The Sound Book: The Science of the Sonic Wonders of the World (18 page)

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.”
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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.
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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.”
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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.
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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 what 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.
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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.