientific opportunism by Rosalind Rolland, of the New England Aquarium in Boston, and colleagues, demonstrated a physiological effect of chronic noise on whales. Rolland's group took advantage of a lull in shipping traffic following the 9/11 terrorist attacks to see how the North Atlantic right-whale population in the Bay of Fundy, Canada, was affected. They monitored whale stress hormones, using sniffer dogs to find floating feces to analyze. After 9/11, the shipping noise reduced by 6 decibels, and Rolland measured a corresponding drop in the whales' stress hormones.
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Trying to determine the long-term effects of this chronic noise exposure on sea creatures is difficult. Bombard fish with loud noise in a tank and they will move away. This reaction suggests that noise might displace fish populations from breeding and spawning grounds, and obscure the communication between animals needed for finding mates, navigating, and maintaining social groups. But what scientists are grappling with is how to measure any harm, when the effects might take years to become apparent and aquatic life can move vast distances.

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here does aesthetics fit into whether a natural sound is good for us? In China and Japan, crickets and other insects used to be kept as pets because of their beautiful sounds. In the Sung Dynasty (AD 960–1279), they were the original portable music player. In the introduction of her book on insect musicians, Lisa Ryan writes, “Fashionable people were never without chirping crickets concealed under robes.”
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Rather than pressing a shuffle button, cricket owners used a tickler to stir the insects and make them perform. For me, however, insects are best heard in choruses, especially when the sound can be embellished by the acoustic of a forest. Chris Watson told me about hearing such choruses in the Congo rainforest in Africa. As the temperature drops at sunset, hundreds, possibly thousands, of species contribute to what he described as an “amazing chorus of sound which just rolls out like a wave from the forest.”
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They create a rich musicality, a “Phil Spector Wall of Sound” that within an hour is gone.
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Chris's best recordings come from sweet spots where any individual insect is not too prominent and the sound “percolates through the acoustic of the environment.”
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The forest changes the calls, and animals have to adapt in response, to compensate for the distortion created by their surroundings. As sound moves through trees, it bounces off trunks and branches. So, in addition to the direct sound propagating straight from the calling animal, are delayed versions that have reflected off the trees.

The similarity between the acoustic of a forest and that of a room has led to scientific papers with titles such as “Rainforests as Concert Halls for Birds.”
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Recently, walking around lakes and forests in Germany, I tested this out for myself. I noticed how the acoustic changed as I left the open meadows and entered the conifer forest, and when no one was nearby I shouted and listened to the sound reflecting back from the trees. The reverberation time in a forest has been measured at about 1.7 seconds, quite similar to that of a concert hall for baroque music.
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Forests transmit bass more easily than treble because at high frequency, foliage absorbs sound. This could explain why rainforest birds tend to produce low-frequency songs with drawn-out simple notes.
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Not only does this form of sound avoid attenuation by the foliage, but the reflections from the trunks amplify the sound of the notes in the same way that room reflections embellish orchestral music. When I shouted in the wood in Germany, I noticed this amplification, but it is subtle because reflections from trees are not as strong as those from the walls of a concert hall.

There is also evidence that birds adapt their singing as their surroundings change. Evolutionary biologist Elizabeth Derryberry examined changes in the calls of male white-crowned sparrows during the last thirty-five years using historical and contemporary recordings from California. In places where foliage has become heavier over the decades, she found that the song pitch has dropped and the birds now sing more slowly.
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In contrast, the songs remain unaltered in the one area where the foliage has not changed.

Forests are not the only determining factor for birdsong. The most extensive research into chronic noise has investigated how birds deal with the rumble of traffic. Great tits in cities such as London, Paris, and Berlin sing faster and higher in pitch compared with those living in forests; urban nightingales sing louder when there is traffic, and robins now sing more at night, when it is quieter.
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For great tits, low-frequency singing is important to demonstrate the fitness of males because bigger, healthier birds can sing lower tunes, but their songs can be drowned out by traffic noise. As Hans Slabbekoorn from Leiden University in the Netherlands puts it, “There is a trade off between being heard or being loved.”
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There are fears that noise changes the balance of species, and consequently the songs we hear in cities. It has been suggested that there are fewer house sparrows because they are unable to adapt their songs to the urban din.
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The adaptation of song to habitat could be one way that birds develop their own dialects. As humans learn to speak, they pick up an accent as they hear other people talk. Similarly, some bird species learn songs by imitation and so can be influenced by their neighbor's singing. The three-wattled bellbird has different dialects in Central America. One, heard in the northern half of Costa Rica, contains loud bonks and whistles. In contrast, the calls from southern Costa Rica and northern Panama contain loud rasping quacks.
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Bird dialects have been extensively studied, not least because they give insight into evolution and how species develop. If the songs of neighboring bird colonies diverge—say, because of changing habitats—then eventually they will stop communicating and mating with each other. Once that happens, their genes will no longer mix, which means the colonies will start going down different evolutionary paths and potentially form different species.

