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

by Trevor Cox

  Similarly, the “brassy” blast of a trumpet or trombone might be wrongly attributed to the metal from which it is usually made. Some historic brass instruments, such as the cornetto, were actually made of wood and yet can still make a “brassy” sound. A musical instrument simultaneously generates many different frequencies, known as harmonics, which give distinct color to the sound. When an oboe plays a tuning note for the orchestra—a concert A at 440 hertz—sound is also produced at 880, 1,320, and 1,760 hertz. These harmonics are multiples of the fundamental frequency, and their strength depends on the instrument’s geometry. When a trombone is played loudly, a shock wave can be created inside the bore similar to that produced by a sonic boom, generating lots of high frequencies. A “brassy” sound is associated with musical notes that have exceptionally strong high frequencies.

  The echo tube at the Science Museum has only a few strong harmonics, and these are not simple multiples of the fundamental. Musical instruments sound beautiful because they have been designed to produce harmonics whose frequencies are regularly spaced. Large pieces of metal tend to radiate at irregular frequencies and sound dissonant. Thus the tube, with its discordant frequencies, adds a metallic quality to voices. Another key feature determining a musical instrument’s voice is how notes begin and finish. A metal chime bar can ring beautifully for a long time; similarly, the air in the echo tube at the Science Museum rang on and on.

  But something else intrigued me about the echo tube: clapping my hands created a zinging sound, the echo starting at a high frequency and then descending in pitch. I talked to a few colleagues, and they were similarly bemused because none of us expected a shift in frequency in a simple tube. One of the fun things about being a scientist is having your expectations subverted and finding something new to understand. Looking through the literature, I found that the descending zing was a culvert whistler. It was first documented a few decades ago by the late American scientist Frank Crawford, who observed a chirp from a pipe under a sand dune in California. In an effort to explain his observation, reported one article, “Crawford has clapped his hands, beat bongo drums and has banged on pieces of plywood in front of culverts all around the San Francisco Bay Area.”47

  Figure 4.4 A single hand clap at one end of a long tube and listening at the other end.

  If you listen to one end of a culvert while someone claps hands once at the other end, as illustrated in Figure 4.4, the first sound to arrive travels straight down the middle of the tube following the shortest distance. The next sound to arrive has reflected once off the side wall and so has traveled a little farther. The next sound has hit both sides once while zigzagging down the tube. Later sounds will follow a longer, more jagged path. If you plot these sounds arriving over time, as illustrated in Figure 4.5, you find that the reflections arrive close together at first, and then gradually are spaced farther apart toward the end of the chirp. At any particular instance, the pitch of the chirp is determined by the spacing between adjacent reflections. When reflections arrive rapidly one after another, as happens initially, a high-frequency sound is the result. As the time between reflections increases, the frequency lowers.48 A similar downward glissando also happens when vibrations pass through a solid like a metal. This might be another reason why the echo tube sounds metallic.

  Figure 4.5 A clap and its reflections from inside a culvert. (Each clap is simplified to a single peak so that the pattern of claps arriving is clearer.)

  Multiple reflections lie at the heart of echoes that create almost-musical sounds. Not long after my canoeing trip, on a hot, sunny afternoon in the city of Angoulême in France, I stood outside the comic book museum while inside my children devoured the extensive Asterix and Tintin collections. Bored, I experimented with clapping and listening to the reflection from the front of the building, a wide, low, and white converted warehouse that had been used to store cognac. But it was the reflection from another structure that caught my attention. There was a high-pitched sound to my right, like someone squeezing a squeaky toy, coming from a staircase. A tonical echo! Boredom turned to an afternoon of fevered experimentation as I recorded and documented the strange reflection from this short flight of stairs.

