by Trevor Cox
The surfaces of the floor and ceiling must also be made from materials that absorb little sound. The tiles of an Islamic mosque are ideal for two reasons: First, they are heavy, which means the sound wave is too weak to physically vibrate the tiles. Second, they are quite impervious to air, which means the acoustic wave cannot easily enter the tile and instead bounces off the front.
Figure 4.2 Focusing effects of two different rooms.
The Brixton Academy in London started life as the Astoria theatre, an art deco wonder from 1929. Film director Alfred Hitchcock was at the opening night, which featured Al Jolson’s The Singing Fool.25 The academy has an echo caused by reflections between the dome and the sloping floor,26 but the echo is heard only during sound checks when the auditorium is empty. When the hall is packed, sound is absorbed by the audience as it squeezes into the pores of people’s clothing, where the wave then loses energy. For poorly attended concerts, the echo has the fortunate effect of compensating for the small audience by amplifying the applause!
Brian Katz, a French academic and acoustic consultant, has been investigating, together with colleagues, an intriguing focusing ceiling from a long-lost room in Paris.27 The room has been linked to the thousands of executions carried out during the French revolution. In the nineteenth century, Auguste Lepage wrote, “To this room [devoted] to meditation and prayer are linked bloody memories. It is there that was sitting the famous court . . . during the September 1792 massacres.” Lepage goes on to describe the room: “Massive pillars supported a roof frame, which was a wonderful construction. This frame, rounded in a dome shape was made from Spanish chestnut, no nails were used and the thousand pieces that composed it were only fixed up by pegs.”28
Though the room was destroyed in 1875, Brian is able to work with a nineteenth-century replica, a small-scale model conserved in the Musée des Arts et Métiers in Paris. The ceiling looks like an upturned wicker basket that has been squashed almost flat. Seen from below, the roof forms rings of beams with gaps between. While the curvature focuses sound, the focal point is at the wrong height for human ears to hear the effect. The secret behind the acoustic is that the beams are spaced farther apart near the middle of the ceiling, coming closer together at the edges of the room. Brian has shown that at some frequencies, the reflections from the different beams combine to amplify the sound in the middle of the room. It is a quirk of geometry. The dome’s wooden lattice resembles a Fresnel zone plate, named after the French physicist Augustin-Jean Fresnel, who studied diffraction in the nineteenth century. Fresnel zone plates use diffraction to focus light. They can be used to focus laser beams and recently have been suggested as a lightweight substitute for heavy lenses used in space telescopes.29 In acoustics, zone plates can be used to focus beams of ultrasound.
Echoes are not just fun phenomena; they can be aids to safety. A couple of years after the Titanic sank, a quick-witted captain described how his freighter had escaped a similar North Atlantic fate while sailing in foggy weather off the Grand Banks of Newfoundland. A five-second blast on his ship’s foghorn resounded from the mist. But was this another steamer’s call? The captain sounded a more elaborate pattern of blasts, which were exactly repeated back to him, revealing that this was an echo. The Day newspaper described how he took evasive action to “avoid hitting a berg that he could feel and hear, but could not see.”30
Another historical example of sailors using echolocation comes from Puget Sound in Washington State. An article in Popular Mechanics magazine from 1927 describes the inside passage from Puget Sound to Alaska as “more crooked than the famous dog’s hind leg; a narrow tortuous channel.”31 In foggy weather, navigators would locate themselves by listening to the echo of their steamboat’s whistle. The strong tides in the channels made it impossible for the ship to move slowly, as would be done in foggy conditions out in open sea. As the same magazine article explained, “The full steam ahead—followed by the full speed astern—is the rule of the echo pilots.”32 If an echo was delayed by 1 second, the whistle would have traveled 340 meters (370 yards), indicating that the boat was 170 meters (185 yards) from shore. Sailors learning to navigate the route had to memorize the reflection delays from key landmarks. On a small island that was too low to produce an echo, an 8-meter-square (25-foot) signboard was erected to aid navigation by reflecting a blast on the whistle.
