by Trevor Cox
For decades, acoustic engineers have been trying to reduce unwanted noise, but many attempts have been defeated by changes in society. A modern car is much quieter than an old clunker, but increased traffic means the average city noise level has remained about the same. As the rush hour extends and motorists seek out quieter roads, peaceful places and times are disappearing. What should we do about this noise? I believe that telling people to stop doing noisy things is futile. It is better to encourage listening and curiosity. Although technology often produces unwanted noise, new devices are also creating bizarre and wonderful sounds every day. Gadgets emit distinctive pings and buzzes that people will cherish and be nostalgic about. The chime of the pinball machine takes me back to hanging out with friends in my youth. My children will likely have fond memories of the iPhone click, long after the device has been supplanted by more sophisticated technologies. With better awareness of the sonic wonders, I hope people will demand better everyday soundscapes.
Since the trip into the sewer, my hunt for sonic wonders has morphed into a full-blown quest. I set up an interactive website (SonicWonders.org) to catalogue my discoveries and serve as a forum for people to suggest enticing sounds for further investigation. After a talk at a conference in London, a delegate told me about a large, spherical room called the Mapparium at the Mary Baker Eddy Library in Boston, where even nonventriloquists can throw their voice. This illusion plays with the mental processing that allows us to locate sound sources, processes that evolved to protect us from predators creeping up on us from behind. A conversation during a TEDx event in Salford made me want to find out more about the moths that have evolved decoy tails to fool echolocating bats. Rummaging through old proceedings from academic conferences has revealed a gold mine of acoustic curiosities, neglected phenomena that devoted scientists have studied alongside their day-to-day research.
Friends and colleagues, and even complete strangers, have given me examples of quirky acoustics and fascinating science. My research has uncovered the ways in which sounds have inspired musicians, artists, and writers: how church acoustics had to be adapted when the liturgy moved from Latin to English, how writers portray subtle acoustic effects such as the indoor feeling at Stonehenge, and how sculptors have ingeniously made sonic crystals that reframe ambient noise.
As a scientist, I want to pick apart what is going on. I went on holiday to Iceland many years ago and marveled at bubbling mud pools, but now I wonder, what caused them to gloop? I have seen Internet videos of a giant Richard Serra sculpture in which a hand clap resounds like a rifle shot. What is happening there? Why does throwing a rock onto a frozen reservoir produce the most astonishing high-pitched twangs? Some of these questions have no readily available answers, but in searching for explanations, I hope to gain insight into how our hearing works, both in these special places and in our everyday lives.
What makes a sound so extraordinary as to be considered one of the sonic wonders of the world? In my quest for these aural gems, I will rely in part on my gut reaction as a trained acoustic engineer: What might be surprising or weird enough to make the experts stop and wonder? One example might be the sonic behavior in an old water cistern in Fort Worden, Washington, which one audio engineer described as “the most acoustically disorienting place I have ever visited.”10 Or perhaps it will be something that takes us back in time to the experiences of our ancestors. Were the Mayan pyramids in Mexico deliberately designed to chirp? Was this sound used as part of their ceremonies? Sonic wonders might also be very rare acoustic effects: Only a few sand dunes sing, droning like a propeller aircraft—a phenomenon that astonished both Charles Darwin and Marco Polo.
Travel guides will be useless. Like most of our texts, they privilege the visual, describing beautiful vistas and iconic architecture while ignoring sounds and unusual acoustics. I was delighted to find the whispering gallery in St. Paul’s Cathedral featured in my London guidebook, but this is a rare exception. The whispering gallery appeals to me as a physicist because the movement of sound around the dome fools listeners into hearing mocking voices emerging from the walls.
