The Sound Book: The Science of the Sonic Wonders of the World (7 page)

The longevity was extraordinary. The 45-centimeter-thick (18-inch) concrete walls mean that there is very little absorption at low frequency when the sound is reflected from the walls. Furthermore, the shipping oil has clogged up the pores in the concrete, creating a smooth surface impervious to air and thus drastically reducing absorption by the walls at high frequencies. The most absorbent substance in the tank was, in fact, the vast volume of air, which caused a quicker decay at higher frequencies. As the sound wave passed from molecule to molecule, tiny amounts of energy were lost. Textbooks show air absorption of tens of decibels per mile at the highest frequency I measured. In most rooms the distance traveled by sound is too small for this to be important. But the oil tanks are about a sixth of a mile long, so at high frequency the absorption by the air was more important than that by the walls.

With six bangs recorded, it was time for a quick analysis. I transferred the measurements to a laptop and ran my program. My initial reaction was disbelief; the reverberation times were just too long. At this point in relating this story, I like to play a game with my fellow acousticians called “guess the reverberation time.” They usually pick an acoustically outrageous number, maybe 10 or 20 seconds. Even so, they always guess far too low. At 125 hertz, the reverberation time was 112 seconds, almost 2 minutes. Even at midfrequency the reverberation time was 30 seconds. The broadband reverberation time, which considers all frequencies simultaneously, was 75 seconds. I called Allan over to give him the good news. We had discovered the world's most reverberant space.

W
hy did we build huge reverberant cathedrals to celebrate the divine? Did our prehistoric ancestors share our appreciation for resonant spaces? These were the questions going through my mind as I stood by the four tall, imposing façade stones outside a Neolithic burial mound, blowing up party balloons and smiling sheepishly at other tourists. When I bought the balloons, I had been tempted to choose the black one with skeletons printed on them. What could be more appropriate for a burial chamber? But reluctantly, I had settled for some big yellow and blue balloons made from thicker latex, because they would go with a bassy bang.

I had abandoned my bulky acoustic equipment for this field trip. Luckily, I could make surprisingly useful measurements with a pin, a balloon, a microphone, and a digital recorder. I crawled between the entrance stones, and a dank, earthy smell filled my nostrils as I entered the cramped tomb. I set up my microphone in one arm of the cross-shaped chamber, ready to record the balloons bursting on the opposite side.

Only in recent years have scientists begun systematically studying the acoustics of prehistoric archaeological sites. And one of the more controversial publications on the subject had brought me to this ancient burial mound only 50 kilometers (about 30 miles) north of Stonehenge.
¹
The whole region is stuffed with prehistoric remains, including the largest prehistoric stone circle in the world at Avebury, which has 180 unshaped standing stones on a 1.3-kilometer (about ¾-mile) circumference, and Silbury Hill, the largest prehistoric mound in Europe. At nearly 40 meters (130 feet) high, made from half a million tons of chalk, the man-made hill has no clear purpose. But I was measuring a smaller monument, Wayland's Smithy, a 5,410- to 5,600-year-old Neolithic long barrow (Figure 2.1).

Figure 2.1 Entrance to Wayland's Smithy.

To arrive at the long barrow, I had trudged along the muddy Ridgeway, an ancient walking highway in central England, on a cold, clear winter's day. Had I been on horseback, I could have avoided the quagmire under my feet and also tested the famous legend of Wayland's Smithy, which claims that if you leave your horse tethered overnight along with a silver coin on the capstone, the next morning your steed will be reshod.

The barrow is a large, low mound, fringed by a circle of beech trees. Most visitors poke their heads inside, snap a few pictures, and move on—examining the ancient monument through twenty-first-century eyes. But I felt compelled to explore the acoustics. I listened to my footsteps and how the sound changed as I crawled about. I talked out loud to myself to test whether my voice became distorted, and I clapped my hands to seek out echoes. I even plucked up the courage to sing a few notes, using the acoustics of the burial chamber to enhance my otherwise feeble bass. And of course, I burst my party balloons.

Acoustic exploration is vital for understanding how our ancestors might have used these ancient sites. Back in Neolithic times, sound would have been even more important than it is today. In a time before writing, being able to listen to someone talking, remember the message, and pass it on was a vital skill. Acute hearing was crucial for avoiding predators, repelling attacks from rivals, and tracking and hunting animals for food. To overlook sound is to render the story of ancient monuments incomplete. We need to explore beyond the visual dominance of modern life and use our other senses: hearing, smell, and touch.

A
n obvious starting point for an exploration of ancient sites is the Greek architectural masterpiece, the theater at Epidaurus. A traveler in 1839 wrote,

I could well imagine the high satisfaction with which the Greek, under the shade of the impending mountain, himself all enthusiasm and passion, rapt in the interest of some deep tragedy, would hang upon the strains of Euripides or Sophocles. What deep-drawn exclamations, what shouts of applause had rung through that solitude, what bursts of joy and grief had echoed from those silent benches!
²

This is a vast, almost semicircular terrace of gray stone seats, banked steeply in front of a circular stage. Even today, tour guides delight in demonstrating the “perfect” acoustics, astonishing visitors as a pin dropped on the stage is heard high up on the huge bank of marble seats. “Few acoustical situations are so enveloped in myth as the antique Greek theatre,” wrote acoustic scientist Michael Barron. “For some, the Greeks are credited with an understanding of acoustics which still baffles modern science.”
³
Unfortunately, no extant documents reveal what the Greeks knew. But we are not completely bereft of written evidence, because Vitruvius, one of Julius Caesar's military engineers, wrote extensively about the design of Greek and Roman theaters between 27 and 23 BC.
⁴
What is striking about Vitruvius's book is that the overriding concern is for good acoustics, with less interest shown for visual appearance.