  What I was hearing was the same phenomenon that creates the chirping Mayan pyramids described in Chapter 2. Staircases can make many different sounds. Acoustic engineer Nico Declercq wrote to me about a quacking staircase: “It is on the Menik Ganga (Gem River) in Sri Lanka, a river you must cross in order to reach the sanctuary of Katharagama . . . [W]hen you cross the water . . . you can hear quacking ducks when you clap your hands or when women hit rocks with clothes they are washing.”49 Back in Europe, artist Davide Tidoni popped balloons to reveal the unusual acoustics of the Austrian city of Linz, including an explosive wheezing sound created by a very long staircase.50

  The strange sounds are made by the pattern of reflections from the treads of the stairs, which distort the sound of the balloon pop or clap, and this pattern can be explained by geometry (Figure 4.6). Figure 4.7 shows the ninety reflections, one from each stair tread, that arrive if you clap your hands once in front of the Mayan pyramid El Castillo. The frequency drops by about an octave because the spacing between the reflections roughly doubles.

  Probably the best way of analyzing a chirp is to look at the spectrogram, as I used previously with bat calls. The top image in Figure 4.8 shows the chirping echo from the staircase. The black, vertical line at far left represents the initial clap. The fuzzy, dark lines that droop to the right show the reflections in which the pitch is decreasing. Compare this sonic fingerprint to the cry of the quetzal bird, the bottom image, which features a similar drooping line. This similar decrease in pitch explains why some people believe the staircase echo resembles a chirping bird.

  Figure 4.6 Sound chirping from a staircase.

  Figure 4.7 Reflections of a single clap from the staircase of El Castillo, the Mayan Temple of Kukulkan.

  Figure 4.8 Acoustic signature of the Kukulkan pyramid (top) and a quetzal bird (bottom). (The echo has been amplified so that the drooping lines of the chirp are easier to see.)

  The particular sound reflected from a staircase depends on where the clapper stands, as well as on the size and number of steps. The squeaky stairs outside the comic book museum were quite short and did not have enough reflections to create the extended sound of a chirping bird. The longest staircase in the world runs alongside the funicular railway up the Niesen, a mountain in Switzerland. It is opened to the public only once a year, for a marathon, and the winner takes about an hour to climb the 11,674 steps. When I simulated the staircase in an acoustic model, it sounded like a wheezy air horn.

  If you’re looking for a staircase to experiment on, I would suggest finding one in a quiet place away from other reflecting surfaces. It does not have to be very long, maybe twenty steps, but the more stair treads there are, the more impressive the effect will be.

  Archaeologists argue about the role of staircases on the sides of Mayan pyramids, and whether they were built to imitate the chirp of a quetzal bird. Leaving aside this debate, what other sounds could the Mayans have made if they had built the stairs differently?

  The sound reflected from a flight of stairs is determined by the pattern of reflections that build up as a clap bounces off each stair tread and returns to the listener. In a normal staircase, the later reflections arrive farther apart than the earlier ones, causing a chirp that descends in frequency. Imagine a staircase constructed by bad builders—one in which the steps are not all the same size. At the bottom of the stairs the steps get smaller and smaller as they go up, creating a series of reflections that are heard with a rising pitch. Then, toward the top, the steps stretch out and get bigger and bigger to create a quick drop in the pitch. With just the right pattern of steps between about 3 and 10 centimeters (1–4 inches) wide, you can get a chirp that rises and then falls in frequency; in other words the staircase would make a wolf whistle. A completely useless staircase, but wha
t a sonic wonder it would be!

  While the embellishment of my voice by a tunnel was not pleasant, it explains why old writings on tonical echoes observe voices modulated into distinct tones. Clapping near a staircase shows how reflections outdoors can sound like a distinct musical note. Occasionally, the old echo tales are fanciful, with the most unlikely one featuring a tune being played on trumpet that resounds at a lower pitch.51 A change in pitch flouts the laws of physics, but then so does the phrase “a duck’s quack doesn’t echo,” and people seem happy to keep repeating that. Maybe the trumpet echo was a simple practical joke, or maybe the basis is a more subtle tonal coloration that has just been overembellished as the story is retold.

  No matter how powerful an echo is, or what type it may be, all the echoes described in this chapter have one thing in common: they can be enjoyed with just one ear; that is, they are monaural delights. Let’s turn now to binaural sonic wonders—those that mess with how our brains use two ears to localize sound.