Music will play an important role in the search, not least because it can provoke strong emotions. Listen to one of Mahler’s grand symphonies in an auditorium like the Great Hall of the Viennese Music Association (Wiener Musikverein) in Vienna, and you might feel shivers down your spine. Music is a powerful research tool, used by psychologists and neuroscientists to tweak the emotions of humans to reveal the workings of the brain. Research into music has taught us a great deal about listening—why some things sound nasty or nice, and how evolution has shaped our hearing. Often the best scientific understanding of sound and how we perceive it comes from research into music. But music and speech engage us on a different level. Indeed, recognizable patterns in music and speech can distract us from acoustics and natural sounds. This book will thus go beyond music and oratory to discover sounds that are overlooked or neglected.
Inevitably, I will have to use words and analogies from the visual world to describe sonic phenomena; we have relied on the visual for too long for our language to have developed otherwise. A newspaper interview with the artist David Hockney once said something about seeing that has stuck with me:
We don’t just see with our eyes, [Hockney] argues, we use our minds and emotions as well. That is the difference between the image which the camera makes—a split-second record from a fixed viewpoint—and the experience of actually looking, of passing through a landscape, constantly scanning and switching our focus. That is the difference between the passive spectator and the active participant he wants us to become. The latter sees not just geometrically, but psychologically as well.11
I want to explore what would happen if we transpose these ideas from the visual to the aural. To see what fascinating sounds reveal themselves, and discover what effect they have on us. This book is about the psychology and neuroscience of hearing as observed and explored by a physicist and acoustic engineer. And no other place embodies the combination of these disciplines better than a concert hall. Strangely, we know more about the human response to classical music in an auditorium than about many more common sounds. Why not, then, start with the most important quality of a concert hall: reverberation?
The Most Reverberant Place in the World
G
uinness recognizes a few world-record sounds: the loudest purr of a domestic cat (67.7 decibels, in case you wondered), the loudest burp by a male (109.9 decibels), the loudest clap measured (113 decibels)—all very impressive. But as a scholar in architectural acoustics, I am more intrigued by the claim that the chapel of the Hamilton Mausoleum in Scotland has the longest echo of any building. According to the 1970 Guinness Book of Records, when the solid-bronze doors were slammed shut, it took 15 seconds for the sound to die away to silence.
Guinness describes this phenomenon as the “longest echo,” but this is not the right term. Experts in architectural acoustics, like me, use the term echo to describe cases in which there is a clearly distinguishable repetition of a sound, as might happen if you yodeled in the mountains. Acousticians use reverberation when there is a smooth dying away of sound.
Reverberation is the sound you might hear bouncing around a room after a word or musical note has stopped. Musicians and studio engineers talk about rooms as being live or dead. A live room is like your bathroom: it reflects your voice back to you and makes you want to sing. A dead room is like a plush hotel room: your voice gets absorbed by the soft furnishings, curtains, and carpet, which dampen the sound. Whether a room sounds echoey or hushed is largely a perception of reverberation. A little bit of reverberation causes a sound to linger—a bloom that subtly reinforces words and notes. In very lively places, such as a cathedral, the reverberation seems to take on a life of its own, lasting long enough to be appreciated in detail. Reverberation enhances music and plays a crucial role in enriching the sound of an orchestra in a grand concert hall. In moderation, it can amplify the voice
and make it easier for people to talk to each other across a room. Evidence suggests that the size of a room, sensed through reverberation and other audio cues, affects our emotional response to neutral and nice sounds. We tend to perceive small rooms as being calmer, safer, and more pleasant than large spaces.1
I got my chance to explore the record-holding mausoleum at an acoustics conference in Glasgow that included in its program a trip to the chapel. Early on Sunday morning, I joined twenty other acoustic experts outside the gates of the mausoleum. Constructed from interlocking blocks of sandstone, it is a grand, Roman-style building rising 37 meters (40 yards) into the air and flanked by two huge stone lions. An uncharitable observer might try to infer something about the tenth Duke of Hamilton’s manhood from the building’s shape, which is a stumpy, dome-topped cylinder. It was built in the mid-nineteenth century, but all the remains have long since been removed. The building sank 6 meters (20 feet) because of subsidence caused by mines, which left the crypt vulnerable to flooding from the River Clyde.
