The Sound Book: The Science of the Sonic Wonders of the World (5 page)

Many centuries ago, the priest would stand in the chancel, almost cut off from the congregation in the nave. Typically, only a small opening above the chancel screen and below the tympanum would allow sound to reach the churchgoers. The priest would chant facing the altar, his back to the worshippers, so any speech reaching the congregation would be a mush of reflections, with all sound arriving indirectly via reflections off the walls and ceiling. Mind you, with the service mostly in Latin it could be argued it was not the acoustics that caused the speech to be unintelligible.

The Reformation of the sixteenth century changed all that: Anglican priests were instructed by the
Book of Common Prayer
to speak from a place where they could be heard more clearly.
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Services in English meant that speech would have to be understood. Innovations such as the pulpit in the nave enabled listeners to hear more clearly. There were still reflections, but because the direct speech reached the congregants' ears quickly and some strong reflections arrived shortly after, the setup tended to aid communication. Late reflections, however, make matters worse.

Why are some reflections useful and others detrimental? It comes down to how our hearing has evolved to cope with a complex soundscape. In a cathedral, like most places, the ear is bombarded with reflections from all around—from the floor, walls, ceiling, pews, members of the congregation, and so on. In a large cathedral there are many thousands of reflections per second.
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Perceiving each individual reflection would quickly overwhelm our hearing. Consequently, the inner ear and brain combine the reflections into a single perceived sound event. Thus, when we clap our hands in a room, we usually hear only one “clap,” even though the ear actually receives many thousands of slightly different reflections of the sound in close proximity. A room does not turn a single hand clap into applause.

The ear is a little bit sluggish, rather like a heavyweight boxer. When the ear receives a very short sound, like a hand clap, or when a boxer is hit by a fast punch, it takes a little time for the system to respond to the stimulus. Both the ear and the boxer also continue to respond after the initial stimulus has gone away: the heavyweight boxer reels and rocks for some time after the punch has landed, and similarly, the hair cells in the inner ear continue to send signals up to the brain for some time after the clap has stopped. On top of this physical sluggishness in the ear, the brain is also constantly trying to make sense of the electrical signals coming up the auditory nerves. The brain employs several tactics to separate the priest's direct speech from the morass of late reflections reverberating around the cathedral.
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If the priest is to one side, the ear nearer the priest receives louder sound waves because the farther ear receives only speech that has bent around the head. The brain thus attends more to the nearer ear, where the speech is louder and easier to pick out among the reflections. Attention focused in this way becomes less effective if there are lots of reflections from many directions, because both ears become overloaded with a wash of unwanted reverberance.

If the priest is straight ahead, another tactic can be used. In this case the brain adds together what is heard in both ears. The speech coming directly from the priest creates the same signal in both ears because the head is symmetrical, so the sound in each ear has traveled an identical pathway. Adding together the signals from the ears boosts the direct sound. Reflections from the side arrive differently at both ears, and when the left- and right-ear signals are added together, some of the reflections cancel out. This binaural processing increases the loudness of the speech relative to the reverberance.
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In big old churches, you often see a small wooden roof (the tester) just above the pulpit. The tester provides beneficial reflections that arrive quickly enough to reinforce the direct sound. The tester also stops the priest's voice from going up to the ceiling to reverberate and return so late that it makes speech less intelligible.

Nowadays, loudspeakers are used to improve speech intelligibility in churches. Like the tester, the loudspeakers direct speech toward the audience, improving the ratio of direct sound to reflections. Older systems used many loudspeakers stacked on top of each other in a line—the idea being that the sound from the loudspeakers adds together to beam the speech toward the audience. More modern systems use sophisticated signal processing to electronically alter the sound coming out of each loudspeaker, creating an especially narrow beam of speech concentrated only on the congregation.
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Whereas large churches are something of a nightmare for speech, they make wonderful performance spaces for organ music, as author Peter Smith writes: “The melody line is dominant, but its chords are sounded against the surviving strains of the preceding chords in declining strength. The result is a measure of clash or discord that adds considerable piquancy to the experience. There is a richness . . . in a great cathedral which is absent from the concert hall.”
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Churches had a profound effect on the development of music. St. Thomas Church (Thomaskirche) in Leipzig, Germany, is an important example. Before the Reformation, the priest's voice took 8 seconds to die away in the church. In the mid-sixteenth century the church was remodeled to help the congregation comprehend the sermons. Wooden galleries and drapes were added that muffled the reverberation, dropping the decay time to 1.6 seconds. Moving forward to the eighteenth century, we find one of the cantors, Johann Sebastian Bach, exploiting the shorter reverberance to write more intricate music with a brisker tempo. Hope Bagenal, the senior acoustic consultant of the Royal Festival Hall in London, considered the insertion of galleries in Lutheran churches, which reduced reverberation, to be “the most important single fact in the history of music because it leads directly to the St Matthew Passion and the B Minor Mass.”
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How reverberant are grand cathedrals? St. Paul's Cathedral in London was built between 1675 and 1710 to replace the predecessor, which had been destroyed in the Great Fire of London. Designed by Sir Christopher Wren, it has a vast volume of 152,000 cubic meters (5.4 million cubic feet). At midfrequency, the reverberation time is 9.2 seconds; at low frequency it rises a little, to 10.9 seconds at 125 hertz.
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These decay times are long, but at low frequency the Hamilton Mausoleum is more reverberant, probably because it has fewer windows (which are quite good at absorbing low frequencies). The values at St. Paul's Cathedral are typical of other large Gothic cathedrals, so the mausoleum appears to beat the sanctuary in terms of reverberation.