The magazine claims that the pilots identified the type of coastline from the echo: “A low coastline returns a ‘sizzling’ echo while a high cliff gives a solid ‘plunk.’ The echo ‘scrapes’ from a beach of sand or gravel, and a forked headland yields a double echo to the expert ear.”33 This claim strained credulity until I heard a talk by Norwegian acoustic expert Tor Halmrast, who had been carrying out experiments into echolocation by blind people.
By making a clicking sound and listening to the sound of the reflections, people can learn to navigate with their ears, mimicking the techniques used by dolphins, bats, and oilbirds. Daniel Kish learned to echolocate from a young age, and in New Scientist he described what a hectic school day was like for him at age 6:
Clicking my tongue quickly and scanning with my head, I move cautiously forward . . . the sounds in front of me take on a softer hue, suggesting there’s a big field of grass ahead . . . Suddenly, there is something in front of me. I stop. “Hi,” I venture, thinking at first that someone is standing there quietly. But as I click and scan with my head, I work out that the something is too thin to be a person.
I realise it is a pole before I reach out to touch it . . . There are nine poles all in a line. I later learn that this is a slalom course, and while I never attempted to run it, I practised my biking skills by slaloming among rows of trees, clicking madly.34
The clicking is usually made by sharply dropping the tongue in the mouth, maybe with an accompanying suck or a short, sharp cluck. The exact sound is very individual, making it difficult for one person to echolocate using someone else’s vocalization.35 The diversity of sounds one can make is amazing. A palatal click is a good one, made by a quick release of the vacuum produced between the tip of the tongue and the roof of the mouth. Being short and loud, it is easier to pick out in noisy places.
The palatal click also contains sound spread across many frequencies, which human echolocators find useful.36 Since they are listening for surfaces only a few yards away, and since most reflections from such surfaces arrive too quickly to be heard distinctly, these people must learn to detect subtle changes between what is heard by each ear. The interference of the click and its reflection might cause coloration (a change in the frequency balance), altering the tonal quality, or what musicians call timbre. The reflection might elongate the original click, for example, suggesting a reflection from a nearby surface. The effect depends on the distance from the reflecting surface, which alters the delay, and also on how the object reflects acoustic waves: larger objects reflect low frequencies more strongly; soft objects tend to absorb sound and so produce weaker reflections. Studies show that even novice echolocators can learn to distinguish square, triangular, and circular shapes with minimal practice.37
Some of the most extraordinary echoes come from man-made structures. Engineered curves can focus echoes, and parallel flat walls can encourage sound to bounce back and forth in a way that is unlikely to happen with natural surfaces. Bridge arches have great potential to be sonic wonders, as I found out on a canoeing trip on the Dordogne River in France a couple of months before seeing Aeolus. One stone arch was just the right size and shape to put the focal point at the water level, and slapping my paddle on the water created a wonderful ricocheting sound. During the lunch break I explored under another bridge over a sandbank. Standing with my back to the edge of the arch and clapping my hands produced a stunning fluttering sound—a multiple echo.
Across the Atlantic in Newton Upper Falls in Massachusetts, a similar fluttering sound under an aqueduct proved so remarkable that denizens named it Echo Bridge. Built in the 1870s, this 40-meter-wide (130-f
oot) arch spans the Charles River and even has steps down to a specially built platform so that visitors can test out the sound effect. The Internet has several videos of dogs being driven mad by their own echoes, believing there to be a rival canine on the other side of the river. Not only does the bridge attract tourists and playful dog owners; it also intrigues scientists. In September 1948, Arthur Taber Jones wrote to the Journal of the Acoustical Society of America, detailing a small study. “A handlcapp [sic] is returned in a series of about a dozen echoes of decreasing loudness, and at a rate of about four echoes per second.”38 Jones describes elaborate experiments undertaken to determine what is causing the reflection.