The eight-sided chapel is on the first floor and is dimly lit by sunlight shining through the glass cupola. The chapel has four alcoves and a black, brown, and white mosaic marble floor. The original bronze doors that start the world-record echo (modeled on the Ghiberti doors at the Baptistry of St. John in Florence, Italy) are propped up in two of the alcoves. Opposite the new wooden doors is a plinth, built of solid black marble, that once supported an old alabaster sarcophagus of an Egyptian queen within which the embalmed duke was laid to rest. The sarcophagus was actually a bit small for the duke, and our guide delighted in relaying gruesome stories about how the body was shortened to get it to fit. On the day I was there, the plinth was covered in laptops, audio amplifiers, and other paraphernalia for acoustic measurements.
The chapel was meant to be used for religious services, but the acoustic made worship impossible. It was like a large Gothic cathedral, and I found it difficult to talk to my acoustics colleagues unless they were close to me, since the sound bouncing around the chapel made speech muddy and indistinct. But was this the most reverberant place in the world? The record is important to me as an acoustic engineer because the study of reverberation marked the beginning of modern scientific methods being applied to architectural sound.
The scientific discipline of architectural acoustics began in the late nineteenth century with the work of Wallace Clement Sabine, a brilliant physicist who, according to the Encyclopaedia Britannica, “never bothered to get his doctorate; his papers were modest in number but exceptional in content.”2 Sabine was a young professor at Harvard University when, in 1895, he was asked to sort out the terrible acoustics of a lecture hall of the Fogg Museum, which (in his own words) “had been found impractical and abandoned as unusable.”3 The hall was a vast, semicircular room with a domed ceiling. Speech in the room was largely unintelligible—a muddy soup of sound more characteristic of the Hamilton Mausoleum than of a well-designed lecture hall. The most forthright critic of the space was Charles Eliot Norton, a senior lecturer in fine arts.
Imagine Norton standing at the front of this vast hall trying to expound on the arts—formally dressed and sporting a large mustache, sideburns, and receding hair. His students would first get the sound traveling directly from the professor to their ears—the sound that goes in a straight line by the shortest route. This direct sound would then be closely followed by reflections—the sound bouncing off the walls, domed ceiling, desks, and other hard surfaces in the room.
These reflections dictate the architectural acoustics—that is, how people perceive the sound in a room. Engineers manipulate acoustics by changing the size, shape, and layout of a room. This is why acousticians like me have an uncontrollable desire to clap our hands and hear the pattern of reflections. (My wife was appalled when I clapped my hands in a crypt of a French cathedral. This must go down as one of the more esoteric ways of embarrassing your spouse.) After clapping my hands, I listen for how long it takes the reflections to become inaudible. If sound takes a long time to die away—if it reverberates for too long—then speech will be unintelligible as consecutive words intermingle and become indecipherable. As Henry Matthews wrote in a nineteenth-century text on sound, reverberation “does not politely wait until the speaker is done; but the moment he begins and before he has finished a word, she mocks him as with ten thousand tongues.”4 This is what happened whenever Norton tried to lecture. Students might quip that most lectures are incomprehensible even before the speech is mangled by the room, but Norton was a good communicator and a popular teacher. In this instance it was indeed the fault of the room and not the performer.
Large spaces with hard surfaces, such as cathedrals, the Hamilton Mausoleum, or the cavernous lecture hall at the Fogg Museum, have reflections that persist and are audible for a long time. Soft furnishings absorb sound, reducing reflections and speeding the decay of sound to silence. Wallace Sabine’s experiments involved playing around with the amount of soft, absorbing material in the lecture hall—a method that makes him seem like an overenthusiastic fan of scatter cushions. Sabine took 550 one-meter-long (about 1 yard) seat cushions from a nearby theater and gradually brought them into the lecture hall in the Fogg Museum to observe what would happen. He needed quiet, so he worked overnight after the students had gone home and the streetcars had stopped running, timing how long it took sound to die away to nothing. He did not use clapping, maybe because it is difficult to clap consistently unless you are a professional flamenco musician, but instead used a note created by an organ pipe.
