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

by Trevor Cox

  We left the Cromarty Firth and entered the larger North Sea inlet of the Moray Firth. We were close to cliffs flecked with smelly white guano from the seabirds, with green slopes above lit up by swatches of yellow-flowering gorse. Then the skipper, Sarah, caught sight of a dolphin doing an arching jump out of the water.

  The engine was switched off to silence its rumble, and I lowered the hydrophone over the side. Initially, all I could hear was the slap of the water on the boat’s hull as it bobbed up and down on the swell. Then I heard it—a high-pitched rapid succession of clicks like a tiny toy motorbike revving up, almost inaudible against the water noise.45

  Next we saw a mother and her calf. The baby dolphin was smaller and a light gray. My thrills were out of sync with the rest of the passengers, as I was the only person with a hydrophone. My fellow passengers were using their eyes to look for dolphins, calling out with excitement whenever one leaped out of the water. But I needed the dolphins to be below the surface for my hydrophones. There was a visual delight having the dolphins so close that I could look them in the eye, but the sound was also magical, because it revealed a little of the underwater world hidden from my fellow passengers.

  Unfortunately, human-made noise is forcing animals to change their calls, including underwater mammals and fish. Are offshore wind farms an environmentally friendly way to make electricity? Possibly not, if you are a harbor seal being bombarded with thumping pile driving as the turbines are installed in the seabed. The number of seals counted on the rocks near Great Yarmouth in England declined during the construction of the Scroby Sands offshore wind farm.46 The noise generated by pile driving is huge—about 250 decibels at 1 meter (3 feet)—and could physically damage the auditory systems of animals.

  In March 2000 there was a mass stranding of a dolphin and sixteen whales in the Bahamas that is widely believed to have been caused by US Navy sonar. Scientists dispute how loud sonar causes strandings. The noise may simply cause the whales to swim away, alter their dive patterns, and suffer decompression sickness. Alternatively, the sound waves could cause hemorrhaging. But conclusively proving that naval sonar causes strandings is problematic, because navies are reluctant to say when and where they are using sonar.47

  A press release published by the environmental lobby group the Natural Resources Defense Council in October 2005 stated, “Mid-frequency sonar can emit continuous sound well above 235 dB, an intensity roughly comparable to a Saturn V rocket at blastoff.”48 While data show the Saturn V rocket producing 235 decibels, numerically the same as the navy’s sonar, the comparison is inapt because of the difference between airborne and aquatic decibels. Similarly, the 250 decibels created underwater by pile driving for wind farms is not the same as 250 decibels in air.

  The decibel is always relative to a reference pressure at which you get zero decibels. In air the reference is the threshold of hearing at 1,000 hertz for a healthy young adult. Underwater, the reference pressure is smaller. It is similar to the differences between Celsius and Fahrenheit temperature scales, where 0°C is the freezing point of water, but 0°F is much colder. In addition, when comparing airborne and underwater acoustics, the differences in the density and speed of sound in air and water must be considered. To account for these factors, acousticians subtract 61.5 decibels from underwater measurements to get an equivalent airborne value.49 Thus, 235 decibels underwater is akin to 173.5 decibels on dry land. In 2008, the New York Times described naval sonar as being “as loud as 2000 jet engines,” a gross overestimation. The sound 1 meter (about 3 feet) away from a sonar is about as loud as a single jet engine 30 meters (about 100 feet) away—not quiet, but by no means as loud as an entire air force squadron.50

  Although some of the decibel analogies are dodgy, the gist of the stories about underwater noise causing harm is correct. Many experts are worried because virtually every aquatic animal uses sound as its main way of communicating. Vision is effective only at short distances underwater. Migrating baleen whales can swim more than 100 kilometers (60 miles) in a day, so they need to chat with others in their pod over long distances. Blue whales can be heard from 1,600 kilometers (1,000 miles) away. Whales achieve such long-distance communication by sending out calls at very low frequency, which are much more efficiently transmitted through seawater than are high-frequency vocalizations.

  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 2007.51 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.52

  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.

  Where 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.”53 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.”54 They create a rich musicality, a “Phil Spector Wall of Sound” that within an hour is gone.55

  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.”56 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.”57 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 m
usic.58 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.59 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.60 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.61 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.”62 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.63

  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.64 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.65 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.66 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.67

  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.”68

  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.

  Echoes of the Past

  T

  here is a saying, “A duck’s quack doesn’t echo and no-one knows the reason why.”1 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”?2 Or even transpose a trumpet tune, with each repeat being at a lower frequency?

  A 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.3 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 prompti
ng 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.4

  But caves and reverberation chambers are not a natural habit for ducks like Daisy. We were curious to know what happens outdoors. To hear a clear single echo from Daisy, I would need a stretch of water with a large reflecting surface, such as a cliff, nearby. In such a place, sound would travel directly from the duck to my ear, followed shortly by the delayed reflection from the cliff. In the taxonomy of echoes, this is a monosyllabic echo, where there is just time to say one syllable before an echo arrives. But Daisy and I could not be too near the cliff, or my brain would combine the reflection with the quack traveling directly from her beak to my ear, and I would hear only one sound.

  I must admit that my field experiments were crude. Though I could not bring Daisy, I did wander around various ponds, canals, and rivers listening to wildfowl. In none of these places could I hear a clear, audible quack separate from the original call. In the end, I came to the conclusion that the phase should say, “A duck’s quack might echo, but it’s impossible to hear unless the bird quacks while flying under a bridge.”