The Sound Book: The Science of the Sonic Wonders of the World
Page 4
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.34
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.35
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.”36
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.”37
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.38 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.
What 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.39
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.”40 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.41
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.”42 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.
In 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.”43
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.”44
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 rese
rvoirs 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.45 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.
Figure 1.2 Wormit water reservoir (using a very long exposure on the camera).
As James and I chatted, the acoustic immediately revealed itself: a rumble began building up and hung about us like a pervasive fog. Many very reverberant rooms are acoustically oppressive, making it hard to have a conversation. But not this reservoir.46 Surprisingly, we could talk to each other even when we were quite far apart—something that was not possible in the similarly reverberant Hamilton Mausoleum.47 It reminded me of a cathedral, with the great advantage that I could shout and clap. Whooping unleashed the full power of the “preposterous” acoustic; the sound rattled around for ages before dying away.
I had a few balloons with me, which I burst to get a rough measurement of the reverberation time. As in mausoleums, the most impressive values were at low frequency: 23.7 seconds at 125 hertz. For the midfrequencies that are most important for speech, the reverberation time was a more modest 10.5 seconds.
Saxophonist John Butcher made recordings in the Wormit reservoir as part of the Resonant Spaces tour. The Wire review of the album describes how he “attacks the spaces.”48 In Butcher’s piece “Calls from a Rusty Cage,” it is often hard to discern the sound of a saxophone among the strange electronic whistles, breathy squeaks, and blasts, which sound like ship horns. Will Montgomery in Wire described how halfway through, Butcher “suddenly leaps into whirling circular breathing with a flamboyant glissando (which . . . recalls the opening to Rhapsody in Blue).”49 This is certainly one musical approach to such a reverberant place: accept the dissonant smog created by lingering notes, and play on.
Another approach is that taken by American trombonist and didgeridoo player Stuart Dempster in his album Underground Overlays from the Cistern Chapel. The chapel in question is the Dan Harpole Cistern in Fort Worden State Park, Washington State, the place Mike Caviezel described as crazy and disorienting. It looks very similar to Wormit, although it is circular rather than square. It was built to supply about 7.5 million liters (2 million US gallons) of emergency water for extinguishing fires. A few websites and books quote a 45-second sound decay. This means it takes about 3 seconds for a note to become half as loud, and musicians can achieve note separation only when they play incredibly slowly.50 Billboard magazine described the recording by Stuart Dempster and his fellow musicians as creating an “intensely serene music in which the slightest changes seem cataclysmic, and gradual swells emerge as tidal waves.”51 Writing in the Times, Debra Craine describes the music as having an “eerie, majestic calm that envelops you with hypnotic elation.”52 Notes played seconds apart form lush layers on top of each other, requiring a player to think about the interaction of notes played far apart; otherwise, intense dissonance is produced. Stuart Dempster commented, “Usually when you stop for a mistake, the mistake has the decency to stop too, but it doesn’t [in the cistern;] it just sits there and laughs at you . . . You have to be a clever composer [or improviser] and incorporate all your errors into the piece.”53
I listened to the album, enjoying the meditative polyphony, but also listening for the ends of phrases, because after the musicians stopped playing, the sound would naturally ring around the cistern. From these parts of the music, the reverberation time can be estimated. For over a decade, colleagues and I have been developing ways of extracting reverberation time from speech and music. The idea is to make measurements in concert halls, railway stations, and hospitals while they are in use. Conventional reverberation measurements require loud sounds: gunshots or loudspeakers blaring out noise or slow glissandos. These are unpleasant to listen to and can damage hearing. Audience members also have an annoying habit of ruining the results by commenting on the noise—“Wow, that was loud”—as the decay is being measured. But the sound of an orchestra in a concert hall, or the speech of a teacher in a classroom—while imperfect for measurement—does include the room acoustic; the difficult part is finding a way to extract the effects of the room from the music or speech. One of the most exciting areas of research at the moment is the use of computer algorithms to extract information from audio. A well-known example is Shazam, an app that identifies music from a brief recording through a mobile phone’s microphone. Other algorithms try to transcribe music automatically or identify the genre of unlabeled audio files.