Sabine called the time it took for the sound to wither to silence the reverberation time, and his work established one of the most important formulations in acoustics. The equation shows how the reverberation time is determined by the size of the room, which is measured by its physical volume, and the amount of acoustic absorbent like the seat cushions from Sabine’s experiments or the wall covering of one-inch-thick felt that he ultimately used to treat the acoustics of the lecture hall. One of the crucial decisions engineers make when designing a good-sounding room—whether a grand auditorium, a courtroom, or an open-plan office—is how long the reverberation time should be. Then they can use Sabine’s equation to work out how much soft, absorbing stuff is needed.5
Alongside reverberation time, a designer has to consider frequency, which directly relates to perceived pitch. When a violinist bows her instrument, the string behaves like a tiny jump rope, whipping around in circles. If she plays the note that musicians call middle C, the jump rope turns a full circle 262 times every second. The vibration of the violin radiates 262 sound waves into the air every second, which is a frequency of 262 hertz (often abbreviated to Hz). The unit was named after Heinrich Hertz, the nineteenth-century German physicist who was the first to broadcast and receive radio waves. The lowest frequency a human can hear is typically around 20 hertz, and for a young adult the highest is about 20,000 hertz. However, the most important frequencies are not at the extremes of hearing. A grand piano has notes from only about 30 to 4,000 hertz. Outside that range we cannot easily discern pitch, and all notes start to sound the same. Beyond 4,000 hertz, melodies are turned into the mindless whistling of someone who is tone-deaf. The middle frequencies where musical notes reside are also where our ears are most efficient at amplifying and hearing sounds. Most speech falls into this range as well, which is why in rooms where music will be played, acoustic engineers concentrate on designing for a frequency range of 100 to 5,000 hertz.
In 2005, Brian Katz and Ewart Wetherill used computer models to explore the effectiveness of Sabine’s treatment in the Fogg Museum. They programmed the size and shape of the lecture hall into a computer and employed equations that describe how sound moves around a room and reflects from surfaces and objects. They then added virtual materials to the walls and ceiling of their simulated lecture hall to mimic Sabine’s felt treatments. Although the absorbers improved the acoustics, the intelligibility of speech was still poor in places. As one
student reported, there were seats where hearing was easy, and conversely, “there were dead spots where hearing was often extremely difficult.”6 Though the treatments were imperfect, Sabine’s experiments opened the door for a wide variety of acoustic exploration. His equations remain the foundation of architectural acoustics to this day.
I love walking into a concert hall and hearing the contrast between the small entrance corridor and the huge expansive space of the auditorium. From the claustrophobic passageway, one enters a palpably vast room, passively perceiving the quiet chatter of anticipation among the audience and the occasional loud sound stirring the mighty reverberation. Entering Symphony Hall in Boston is particularly exciting for me. Symphony Hall is Mecca for many acousticians, for it was in this very hall that Wallace Sabine applied his newfound science to create an auditorium still considered to be one of the top three places to hear classical music in the world. Completed in 1900, it has a shoe box shape—long, tall, and narrow—with sixteen replica Greek and Roman statues set into the walls above the balconies. On my visit I settled into one of the creaky black leather seats while the Boston Symphony Orchestra was tuning up on the raised stage in front of the gilded organ. As the first piece began, I could immediately understand why audiences and critics wax lyrical about the place. The hall beautifully embellishes the music, having a reverberation time of about 1.9 seconds.7 When the orchestra stopped playing at the end of a moderately loud phrase, it took nearly 2 seconds for the sound to become inaudible.