The question Jones was trying to answer was whether the sound was skimming around the inside of the curved arch, like the whispering galleries I describe in the next chapter, or propagating horizontally just above the water. He unsuccessfully tried using listening trumpets to determine which direction the sound was coming from. Further attempts using blankets to block off sound going around the arch failed because of high winds.
Unable to get to the bridge myself, I found photos and postcards that allowed me to estimate the shape of the archway. I calculated the echo delay from video soundtracks of dogs barking. And finally, modern prediction methods allowed me to visualize how the sound moves.
Figure 4.3 shows twelve frames from an animation I made to understand the bridge. Each snapshot shows the roughly semicircular shape below the arch, where the platform is to the left and the water is the long flat line at the bottom. Starting from the top-left frame, the dots show how sound moves from the talker, across to the far side of the bridge, and back again.
Figure 4.3 Snapshots from an animation showing sound moving under Echo Bridge.
To make this animation, I modeled sound as lots of tiny snooker balls, which are fired in all directions from the platform. The computer works out how the balls bounce around the odd-shaped billiard. For images 1–6 in Figure 4.3, the sound is moving from left to right; it then reflects from the right side and travels back in the opposite direction. The answer to Jones’s question is that the sound both hugs the inside of the curve and skims along the water’s surface.
Early writers on echoes were keen to find extraordinary multiple echoes with the greatest numbers of repeats—echoes that would turn a “ha” into laughter. This endeavor was taken to the absurd by the echo collector in Mark Twain’s short story “The Canvasser’s Tale”:
You may know, sir, that in the echo market the scale of prices is cumulative, like the carat-scale in diamonds; in fact, the same phraseology is used. A single-carat echo is worth but ten dollars over and above the value of the land it is on; a two-carat or double-barreled echo is worth thirty dollars; a five-carat is worth nine hundred and fifty; a ten-carat is worth thirteen thousand. My uncle’s Oregon-echo, which he called the Great Pitt Echo, was a twenty-two carat gem, and cost two hundred and sixteen thousand dollars—they threw the land in.39
In the seventeenth century, the real-life collector and myth buster Marin Mersenne analyzed the claim that a tower near the Aventine Hill in Rome would repeat the entire first line of Virgil’s Aeneid eight times.40 Since it takes nearly 40 seconds to hear eight repeats of the phrase, the farthest reflection would have had to travel a round-trip of 14 kilometers (8½ miles), which is too far for the voice to carry and still be audible.
More believable are the stories about the sixteenth-century Villa Simonetta in Milan. The great eighteenth-century mathematician Daniel Bernoulli stated that he could hear up to sixty repetitions from the echo.41 Twain wrote about this in his travel book The Innocents Abroad, which includes a plate showing a woman entertaining two gentlemen by blasting a trumpet to excite the echo. Iris Lauterbach, writing on Italian gardens, noted that the villa was famous well into the nineteenth century “but not for its garden: the attraction was an echo.”42
The villa was a rectangular horseshoe shape, with two large wings, exactly parallel to each other, spaced 34 meters (110 feet) apart. The semi-enclosed courtyard used to open up to a luxuriant garden. On the first floor there was a single window up near the roof on one of the wings. Speak from this window and the words would bounce back and forth across the courtyard between the parallel wings. The sound took 0.2 second to complete the round-trip, which meant a very short blast repeated many times. Old reports claim that a pistol shot would repeat forty to sixty times.43 Seventeenth-century engravings of the villa show the upper walls of the wings to be very simple flat surfaces, ensuring that the sound could bounce back and forth without being scattered in other directions and lost from the echo path.
In the engravings, the echo window looks odd—the only opening on the upper walls of the wings, and ruining the architectural symmetry. It makes me wonder whether the window was deliberately placed to take advantage of the acoustic phenomena. Unfortunately, the villa was extensively damaged by bombing during the Second World War, so the courtyard now lacks the grand colonnades and vistas, and disappointingly, the echo has been dulled to a single retort.44
Is it just me, or is it virtually impossible not to shout and whoop when entering a tunnel? Some are better than others, with the foot tunnel under the Thames near Greenwich, London, being one of my favorites. Finished in 1902, it was built to let South London residents walk to work across the river in the Isle of Dogs. I went back there a few months after my trip to France, on a cold winter’s night, to see if my childhood memories of the acoustics were right. Despite being a foot tunnel, nearly everyone passing through seemed to be on a bike. I spent some time wandering up and down the 370-meter-long (400-yard) tube. It is a squat cylinder covered in off-white glazed tiles.
The poorly lit tunnel is only about 3 meters (10 feet) in diameter. As sound waves bounce back and forth across the width, they tend to distort dramatically. If I stood right in the middle, my voice resounded with a metallic twang. The resonances of the tunnel were overamplifying certain frequencies in my voice, making it sound unnatural. I asked the sound artist Peter Cusack about his impressions of the place:
Sometimes there is a busker in the middle of it, and if you listen from the end . . . then there’s no way of telling what the tune is or even what instrument they are playing. It is just this musical mush that comes up one end, which is actually quite pleasant. And as you walk down the tunnel and get closer and closer, and it becomes clearer and clearer, it is often a bit of a disappointment when you get there.45
At one point I was alarmed to hear what I initially thought was an approaching freight train. I was relieved to see it was just the rumble of a skateboard being amplified by the tunnel. After passing me, the skateboarder flipped up his board but missed catching it, causing an impressive clash as though someone were slamming the doors of a large cathedral. The initial crash traveled hundreds of yards to the end wall and returned with an audible echo. The hard-tiled surfaces allow the sound to rattle around the tunnel for a long time before dying away.
Engineers at Bradford University, England, have been using the ability of tunnels to carry sound long distances to find obstructions in sewers. Noise is played down the pipe, and a microphone records any echoes heard. The time it takes for the echo to arrive reveals how far away the blockage is, and the acoustic characteristics of the reflection tell the scientists about the size and type of obstruction.
One reason tunnels often have impressive acoustics is that sound can travel an unusually long way in them. If someone is talking to you outdoors, the farther apart the two of you are, the quieter the other person sounds. Imagine blowing up a balloon: as it expands, the rubber gets thinner as it spreads over a larger surface area. Being farther from a sound source outdoors is like being on the edge of the balloon; the energy is spread thinly like the balloon’s rubber, so it is quieter. But in a tunnel the acoustic wave is spread across the width of the tube, which does not change in size as you get farther away from the sound source. The only way energy is lost is through absorption by the tunnel walls.
If the walls are made of hard materials such as tiles, brick, or sealed concrete, sound can carry for huge distances.
Still curious about why my voice sounded so metallic in Greenwich, I sought out another example to experience—one where the effect is even stronger. London’s Science Museum has a hands-on gallery full of children noisily enjoying science. Across the back wall is a long, sloping industrial tube, 30 meters (100 feet) long and about 30 centimeters (12 inches) in diameter. “Sounds like gunfire,” suggested a young boy just before I started to experiment. This was a good description. A clap of my hands sounded like a cross between a sheet of metal being struck and a laser gun from a sci-fi movie with a slow recoil.
It is easy to assume that the tube’s material dominates the sound. But while the pipe was made of metal, the material has very little to do with why my voice or clapping developed a robotic quality. The tube could have been made of any hard material, such as concrete, metal, or plastic, and it would still have made a twang, as happened within the tiled Greenwich foot tunnel. What is most important is the geometry of the bore, because it is the air that is doing most of the vibrating, not the tube walls. The same confusion exists with musical instruments. I learned the clarinet when I was younger, and the lower notes on the instrument are often described as being distinctly “woody,” which you might assume comes from the black ebonite tubing. However, my colleague Mark Avis once played a brass clarinet and noticed how remarkably “woody” that sounds. The great jazz player Charlie Parker famously used a plastic saxophone for some gigs, yet he still created his distinctive sound.46