  Going round the Bend

  W

  hispers reflected from a giant hemispherical ceiling were described by Wallace Sabine, the grandfather of architectural acoustics, as “the effect of an invisible and mocking presence.”1 In the huge dome of the Gol Gumbaz mausoleum in India, “the footfall of a single individual is enough to wake the sounds as of a company of persons,” reported the celebrated physicist C. V. Raman, and “a single loud clap is distinctly echoed ten times.”2 When I was in the sewer (see the Prologue), my speech appeared to hug the walls of the tunnel, spiraling around the inside of the curve as the sound slowly died away. Some of the strangest sound effects can be created by simple concave surfaces.

  In 1824, naval officer Edward Boid described how a curve can dramatically amplify sound, and not always for the best. He wrote, “In the Cathedral of Girgenti, in Sicily, the slightest whisper is borne with perfect distinctness from the great western door to the cornice behind the high altar—a distance of two hundred and fifty feet.” Unfortunately, the confessional was badly sited: “Secrets never intended for the public ear thus became known, to the dismay of the confessors, and the scandal of the people . . . till at length, one listener having had his curiosity somewhat over-gratified by hearing his wife’s avowal of her own infidelity, this tell-tale peculiarity became generally known, and the confessional was removed.”3

  Figure 5.1 The cat piano.

  For centuries, people have known that curved surfaces amplify sounds and allow covert listening. Athanasius Kircher gave a good explanation in the seventeenth century. We met Kircher in Chapter 4 because he wrote extensively on echoes. His publications also document some fantastical devices, including giant ear trumpets built into the walls of royal chambers for eavesdropping. Probably his most famous—or infamous—device is the Katzenklavier (literally, “cat piano”; Figure 5.1). It has a normal piano keyboard in front of a line of cages, each of which has a cat trapped inside. Every time a piano key is pressed, a nail is driven into the tail of one unfortunate feline, which naturally screeches. With the right set of cats, ones that shriek at different frequencies, a sadistic musician could play a tune on the instrument. The sound would have been excruciating, but then it was designed to shock psychiatric patients into changing their behavior, rather than being a genuine instrument for playing Monteverdi or Purcell. Fortunately, it is unlikely that it was ever built.

  At this point you might be doubting the sanity and rationality of Kircher. Yet he drew diagrams that illustrated a good scientific understanding of how an elliptical ceiling can enhance communication between two people (Figure 5.2).

  The lines in the diagram show the paths that sound “rays” take when going from the speaker to the listener. These ray paths can be worked out using a ruler and protractor. Alternatively, by treating the room as a weird-shaped pool table, the paths can be worked out by following the line a cue ball would take (ignoring gravity). If the cue ball is placed at the speaker’s mouth and fired toward the ceiling, it will always go to the listener. So all the sound going upward is focused at the listener, allowing even quiet whispers to be heard across a large room.

  Figure 5.2 Simplified tracing of a plate from Athanasius Kircher’s Phonurgia Nova (1673).

  The problem with this design is that the listener and speaker have to stand in particular places—the foci of the ceiling ellipse. The design is not very useful if one person wants to talk to an audience of listeners scattered around the room. In 1935, the Finnish modernist architect Alvar Aalto tried to overcome this problem using a wavy ceiling for the Viipuri Library. (Originally, the library was in Finland, but the town of Viipuri was subsumed into the Soviet Union after the Second World War.) From the speaker’s podium at one end of the room, the ceiling looks like gentle undulating waves coming in from the sea. The wave troughs form concave curves, each designed to amplify the sound for particular listeners. Unfortunately, each wave crest also reflects sound back toward the talker, weakening the strength of the reflections to the back of the room and making it harder for those at the rear to hear the speaker. In reality, using curved focusing ceilings to improve communication in a room rarely works as intended.4

  Elliptical ceilings work rather like a shaving mirror, a simple curved reflective surface that brings light rays together to a point. Both the ceiling and shaving mirror achieve magnification, but whereas for light the result is a bigger image, for sound the result is increased loudness. In the shaving mirror, the reflections that meet your eyes are distorted so that you see an enlarged picture of your face. But with hearing, the reflections coming from different parts of the ceiling add together at the entrance of the ear canals and are treated holistically by the brain. The overall effect is a louder sound, which can make distant objects appear closer than they really are.

