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

by Trevor Cox

  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.28 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.29 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’?”30

  There 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.31

  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.32 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.33 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.34

  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.35 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.36 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.37 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.

  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.38

  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.39 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.40

  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.”41

  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.42

  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.

  Wildlife 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.”43 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.”44 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.