The Sound Book: The Science of the Sonic Wonders of the World (13 page)

Since the echolocation calls are at too high a frequency to be heard by humans, we needed some electronic assistance. Clare handed out bat detectors: black boxes about the size of an old brick mobile phone with two controls—one marked
gain
, the other marked
frequency
. As darkness started to fall, our small group of bat hunters set off down a tree-lined path, clutching the hissing detectors. Near an old railway bridge, my detector spurted out a fast series of clicks, like someone rapidly clapping hands in an erratic rhythm. “Pipistrelle,” Clare announced, identifying the species from the call pattern. Each click is actually a chirp, a short, sharp yelp descending in frequency. The rate at which chirps are produced changes as a bat approaches an object, to the point where each individual chirp cannot be heard. When this happened, the detector sounded like it was blowing a raspberry.

The next day I examined some recordings of a common pipistrelle. The best way to view each chirp was a spectrogram, because it would show how the frequency of the sound changed over the length of the call. More often used to examine speech, the spectrogram is a wonderful tool for visualizing sounds. In Figure 3.2, the dark descending lines illustrate how the frequency dropped from 70 kilohertz to just under 50 kilohertz over a short (7-millisecond) call.

But how could I have heard this call on the bat monitor, when the frequency is far too high for my hearing? An ultrasonic microphone on the bat monitor picks up the chirps of the bat, and the detector adjusts the tone to be within human hearing range.
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Clare was able to identify the bat as a common pipistrelle because each species uses different frequencies to echolocate, producing contrasting sounds from the detector. The noctule bat, for example, produces jazzy, rhythmic lip smacks with a distinct groove. Experts can also tell whether the bat is emerging from its roost, feeding, flying by, or having a chat with a friend from the different sounds of the calls.

I find it particularly remarkable that bats do this with basically the same vocal and hearing apparatus that humans have. To make such high-frequency noise, bats have to push the mammalian body to its extremes. Some bat species produce sound at 200 kilohertz, which means they are opening and closing the gap between their vocal folds 200,000 times a second, although they boast an important modification: thin and light membranes attached to their vocal folds that can vibrate very quickly.

Figure 3.2 The call of the common pipistrelle.

Bats not only hit extremely high notes, but they also routinely generate extraordinarily loud calls. The calls might reach 120 decibels—analogous to the sound reaching your ear from a smoke alarm going off just 10 centimeters (4 inches) away.
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These are levels that can damage a mammalian hearing system, so bat ears have a reflex to protect themselves: muscles contract when the bat is calling and displace the tiny bones in the middle ear, thereby reducing how much vibration is transmitted from the eardrum to the inner ear. Humans also have this acoustic reflex, but the evolutionary purpose is still being debated. Maybe as in bats, the reflex protects our hearing against loud sounds. Or maybe it reduces the volume of our own speech so that other sounds are more audible.
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Leaving the path, we headed through woods toward a small reservoir, stumbling over tree roots as we went (not taking a flashlight along on a nighttime walk was a mistake). But it was worth blundering through the dark to hear the Daubenton's bats hunting insects just above the water. Their roost was under a gigantic brick bridge, and the bat detectors periodically burst to life sounding like distant machine-gun fire. Armed with a detector, I could appreciate the huge number of bats that lived in the valley. It is astounding that up until then I had been totally unaware of these sounds around me. In a radio interview, sound recordist Chris Watson explained how listening to bats hunting down prey changed his perception of Lake Vyrnwy in Wales: “The place was actually turned on its head from being this peaceful tranquil environment to human ears, to being the carnage of a battle above my head in the ultrasonic region.”
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What else
aren
't
we hearing? Marc Holderied at his laboratory in Bristol University, England, is another bat expert with infectious enthusiasm, who answered my questions so extensively that I almost missed my train home. He explained to me that the bats are not only hearing the insects and each other; they are also listening for sound reflecting from plants. Marc and colleagues had been researching
Marcgravia evenia
, a Cuban vine with a leaf that is especially good at reflecting sound and so stands out from the rest of the vegetation in the rainforest. The vine has an arching stalk with a ring of flowers at the end. The last leaf on the branch hangs vertically above the flowers and forms a concave hemisphere to reflect a bat's ultrasonic chirps.

As a bat flies around the rainforest, it hears a very complicated pattern of reflections from all the vegetation. The echoes shimmer and are continually changing. In contrast, the pattern of sound reflections from the convex vine leaf stays almost the same, no matter what the angle of the bat to the plant. So the vine stands out as the only thing in the rainforest that gives a constant reply to the echolocation signal. Moreover, the hemispherical shape of the leaf focuses and amplifies the echolocation signal so that the bat can hear the plant from farther away. Marc and his collaborators confirmed these acoustic properties with laboratory measurements using a tiny loudspeaker to radiate ultrasound and a microphone to sense the reflections off the leaf.

But what evidence suggests that bats take any notice of the reflections from the leaf? By training bats to search for a feeder in a laboratory full of artificial foliage, the researchers demonstrated that the animals found food twice as fast when the hemispherical leaf was in place. In the rainforest, the vine increases its chances of being pollinated by attracting bats with its concave leaf; in exchange, the flying mammals get nectar.
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In Marc's laboratory there was a set of dried moth samples, some with extravagantly long tails. Like the vine, these moths have changed because of bats using echolocation. Some moths have evolved high-frequency hearing solely to listen for predator bats. The long moth tails are ultrasonic decoys. A fighter jet can drag a decoy behind it to lure radar-controlled missiles away from the plane. Similarly, a moth sacrifices a decoy tail to protect itself from the bats. The yellow Madagascar moon moth in Marc's laboratory had two swallowtails, each six times the length of the main body. The tails ended in twists, and Marc's measurements show that this means the tails very effectively reflect the bats' calls from all directions, mimicking the ultrasound reflections from the wings of a smaller moth. Marc has shown that 70 percent of the time the bat attacks a streamer tail rather than the insect's body; the moth loses its tail but lives.

W
ildlife recordist Chris Watson describes the oceans as “the most sound-rich environment on the planet,” adding, “arrogantly we think we're on planet earth, and of course we aren't, we live on planet ocean, 70% of the planet is ocean.”
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To illustrate his point, Chris told me about an expedition to the Arctic, where, off the coast of the island of Spitsbergen in the Svalbard archipelago, he had encountered bearded seals singing underneath thick sea ice. He lowered hydrophones (underwater microphones) through the ice holes made by the seals into the still, inky blackness of the water. To Chris, the seal calls were mesmerizing to listen to because they appeared to be from another planet: “It's almost beyond description. To use lots of clichéd terms, it sounds like a choir of alien angels.”
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The seals make long drawn-out glissandos lasting many tens of seconds. I could do a good impersonation with a Swanee (slide) whistle by gradually pulling out the plunger. It seems that longer glissandos are more appealing to females, so length (of call) matters.

Chris's vivid description of aquatic acoustics made me want to experience these wonders firsthand, which I managed a month after my bat expedition. On a cold, wet, and windy day, I boarded a small boat with a dozen other passengers also swathed in rain gear, clutching a hydrophone and a sound recorder. We were on a trip around the Cromarty Firth in Scotland to meet the resident bottlenose dolphins.

The Cromarty Firth is quite industrialized, and we started by cruising around the gigantic, yellow, rusting legs of an oil rig. In the distance, two other platforms were being repaired, and a cruise liner was moored alongside so that her passengers could hunt for Nessie in nearby Loch Ness. But the dolphins were being as elusive as the fabled monster.

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.
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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.
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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.
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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.”
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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.
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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.
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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.