  In Elements of Physics (1827), Neil Arnott writes:

  The widespread sail of a ship, rendered concave by a gentle breeze, is a good collector of sound. It happened once on board a ship sailing along the coast of Brazil, far out of sight of land, that the persons walking on deck, when passing a particular spot, always heard very distinctly the sound of bells, varying as in human rejoicings. All on board came to listen and were convinced; but the phenomenon was most mysterious. Months afterwards it was ascertained, that, at the time of observation the bells of the city of Salvador, on the Brazilian coast, had been ringing on the occasion of a festival; their sound, therefore, favored by a gentle wind, had traveled 100 miles [160 kilometers] by smooth water, and had been brought to a focus by the sail on the particular spot where it was listened to.5

  Is this story true? Can an acoustic mirror pick up bells 100 miles away? One way to answer this question is to look at some more modern examples. Just south of Manchester in England stands the gigantic dish of the Lovell Telescope at Jodrell Bank Observatory. This telescope uses the same process of focusing to collect and magnify radio waves, and in the past it played an important role in the space race. When the Soviet probe Luna 9 surprised the West by landing on the moon in 1966, the observatory intercepted the spacecraft’s transmissions. Feeding the signal into a fax machine revealed pictures of the lunar surface that were then first published in a British newspaper before they appeared in the Soviet Union.

  Two whispering dishes stand in the shadow of the giant telescope. (There are other, similar whispering dishes at other science museums and sculpture parks.) The last time I visited, my teenage sons entertained themselves by whispering insults at each other using the dishes. The mirrors are 25 meters (80 feet) apart, yet the sniping siblings were very loud. But Arnott’s sailing ship was much farther from Salvador than a few tens of meters.

  Around the coast of England are remnants of acoustic mirrors designed to work over relatively long distances. These are large, ugly concrete bowls, typically 4–5 meters (13–16 feet) in diameter, which face the sea. Built in the early twentieth century, they were intended as an early-warning system for enemy aircraft. Most are bowl-shaped, but in Denge, Kent
, there is also a vast, sweeping arc of discolored concrete. The arc is 5 meters (16 feet) high and 60 meters (200 feet) wide—the equivalent of about five double-decker buses parked end to end. It is curved both horizontally and vertically to magnify the engine noise from approaching aircraft.

  Military tests showed that the large strip mirror could detect aircraft 32 kilometers (20 miles) away, when enemy planes were roughly a third of the way across the English Channel. But in poor weather conditions, aircraft might get within 10 kilometers (6 miles) before detection, and listeners struggled to hear planes with quieter engines.6 Even on a good day, these acoustic mirrors provided a measly ten minutes of extra early warning. Once a working radar system was developed in 1937, the plan to build an extensive network of mirrors was dropped.

  The short detection ranges of the concrete acoustic mirrors makes the claim of a ship sail focusing sounds from a festival 100 miles away seem fanciful. But a catastrophic event several years ago in England hints at an explanation.

  In December 2005 an overflowing storage tank caused a giant explosion at the Buncefield oil terminal in the UK and shook glass doors in Belgium 270 kilometers (170 miles) away.7 This was one of the biggest explosions in peacetime Europe, measuring 2.4 on the Richter scale.8 Although the bang at Buncefield must have been very powerful, the initial loudness alone does not explain the huge distances the noise carried.

  The catastrophe happened on a still, clear, and frosty morning when a layer of cold air was trapped close to the ground by warm air above. Without this temperature inversion, the Belgians would have been left undisturbed. When the oil refinery blew up, sound waves would have been sent out in all directions, rather like the ripples created when a rock is lobbed into a pond. Much of the noise would have headed upward toward the heavens and, under normal conditions, would never have been heard again. But with the temperature inversion, the sound heading upward was refracted back down to Earth and could be heard far away.