The nightingale is a plain-looking bird, but its singing is commonly cited as among the most beautiful in Europe. Listen to a few recordings of a nightingale, and you will notice how many different songs the male can produce. Because they live in thickets, the large vocal repertoire is a more effective display of prowess than is something visual.
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In 1773, English lawyer, antiquarian, and naturalist Daines Barrington put the nightingale as top-of-his-pops, based on scoring different British birds for their sprightly notes, mellowness of tone, plaintive notes, compass, and execution.
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A duet between acclaimed cellist Beatrice Harrison and a nightingale was the very first live outside broadcast on BBC radio, in 1924. The nightingales in the woods around Harrison's home in Oxted, England, had taken to echoing her cello practice. The broadcast was almost a failure, however, because the birds were initially microphone shy. But they did eventually sing, and the program was so popular it was repeated for the next twelve years and became internationally renowned.
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The nightingale has a beautiful song, which means it is a potentially restorative sound; however, our response to animal calls goes beyond aural aesthetics. When people wrote to Andrew Whitehouse about their experiences of birdsong, the iconic nightingale and its wonderful warbling rarely appeared in the stories. People more often wrote about the stuttering long cry of herring gulls in seaside towns or the excited screeching from flocks of swifts. Sometimes these calls brought back childhood memories: “A Common Gull, just this moment, cried outside my room window. My instant response to that is clear pictures of massed trawlers at Point Law (Pint La) where I spent school holidays.” Or songs that signified the seasons: “The birdsong I love the best is the scream of the swift, because of its associations with summer.”
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Thus, the natural sounds most likely to be restorative and best for our health are those that are familiar and bring back happy memories. When I asked Chris Watson for his favorite sound, he did not choose something exotic from his recording trips around the world; instead he described the complex, rich, and fruity song of the blackbird—something he could hear in his back garden. Hearing nature is different from viewing it, however, so we need new theories to explain which sounds are good for us and why. I enjoy hearing the sound of ducks, not because I think quacks are especially beautiful, but because the sound brings back fond memories of measuring echoes.

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here is a saying, “A duck's quack doesn't echo and no-one knows the reason why.”
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Hoping to disprove this one slow afternoon at the office, I found myself semiprone on a grassy knoll, pretending to interview a duck named Daisy. Every time she quacked or stretched and opened her wings, camera shutters fluttered like castanets. My colleagues stood close by, unable to contain their laughter. The press had caught wind of our modest attempt to correct the misconception about the supposed non-echoing quack and were doing their best to turn it into an international news event.

Little did I know that, a few years after fronting this frivolous science story, I would once again become engrossed in echoes, rediscovering the childlike pleasure of finding places where a yell resounds with satisfying fidelity to the original. But there is more to echoes than shouting in tunnels or yodeling in the mountains; depending on the type of echo, sound can return magically distorted—claps turning into chirps, whistles, or even zaps from laser guns.

Early documenters of natural phenomena, such as the seventeenth-century English naturalist Robert Plot, used fantastic terms such as
polysyllabical
,
tonical
,
manifold
, and
tautological
to describe the mystery of echoes. But while the cataloguing of animals and birds has survived to the present day and still captures interest, the same is not true of echoes. It is time to revive the echo taxonomy. Can an echo turn a single word into a sentence? Or return the voice “adorned with a peculiar Mu[s]ical note”?
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Or even transpose a trumpet tune, with each repeat being at a lower frequency?

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few months before the photo shoot with Daisy, Danny McCaul, the laboratory manager at Salford University, had been approached by BBC Radio 2 to find out whether the phrase “a duck's quack doesn't echo” was true or false. Ignoring Danny's careful explanation of why a quack will echo, the factoid was still broadcast. Annoyed that his acoustic prowess had been overlooked, Danny and some of his colleagues, including me, decided we needed to gather scientific evidence to prove the point.

Convincing a farm to lend us a duck and transporting it to the laboratory were probably more time-consuming than the actual experiments. First we placed Daisy in the anechoic chamber and made a baseline measurement of an echo-free quack. The
anechoic chamber
is an ultrasilent room where sound does not reflect from the walls; it is without echoes, as the name implies.
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It was important to have a reference sound without echoes; after all, this was a serious piece of science and not a bit of Friday afternoon fun. After a brief comfort break for Daisy, she was carried next door to the reverberation chamber, which sounds like a cathedral with a very long reverberation time, despite being little bigger than a tall classroom. Normally, the chamber is used to test the acoustic absorption of building parts like theater seating or studio carpets. In this room, Daisy's quacks sounded evil and ghostly as they echoed around the room, the noise prompting her to cry out again and again. We had created the ultimate sound effect for a horror movie, provided the film featured a vampire duck.

An echo is a delayed repetition of sound, which for a duck might be caused by a quack reflecting off a cliff. The vampiric cry in the reverberation chamber demonstrated that quacks reflect from surfaces like every other sound. We were not surprised by the result, not least because there are bird species that echolocate, using wall reflections to navigate caves. The great Prussian naturalist and explorer Alexander von Humboldt wrote about one of these species, the oilbird, a nocturnal frugivore (fruit eater) from South America. On a visit to the Guácharo Cave in Venezuela in the late eighteenth century, Humboldt experienced the squawking and clicking of the roosting birds. The clicks are the echolocation signals; the birds listen to the reflections to navigate in the dark.
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