Vitruvius provides simple design principles that still apply today. Greek theaters bring the audience close to the stage so that they can hear the sound as loudly and clearly as possible. This is why the audience seating is roughly semicircular. However, for the audience seated to the side of the stage in Epidaurus, the actors' words would still have been rather quiet, because the voice naturally projects forward.
⁵
The solution to this problem was to give the side seats to foreigners, latecomers, and women—the ancient equivalent of cheap seats.
⁶

Ancient theaters were built in very quiet locations, so that unwanted noise wouldn't drown out an actor's voice. The designs exploited sound reflections, including those from the circular stage floor and scenery. All these reflections reinforced the sound of people speaking on the stage. As the Roman scholar Pliny the Elder noted, “Why are choruses less distinct when the orchestra [stage floor] is covered with straw? Is it due to the roughness that the voice, falling on a surface which is not smooth, is less united, so that it is less? . . . Just in the same way light shines more on a smooth surface because it is not interrupted by any obstruction.”
⁷
The straw probably quieted the sound by absorption rather than scattering. Pliny the Elder's comments are relevant to modern homes, which have become much more reverberant, now that wooden flooring is more fashionable than carpet.

The ancient theaters themselves provide compelling archaeological evidence of an empirical trial-and-error development of good acoustic design.
⁸
But there is no indication of anything like a modern scientific understanding. Writing about Vitruvius, academics Barry Blesser and Linda-Ruth Salter conclude, “Although some of his insights would be confirmed by modern science, others would prove to be nonsense.”
⁹
The more dubious ideas included the suggestion that a few large vases dotted around the theater would enhance an actor's voice.
¹⁰
As a translation of Vitruvius's writings states, “The voice, uttered from the stage as from a center, and spreading and striking against the cavities of the different vessels, as it comes in contact with them, will be increased in clearness of sound, and will wake an harmonious note in unison with itself.”
¹¹

If only acoustic-engineering solutions were that cheap and easy. Unfortunately, the vases would have made little difference to the acoustic. Blow over the neck of a large beer bottle, or more fittingly a large Roman wine jug (say, 40 centimeters, or 16 inches, tall), and you might hear a low, resonant hum. This is the resonant frequency of the air enclosed within the jug. Objects have particular frequencies at which they like to vibrate; flick a champagne flute with a finger and a distinct tone is heard at the glass's natural resonant frequency. But set a wine jug on the floor next to you at Epidaurus, and what you hear is unlikely to change. Any energy used to get the air in the jug resonating will be lost within the vessel. When you walk past empty beer bottles at a pub gig, the sound does not change.

Intriguingly, resonant vases can be found in about 200 churches and mosques built between the eleventh and sixteenth centuries in Europe and in western Asia. These range from 20 to 50 centimeters (8–20 inches) in length, with openings between 2 and 15 centimeters (about 1–6 inches) in diameter. Unfortunately, there are no contemporaneous writings explaining their purpose. High up in the vast Süleymaniye Mosque in Istanbul, you can see a ring of sixty-four small, dark circles just below the ornate ceiling of the dome, which are openings for resonators.
¹²
In St. Andrew's Church, Lyddington, UK, there are eleven jars high up in the chancel—six in the north wall and five in the south.
¹³
In the church of St. Nikolas in Famagusta, North Cyprus, holes can be seen that connect to hidden pots and pipes. However, scientific studies have shown they would have been useless.
¹⁴
The natural resonant frequencies of some vases do not match the frequencies of speaking or singing, and hundreds of vessels would be needed to have a significant effect.

Such myths probably arise and persist because sound is invisible, so the cause of an aural effect is not always obvious. Before the twentieth-century advent of electronic equipment to record and analyze acoustics, it was impossible to calculate a complicated sound field such as a church. The eminent architectural acoustician Leo Beranek documented some of the myths of acoustics.
¹⁵
My favorite is the story of the broken wine bottles found under the stages and in the attics, walls, and crawl spaces of some of the great European concert halls. Were these artifacts evidence of an ancient technique for improving acoustics, as some have claimed? No, just evidence of the drinking habits of construction workers.

Another myth Beranek notes is the assumption that wooden auditoriums are best because the walls vibrate like the body of a violin. But actually it is better to make the surfaces hard so that sound is not needlessly absorbed. Newer halls that are lined with wood, such as the Tokyo Metropolitan Art Space concert hall, actually use thin veneers of wood glued solidly onto concrete or other heavy and thick substrates.

Greek and Roman theaters are remarkable sonic wonders in which thousands of spectators can hear without the aid of modern electronics. They were clearly designed to achieve good acoustics, but were the Greeks the first skilled acoustic craftsmen?

S
ound is ephemeral, disappearing as soon as it's made, so it is difficult to know exactly what our ancient ancestors heard. Evidence of prehistoric acoustics is very sketchy. Musical artifacts provide some of the most robust evidence of our ancestors' sonic world.