The Sound Book: The Science of the Sonic Wonders of the World (21 page)

Figure 5.5 Teufelsberg.

Before entering, I had to sign a liability waver because the derelict buildings have many holes and missing walls with no protection from the shear drops below. My German tour guide was Martin Schaffert, a young history scholar sporting a neat beard, small ponytail, glasses, and a flat cap. As Martin explained the history of the site, I surveyed what remained of the buildings. Doors and walls were missing, with the debris of the crumbling buildings lying on the floor mixed with broken glass from semi-illicit parties. Where walls were intact, they were covered in graffiti. My eye was drawn upward, where on top of the main building were three domes; some were vandalized, with their walls partly in tatters, but the topmost one, sitting on top of a five-story tower rising out of the roof, was intact.

These were
radomes
(a contraction of the words
radar
and
domes
), used to hide the spying activities from prying eyes as the British and Americans listened in on broadcasts and wireless communications from East Germany, Czechoslovakia, and the Soviet Union. The spherical domes were also used to protect the listening equipment from the elements, especially wind and ice. All that remains now are the concrete plinths to which the antennae were bolted. The domes were constructed of triangular and hexagonal fiberglass panels stretched between a scaffold, looking like giant soccer balls. Fiberglass is transparent to electromagnetic waves and so ideal for radomes; this was one of the reasons the material was developed during World War II.

The tower supporting the top dome was missing all its walls, but the dome itself was virtually complete because it had been rebuilt and reused for air traffic control over Berlin. Every surface of the dingy stairwell up the center of the tower was coated in graffiti. As I climbed the stairs to the dome, I could hear the reverberated voices of other visitors enjoying the acoustics. With a midfrequency reverberation time of about 8 seconds, the radome sounds a bit like a cathedral. Musicians come to perform music in the space. But there is more to enjoy than just the reverberance.

Once inside, I stood and watched other visitors; it was wonderful to see their faces light up when they realized how odd the acoustic was. The slightest sound, even a simple footstep, created a ricocheting effect. Some were prompted to explore with gusto (a good hard stomp could be heard to repeat eight times, sounding like distant firecrackers), while most contented themselves to playing more subtly, almost treating the space with the same reverence as they would a place of worship.

I then climbed onto the old antenna plinth to get to the center of the room. The dome is roughly the top two-thirds of a sphere about 15 meters (50 feet) in diameter, constructed from yellowing hexagons. A band of graffiti 2 meters (about 7 feet) high ran above the floor, interrupted only by a second small opening, through which was an unprotected shear drop to the roof of the building five stories below. I got out my recorder to dictate my impressions of the place, and noticed how every word was doubled up by the reflections from the radome.

Here in Teufelsberg I wanted to explore an effect that happens in the spherical Mapparium.
²¹
The unusually strong focus makes it possible to experience the strange sensation of whispering into your own ear. Or, as Hartmann puts it:

As you approach the exact center of the Mapparium sphere you suddenly become aware of strong reflections of your own voice . . . If you sway to the left, you hear yourself in your right ear. If you sway to the right, you hear yourself in your left ear.
²²

In Teufelsberg the effect is strongest if you look upward while whispering, because there is a larger concave area to focus the sound. Thus, with my head tilted backward, five stories up in a fiberglass radome in Berlin, I discovered an exciting binaural sonic wonder, an effect that reveals how we work out where sound is coming from. Having two ears enables mammals to perceive the locations of sound sources. Hearing evolved to enable animals to sense danger, alerting them to predators sneaking up and trying to turn them into lunch. Humans have good vision, but our eyesight cannot detect threats from behind, so being able to hear and locate danger is crucial.

There are a couple of main ways in which we sense where sound is coming from. Imagine that someone is talking to you from your left side. The sound arrives first at your left ear because it takes slightly longer to reach your right ear. Your brain also makes fine distinctions in loudness. The sound has to bend around your head to reach the right ear, causing it to be much quieter at high frequency. (The loudness of low frequencies coming from far away is minimally affected by your head itself). Your brain compares the timing in each ear at low frequency and the relative level of high frequencies to decide where the sound is coming from.

Spherical rooms can mess with both of these cues. The loudness cue can be distorted, creating an unexpected localization. It makes sound appear to come from the wrong direction, as Hartmann describes: “Suppose you are on the Mapparium bridge facing South America. There is a source of noise to your right, but you discover that you hear the noise coming from your left!”
²³
Strong, focused reflections from the sphere create a loud reflection at the left ear, fooling the brain into localizing the noise on the left side.

Localization is usually based on the first sound arriving at the ears (the precedence effect). This rule of thumb serves us well because the earliest sound takes the quickest route, which is usually a straight line between talker and listener. You may have sat through a church service in which the sermon appeared to be delivered by the loudspeakers rather than by the preacher. The reason for this impression is that the sound from the loudspeakers reaches the listener first. Adding a little delay electronically in the public address system, so that the first sound wave to reach the listener comes directly from the preacher's lips, solves this problem.

But adding a delay is ineffective if the loudspeakers are turned up too high, because the precedence effect can be overruled by a loud sound that arrives later—a situation that prevails at most rock concerts. Without electronic amplification, though, reflections from walls are usually too quiet to cause a problem. But in the case of the Mapparium or Teufelsberg, where the dome's focus greatly amplifies the delayed sound, the reflections are so strong that we are fooled by false localization. When I burst my balloons in Teufelsberg, the first reflection from the ceiling was 11 decibels louder than the sound coming directly from the balloon (Figure 5.6). (A useful guide is that an increase in 10 decibels is roughly a doubling in perceived loudness.) When I knelt down to unzip my knapsack, it sounded like someone was opening the bag above my head!

Figure 5.6 Direct sound and reflections from bursting a balloon in the center of the Teufelsberg radome.

Barry Marshall, from the New England Institute of Art in Brookline, Massachusetts, who used to be a guide at the Mapparium, told me how he used the acoustic to play practical jokes on visitors and “blow their minds.” The strong focus meant that he could stand far away from visitors and surprise them by calling “over here,” and they would look in the wrong direction.
²⁴
In Teufelsberg, I contented myself with spying on other conversations by finding the point where the speech of other visitors was being focused.

Long-distance whispering and sound focusing tend to unnerve people because they hear something that seems supernatural. If I were to chat with you in a normal room, the low frequencies in my voice would have roughly the same loudness in both your ears, whichever way you faced, because they can easily bend around your head by diffraction. Normally, low frequencies become much louder in just one of your ears only if I get very close, maximizing the “sound shadow” cast by the head. The effect reduces low frequencies in the far ear, making you think I am next to you. But in the Mapparium, the sphere can focus sound so intensely on one ear that it can trick the brain into thinking that I must be close by. Not only can I whisper sweet nothings to a loved one yards away; I can even narcissistically whisper them to myself!

T
he point of whispering, of course, is to say something so quietly that unintended listeners cannot hear it. Apparently, the original dome of the Capitol allowed members of the House of Representatives to whisper private messages to each other. But the amplification worked both ways: the congressmen could also overhear their colleagues' secrets. We naturally seem to associate sound amplification by curves with spying, subterfuge, or illicit liaisons. Fellini used this link to dramatic effect in his film
La Dolce Vita
, in which a concave basin allows eavesdropping on conversations from the lower floor of a villa.
²⁵
But the most curious spying legend concerns a large limestone cave near Syracuse in Sicily called the Ear of Dionysius. The story goes that the tyrant Dionysius (ca. 430–367 BC) used the cave as a prison and exploited the acoustics to find out what hapless detainees were whispering to each other.

The cavern is shaped like a tall, pointy donkey's ear and narrows dramatically at the top. The wedge shape acts like a funnel to sound, as Figure 5.7 illustrates, potentially collecting whispers from the ground level and concentrating them at the roof of the cave, 22 meters (72 feet) above. Legend has it that Dionysius spied on prisoners from a listening chamber at the top, picking up the amplified sound through a small hidden opening at the top of the cavern.

The cave is a popular tourist venue, and in the past, the listening chamber could be visited—as one traveler noted in 1842, “the only . . . mode of access being by means of a rope and pulley, the adventurer hazarding his life in a little crazy chair.”
²⁶
Despite the legend being recounted to tourists, some reports cast doubt on whether spying was possible. In 1820 the Reverend Thomas Hughes wrote, “A very low whisper is heard only as an indistinct murmur; the full voice is drowned in the confusion of the echoes. The voices of several persons speaking at the same time are as unintelligible as the cackling of geese, so that if the ancient Sicilians were half as loquacious as the modern, who always chatter in concert, they must very often have put the listening tyrant to a nonplus.”
²⁷

Figure 5.7 Sound in the Ear of Dionysius.

With modern health-and-safety precautions, visitors can no longer enter the upper chamber. A listener today is simply left to enjoy the reverberation at ground level, marvel at the legend, and gaze at the cavern's large, ear-like shape. (A different sound-related tradition that has been stopped is the firing of firearms for tourists: according to another nineteenth-century visitor, “A pistol was fired, and the report was like the discharge of an eight-and-forty pounder.”
²⁸
)

Recently, Gino Iannace, from the Second University of Naples, and collaborators persuaded the cave owners to let them into the listening chamber to survey the acoustic. Just as my team rates theaters, classrooms, and railway stations, Iannace's group took a battery of measurements to assess speech intelligibility in the cave. The results were “on the bad side of average,” indicating that the cave's reverberation makes speech muddy and incomprehensible. Undeterred, Iannace then carried out a series of perceptual tests, asking listeners to transcribe phrases recounted in the cave, but nobody could get a single word right. Disappointingly, the scientific measurements failed to support the legend.

Utterly overcome by pain and grief, I crouched against the granite wall.

I just commenced to feel the fainting coming on again, and the sensation that this was the last struggle before complete annihilation—when, on a sudden, a violent uproar reached my ears. It had some resemblance to the prolonged rumbling voice of thunder, and I clearly distinguished sonorous voices, lost one after the other, in the distant depths of the gulf.

Suddenly my ear, which leaned accidentally against the wall, appeared to catch, as it were, the faintest echo of a sound. I thought that I heard vague, incoherent and distant voices. I quivered all over with excitement and hope!