The Sound Book: The Science of the Sonic Wonders of the World (34 page)

Figure 8.8
Organ of Corti
.

A conversation with acoustics expert Jaime Llinares made Meseguer realize that if photonic structures were scaled up, they would make a sonic crystal, stopping sound at particular frequencies from passing through. In 2011, I showed that sonic crystals also reflect some frequencies intensely, mimicking the iridescence of butterfly wings (unfortunately making an unpleasant sound).
¹²
Órgano
was just the right size for Meseguer and Llinares to test out their idea, being about 4 meters (13 feet) across, with the cylinders regularly spaced about 10 centimeters (4 inches) apart.
¹³

Meseguer placed a loudspeaker on one side to produce noise. A microphone on the other side confirmed their hypothesis: there were band gaps, frequencies of sound that did not go through the array of cylinders. The effect is caused by interference, a phenomenon first explained by British physicist Thomas Young in 1807. Young was a child prodigy who could speak fourteen languages before he was nineteen and first trained as a physician. His classic double-slit experiment, which is still used in schools to teach physics, is illustrated in Figure 8.9. When a monochrome light is shone through two slits, a pattern of light and dark appears on a screen. In some places waves from the two slits arrive with their peaks and troughs aligned and there is constructive interference, giving a bright patch. Sometimes the two waves are misaligned and they cancel each other out, leading to a dark patch caused by destructive interference.

Figure 8.9 Thomas Young's double-slit experiment.

The same effect can be demonstrated with sound if you use a loudspeaker and a screen with the slits farther apart. You can go further and add more slits in the screen, and even many perforated screens spaced out one in front of the other. If you then remove the screens and place a cylinder where each slit was, you have made yourself a sonic crystal like Eusebio Sempere's sculpture. As in the double-slit experiment, what is heard passing through the forest of cylinders is determined by constructive and destructive interference, with sound of some frequencies getting trapped inside, rattling back and forth between cylinders and failing to emerge.

As soon as people discovered that sonic crystals block sound, experimenters began testing their use as noise barriers. But crystals attenuate only a few select frequencies. Consequently, solid barriers made of wood and concrete are nearly always more effective at stopping broadband noise. A colleague of mine at Salford, Olga Umnova, has been experimenting with an acoustic black hole that absorbs a wider range of frequencies. The black hole is made by gradually decreasing the diameter of the cylinders at the edge of an array of crystals. The result is an outer shell that guides the sound to the core, where it can be removed using conventional absorbants.
¹⁴
Sonic crystals have also caught the media's attention as a way of making the sound equivalent of Harry Potter's cloak of invisibility. Normally, we can sense the presence of an object by hearing the sound reflecting from it. The “cloak of inaudibility” surrounds the object, gracefully bending acoustic waves around it so that it cannot be heard. Unfortunately, sonic crystals are often too big to be practical, because sound waves are quite large, especially compared to light.

Organ of Corti
had some of its acrylic cylinders missing to form a sinuous path through the middle of the sculpture. Being a regular array of cylinders, the artwork should have amplified certain sound frequencies and attenuated others. But I had chosen a bad day for listening in London. Workers were digging up the road using jackhammers, and as I tried to listen for subtle changes in the noise, the power tools stopped and started randomly, making it impossible to tell what the sonic crystal was doing.

Later that summer the sculpture was placed near a weir on the River Severn in England. The more constant noise from the waterfall made it easier to hear the sound being sculpted. Francis Crow told me that the effects were most obvious as you exited the sculpture. Walking into the structure, the forest of cylinders subtly removed selected frequencies from the noise, but such absences could be hard to hear. Walk out of the artwork and these removed frequencies reappeared and could be heard. This makes sense, because our ears are designed as an early warning system to listen for new sounds, not subtle absences.

Francis explained to me that some of the motivation behind the work was to change how people listen: “It's hearing what's already there but framed through the structure.”
¹⁵
It was the sonic equivalent of
Skyspace
, a series of works by James Turrell. These are large rooms where visitors view the sky through an aperture in the ceiling, framing light and space.
Organ of Corti
was framing how we listen to sound. Appreciating the work takes time; near the weir, people tended to linger and enter a meditative state. One visitor took this idea to an extreme and spent over half an hour inside the sculpture, commenting, “I made my own symphony.” Another described the subtle ebb and flow inside as “disorienting.” It is difficult to experience sound art quickly. A brief glance at a piece of visual public art is likely to yield more than a brief encounter with a sonic artwork.

Many minimalist sculptures distort sounds. Several of Anish Kapoor's artworks are large, concave light mirrors. I went to see
Her Blood
(1998) at the Manchester Art Gallery in England. The work is made from three huge, concave dishes 3.5 meters (11 feet) in diameter propped vertically against the gallery walls. Two were highly polished mirrors; the third was stained deep red. As visitors walked toward a dish, the visual reflection of their figure was distorted. Some distance away, they could see themselves smeared across the lower half of the dish; as they walked closer, suddenly their image formed rings going completely around the dish. At this position they were at the focal point for both light and sound. Attendants had noticed the way the reflection distorted voices, and they were encouraging visitors to speak at the dishes.