W
hat about natural spaces, such as caves? The US military became very interested in the acoustics of caves and tunnels during the hunt for Osama Bin Laden in Afghanistan. The idea was to give troops a better understanding of the layout of subterranean passages before they entered. David Bowen, from the acoustic consultancy Acentech, investigated the feasibility by having soldiers fire a gun four or five times at the mouth of a cave and recording the acoustic result. The branches, constrictions, and caverns would alter the way the sound reverberates. This information would reflect back to microphones at the entrance, allowing the cave's geometry to be inferred.
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Cave geometry can produce wonderful reverberations. Smoo Cave on the north coast of Scotland emerges among some of the most spectacular and rugged terrain in Britain, with stony green mountains and glorious sandy beaches being bombarded by crashing waves. Nine months after hearing the Hamilton Mausoleum, I went to visit the cave in the hope of finding a more reverberant place. I entered through a large, gaping arch in a sheer limestone cliff cut by the sea. But the first chamber was not as reverberant as I had hoped, because the entrance was very open and there was a large hole in the roof, so the sound quickly disappeared. The second chamber was much more interesting, with a waterfall crashing through a hole in the ceiling, falling 25 meters (about 80 feet) to the flooded cavern floor. The sound was loud and overpowering; when I closed my eyes, it was hard to work out where the noise was coming from as the roar of the cascade reverberated around the cavern.

Basalt columns impressively bedeck Fingal's sea cave on the island of Staffa, Scotland, about 270 kilometers (170 miles) southwest of Smoo Cave. In 1829, the composer Felix Mendelssohn took inspiration from the sound of the Atlantic swell rising, falling, and echoing around the cave. Enclosing the first twenty-one bars of his overture
The Hebrides
, he wrote to his sister Fanny: “In order to make you understand how extraordinarily The Hebrides affected me, I send you the following, which came into my mind there.”
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David Sharp, from the Open University in the UK, measured a reverberation time of 4 seconds in the cave—somewhere between a concert hall and cathedral in the pecking order of reverberance.
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In general, although caves can be very large, it seems that the biggest do not reverberate more than grand cathedrals. Writing about a performance of postmodern compositions by Karlheinz Stockhausen in the Jeita Grotto in Lebanon, acoustician Barry Blesser notes that although caves are large, correlating to long reverberation times, they are usually made up of multiple connecting spaces, meaning that the sound decay is “softened, reaching only a modest intensity.”
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Every time a sound wave bounces or reflects, it loses some energy. In a cave, there are lots of side passageways where walls are rough and uneven. The lumps and bumps disrupt the sound, forcing it to bounce back and forth across these passageways and die away faster. The most reverberant spaces have not only smooth walls, but also very simple shapes; this means they are man-made.

I
n 2006, the Japanese musician, instrument builder, and shaman Akio Suzuki and saxophonist, improviser, and composer John Butcher went on a musical tour of Scotland called
Resonant Spaces
. According to the publicity material, the tour aimed to “set free the sound” of exciting and incredible locations, including the old reservoir in Wormit: “My God it's got a preposterous sound, a huge booming decay and . . . echoes, careening around off its concrete walls. Normally I suppose that would be the worst thing you could think of in a performance venue, but for this tour it's pretty ideal.”
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An earlier conversation with Mike Caviezel, head of audio for Microsoft games, had piqued my interest in such spaces. After I gave a keynote address at a conference in London, Mike had approached me to tell me about his visit to a similar water reservoir in the US. He described how the acoustics and darkness make it “one of the most crazy, sort of physically disorienting spaces I've ever been in.” Mike also described how the reflections affect speaking: “You immediately lose track of what you're talking about, and all you can focus on is just the acoustics of the space.” The reverberation is so powerful that “it's very hard to get out . . . clear thoughts or sentences,” he said, “and everything quickly devolves to people either whistling, or clapping their hands, or testing the space.”
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Curious to experience such an odd-sounding space, I decided to visit Wormit a couple of days after I had been in the Hamilton Mausoleum. The arts company that had organized the
Resonant Spaces
musical tour, Arika, gave me contact details for the owner, James Pask, who was only too delighted to show me around. In a gentle Scottish accent, he explained that he had acquired two underground reservoirs when he bought the land; the smaller one had been turned into a vast garage under his house, but the larger one just lay empty underneath his lawn.

We wandered out into the garden, chatting about structural loads and the history of Wormit's municipal infrastructure. The reservoir had been built in 1923 with the intention of serving a large town, but the war intervened and Wormit never grew very large. Eventually, the cost of maintaining the oversized reservoir led it to be decommissioned.

It was very windy that day, with the autumn sun glinting off the Firth of Tay down the hill, and the city of Dundee in the distance across the water. The lawn was extraordinarily flat. Black ventilation pipes poked out of the ground and hinted at what lay below. James opened up a very overgrown manhole cover and asked me if I was worried about health and safety, before disappearing down a ladder into the dark to turn on the light.

The ladders resembled those on ships. The first led down to a small platform, and then I had to climb precariously over a chain fence to reach a second ladder, which led to the floor below. The vast space, illuminated by the light streaming through the manhole cover and a single lightbulb, had few visual charms. It was just a concrete box, about 60 meters (200 feet) long, 30 meters (100 feet) wide, and 5 meters (15 feet) high.
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The concrete on the walls had the texture of the wood shuttering used during the construction imprinted on them (like the walls of the National Theatre in London). It reminded me of a municipal garage, with a forest of concrete pillars regularly spaced about 7 meters (23 feet) apart holding up the concrete ceiling (Figure 1.2). The floor was wet here and there, and it was pleasantly cool, like a natural cavern.