The Sound Book: The Science of the Sonic Wonders of the World (19 page)

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.

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
K
atzenklavier
(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

E
lliptical 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.

Intriguingly, Arnott's story of the sailing ship mentions the weather as being a crucial part of the tale. The report might well be correct if a temperature inversion helped direct sound to the concave sail.

A
few years ago I presented two science shows at the Royal Albert Hall in London to thousands of children. Though better known as a music venue, the hall is actually dedicated to the promotion of art and science, and it was built on land purchased with the profits of the Great Exhibition of 1851. For an amateur performer like me, a complex science show is a daunting challenge, made all the worse in this case by the vastness of the arena. Fortunately, the acoustics have been significantly improved since the hall opened 130 years ago. Indeed, the Prince of Wales struggled with his opening speech. According to the
Times
(1871):

The address was slowly and distinctly read by his royal Highness, but the reading was somewhat marred by an echo which seemed to be suddenly awoke from the organ or picture gallery, and repeated the words with a mocking emphasis which at another time would have been amusing.
⁹

The hall's ubiquitous curved surfaces are probably what caused the mocking echoes. From above, the floor plan appears as an ellipse, and the whole structure is topped with a large dome. The curved surfaces focus sound like Kircher's elliptical ceiling. But how such reflections are perceived depends on the size of the room. In the vast Royal Albert Hall, the curves cause disastrous echoes. Sound appears to come from several places in the room and not just the stage. In a small room the focused sound arrives quickly; in a larger room the reflections are delayed.