The concave dishes of
Her Blood
did not behave much differently than the radome in Teufelsberg. In contrast, Richard Serra's giant works at the Guggenheim Museum in Bilbao, Spain, produced an astonishing diversity of sounds. It was like having a giant sound-effects unit to play with.
The Matter of Time
(2005) is an installation of seven giant sculptures made from spirals, serpentine ribbons, and sinuous curves of rusting steel that rise many yards overhead. The brown metal walls rise at angles, forming narrow passages that twist and turn, sometimes closing over in an inverted
V
, disorienting a sense of space and balance as you walk through. It is like being in a giant steel maze; around the corners I expected to bump into Alice from Wonderland.

When I visited, the gallery echoed with the hubbub of chattering schoolchildren. As I entered some of the works, I could hear the effects of going into a partial enclosure: the ambient noise quieted, and my ears sensed the quick reflections from the steel walls. The sound was being reframed as happened with
Organ of Corti
.

I was fortunate to have a press pass, which meant I could take out my digital recorder, wait for a moment when I would not disturb others, and clap my hands to reveal the acoustic. In the middle of the giant spirals were large, round arenas about 8 meters (30 feet) across. In these spaces the reflections focused into the middle. This focusing created Gatling gun echoes, as the claps whizzed past me every twenty milliseconds. In some places, stamping my feet made repeated twangs like vibrations propagating up and down a very long spring. As I mentioned in Chapter 5, some were very good whispering galleries, carrying my voice efficiently from one end to the other, with the sound hugging the walls of steel.

Best of all was
Snake
, a work made from three long, tall, twisting metal sheets forming two narrow corridors about 30 meters (100 feet) long. The passageways were only about a meter wide, and resonances across the narrow width colored my voice. When I stood in just the right place, with a flat bit of the ceiling high above the sculpture, the sound would ricochet back and forth between the ceiling and the floor. Sound would also go along the narrow channel and be reflected from other sculptures at the end, before being returned as a diffuse echo. Stamping my feet on the floor was very satisfying because in just the right place I could impersonate a rifle shot. I was not the only person enjoying the acoustic distortions; others were calling “hola,” “echo,” and “boo” as they walked around.

W
hen Peter Cusack asked Londoners about their favorite sounds, often the answers were about mundane and everyday sounds. The favorite-sound question is personal, so the semantic meaning of what is heard usually trumps the raw physical characteristics of the acoustic wave. If you take a moment's break from reading and just listen, what do you hear? I can hear voices from the office next door, rain falling on the pavement outside, and footsteps in the corridor. Did you also come up with a list of what made the sounds? I thought “voices, rain, and footsteps,” not “mumbling, pitter-patter, and tap tap.” We describe what we hear mostly in terms of sources and metaphorical meanings, not their intrinsic sound.

But sometimes the physical characteristics of what we hear do matter. Something loud like an explosion triggers a rapid fight-or-flight response. A plane flying overhead might not be as loud as an explosion, but the sheer volume of the noise might still drown out a conversation. A melody is just a sequence of abstract tones, yet it can tap deeply into our emotions, evoking joy, sorrow, and love. But mostly, for everyday sounds, it is the cause of what we hear that matters. Where possible, the brain identifies the source, and our response is then colored by how we feel about what caused the sound. If you hear a bus in a city square, your response is likely to be strongly influenced by whether you want to get on it, or by your attitude toward public transport: are buses a waste of taxpayers' money clogging up the road or a public good easing pollution and congestion?

This is why Peter's question about sonic favorites reveals sounds that, at face value, have little aesthetic appeal: the announcement “Mind the gap!” in the London Underground, the siren of a New York Police Department squad car, or Turkish market stall holders shouting out offers on Turmstrasse in Berlin.

I am struck by the similarity between the favorite-sound answers and what correspondents told Andrew Whitehouse about birdsong. Many of the stories about city sounds and birdsong are not about the awe-inspiring, surprising, or most beautiful. They are not the sonic equivalent of the Taj Mahal, the Golden Gate Bridge, or the Grand Canyon; instead they are about the sounds that remind us of special places and times, or the sounds likely to be heard in our everyday lives. People often mentioned transportation sounds. After all, getting around a city is such an important part of living and working there. Imagine if the study had asked for favorite images of London; respondents would probably have listed visually appealing or striking attractions such as St. Paul's Cathedral, the London Eye, or Tower Bridge. The ringing of Big Ben is an exception, a sound that has both aesthetic beauty and deep historical, personal, and social meaning for the British.

Peter has been running the Favourite Sound Project for over a decade, so some of the original sounds have disappeared. When trains used to arrive in London, there was a staccato cascade of door slams as passengers disembarked. This sound disappeared as the old rolling stock was decommissioned. What replaces these distinctive sounds is similar across the world because of globalization of technologies and products. Regretfully, cities are becoming more sonically similar and less individual, mimicking the visual homogenization of main street.

Andrew Whitehouse found that birdsong heightened some emigrants' sense of being in an alien country. I got a similar feeling when I traveled to Hong Kong, but with a different sound. My strongest sonic memory was the hubbub of the massed crowds of Filipino women along walkways and crowded into shopping malls. Under the HSBC tower block is a large, covered plaza, which was alive with the high-pitched chatter of females socializing. To Hong Kong residents this is a common sound, as domestic workers gather on Sundays around the city center, lay out their picnic blankets, and socialize with friends. But to me as an outsider, it stood out as something special and unique in Hong Kong.