Applying our algorithm to Stuart Dempster’s recording gave an estimated reverberation time of 27 seconds over the low frequency ranges of the trombone and didgeridoo.54 This is a good indication that the American cistern beats the Scottish reservoir. But to be sure, I wanted a conventional impulse response. When creating a new auditorium, acoustic engineers work from graphs and tables of reverberation times and other parameters to check that the hall meets design specifications. However, these scientific charts and parameters mean little to architects, so acousticians are increasingly making audio facsimiles of a proposed auditorium and getting clients to listen. This auralization starts with a piece of music that has been recorded in a completely dead space, like an anechoic chamber (described in Chapter 7). In other words, it is the sound of the orchestra without any room. Acousticians then combine this music with a model of how sound will move in the future place. In the past, impulse responses came from scale models of the auditorium at one-tenth or one-fiftieth of the full size, but nowadays they are more often predicted by computers.
Auralization also works with impulse responses measured in real rooms, so it has also been encoded into artificial reverberation algorithms used by musicians and sound designers creating film and game soundtracks. In one of these reverberators I stumbled across a library of impulse responses that included three measured in the American cistern. At low frequency the Dan Harpole Cistern has the same reverberation time as the Wormit reservoir: 23.7 seconds. But at midfrequency the America cistern wins, with a reverberation time of 13.3 seconds. These reverberation times are longer than those found in even the biggest cathedrals in the world.
Entering the oil storage complex at Inchindown, near Invergordon, Scotland, felt like walking into a villain’s secret lair in a James Bond movie. The 210-meter-long (230-yard) entrance tunnel was narrow, concrete lined, and not much taller than me. And as I walked up the gradual incline from the entrance in the hillside, the daylight steadily faded behind and my torch vainly attempted to illuminate the way. The concrete lining ended, the tunnel became bare rock,
and an alcove on the left revealed the entrance to the number one oil tank. But this was not a door, because the only way to get through the 2.4-meter-thick (8-foot) concrete wall and into the gigantic storage tank is via one of the four oil pipes, each only 46 centimeters (18 inches) in diameter. This was no time to worry about claustrophobia, because at the other end of the pipes was hopefully the most reverberant space in the world.
I was in Inchindown nine months after experiencing Wormit, visiting tanks that had once held heavy, crude shipping oil. These reservoirs supplied the naval anchorage in the Cromarty Firth at the bottom of the hill. The tanks were constructed in great secrecy amid concerns about the strengthening of Germany’s armed forces during the 1930s and the threat posed by long-range bombers, which is why the tanks were dug deep into the hillside. The vast complex took three years to complete. The whole depot held 144 million liters (38 million US gallons) of fuel—enough to fill up two and a half million diesel cars.
My guide was Allan Kilpatrick, an archaeological investigator for the Royal Commission on the Ancient and Historical Monuments of Scotland. Allan is incredibly passionate about the oil tanks, having learned about the secret tunnels as a local boy. With us were about eight other people, taking advantage of a rare opportunity to see the place, although some never got into the main storage tanks, because they found the entrance too claustrophobic.
I was about to enter one of the big tanks, designed to hold 25.5 million liters (about 7 million US gallons) of fuel. I lay down on a trolley, a narrow sheet of metal about 1.5 meters (5 feet) long, and was pushed into the pipe like a pizza being put into a deep oven. The entrance holes looked even smaller when I was waiting to be dispatched, and as I entered I could feel the walls of the pipe pushing into my shoulders, compressing and squeezing me. The helpers kept shoving, my hard hat fell off, and then I was in. It was an undignified landing as I stopped at an angle, with my feet on the floor of the storage tank and my torso still half in the pipe. I struggled upright with a helping hand from Allan—dressed like a climber and looking at home in the dark underground world. Soon afterward, my acoustic measurement gear was pushed through, all carefully selected to ensure it would fit the narrow pipework.