The Sound Book: The Science of the Sonic Wonders of the World (16 page)

On the TV show
Candid Camera
, echo prankster Bob Perry stood at Coit Tower, which affords great views of San Francisco, next to a false sign saying, “echo point.” Standing alongside his unsuspecting victim, Bob shouted to create the illusion of sound bouncing off the tower, imitating an echo delayed by about one-fifth of a second. The joke was that whenever the victim tried shouting, there was no echo.

Bob Perry is impersonating what music producers would call a
slapback echo
, which is a single loud and delayed repetition. This effect was popularized in rock 'n' roll recordings from the 1950s and helped create the characteristic sound of famous singers like Elvis Presley. The audio engineers used two tape recorders to produce electronic echoes. A single big loop of the magnetic tape was fed through both recorders; the first machine would record the music onto the tape, and the second would pick up the sound from the tape a short time later, thus producing a delayed, slapback echo. The time between the passing of the tape under the record head on one machine to its reaching the pickup on the other machine determined the delay of the echo. On tracks such as “Boogie Disease” by Doctor Ross, the echo delay is about 0.15 second, creating the impression that the electric guitar on this blues recording is playing at double speed, as every strum is repeated.

The same effect gave Elvis's vocals a distinctive sound on his recordings with Sun Records, such as “Blue Moon.” When Elvis switched to the RCA record label and achieved global hits with songs like “Heartbreak Hotel,” the sound engineers could not work out how to reproduce the slapback echo, and they resorted to adding heavy reverberation from a hallway outside the studio.
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Nowadays, it would be simple to reproduce this effect digitally, as delay is one of the cornerstones of modern pop production. To create the effect without electronics, RCA's engineers would have had to record Elvis in a studio next to a long tunnel or a tall room with a domed roof that had a slapback echo (remember, one of the dimensions would have to be at least 33 meters (110 feet), which would make it a rather large recording studio).

T
he Imam Mosque in Isfahan, Iran, might have worked for Elvis's voice as, according to the old writings on echoes, it is a
centrum phonocampticum
, the object of an echo. Constructed in the seventeenth century, the building is visually stunning, with dazzling blue Islamic tiles. A huge dome rises to an exterior height of 52 meters (170 feet) and, as one travel guide states, “replicat[es] individual sounds in a series of clear echoes.”
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Tour guides delight in standing underneath the dome and snapping or flicking a piece of paper, which creates a short, sharp “clack, clack, clack, . . .” The room immediately responds with about seven quick-fire echoes.
²²
Sound bounces back and forth between the floor and ceiling, with the curved dome focusing the sound, forcing it to keep moving vertically up and down in a regimented fashion. Without a dome, the echo from the ceiling would be lost among all the other sound reflections in the mosque.

Luke Jerram is an artist who often uses sound as an art medium. His work
Aeolus
was inspired by a visit to Iran, where he heard the echoes in the Imam Mosque. I first met Luke about seven years ago, when we both reached the finals of FameLab, a pop idol–style competition to find science presenters for the media. I caught up with Luke when
Aeolus
, or what he called “my ten-ton musical instrument,” was being installed outside my university's building at MediaCityUK in 2011.

Aeolus
looks like a section through a giant steel hedgehog, an arch 4–5 meters (13–16 feet) high, with 300 long, hollow steel pipes sticking out of the top and sides (Figure 4.1). The shape was inspired by the twelve stuttering echoes that Luke had heard when he clicked his fingers in the mosque. Stand in the right place under
Aeolus
, and you can hear the focus from the arch subtly amplifying your voice. The light coming through the mirror-lined steel pipes creates geometric patterns echoing the decoration of the mosque.

While the arch is the most obvious visual feature of the sculpture, the main sound effect is created by long wires that stretch almost invisibly from supporting poles to the hedgehog. Each of the wires is driven to vibrate by the wind. Pieces of wood act like violin bridges, transferring the string vibrations to membranes stretched across the ends of the pipes. The membranes then cause the air in the pipes to resonate. Overall, the result is an eerie, pulsing sound like a minimalist piece of music by American composer Steve Reich in which tones come and go depending on the changing wind.

Figure 4.1
Aeolus
.

The work is named after the ruler of the four winds in Greek mythology. Luke's intention is to use “sound to paint pictures in people's imaginations,” allowing visitors to “visualize the changing landscape of wind around the artwork.”
²³
The sound is hard to locate, appearing to be flowing vaguely from above. The lengths of the pipe are carefully selected to form a musical scale. Appropriately, the Aeolian mode is used, which, as a minor scale, lends a malevolent, spooky character to the sound.
²⁴
If I shut my eyes, I could imagine I was in a B movie during a Martian invasion.

Luke decided to construct
Aeolus
after meeting a master digger in Iran who had described the construction of underground irrigation canals called qanats. The digging is wet, claustrophobic, and dangerous. The worst task is probably “devil digging,” in which they mine up to a well of water from underneath. Just imagine being in a cramped passageway at the moment the digger breaks through and the water comes cascading down on top of you. What inspired Luke to create a singing building was the howling of a qanat's air vents in the wind.

Like the Iranian mosque, many grand buildings feature domes, but only rarely do they have the right curvature to achieve distinct echoes. The room diagrammed on the left in Figure 4.2 has a focal point that is too high; the building on the right brings amplified sound back to the listener at ground level and produces a pattern of repeated echoes. By measuring the time between echoes in a recording, I estimate that the interior height in the Imam Mosque is 36 meters (120 feet). The place to stand in the mosque is marked on the floor; this spot is called the
centrum phonicum
, according to old writings on echoes.

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
.
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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.”
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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.
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In acoustics, zone plates can be used to focus beams of ultrasound.

E
choes 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.”
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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.”
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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.”
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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.”
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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: