The Sound Book: The Science of the Sonic Wonders of the World

by Trevor Cox

  Others have disputed whether a layered dune is needed, however. Simon Dagois-Bohy and colleagues re-created the booming sound in the laboratory by tipping a small sample of sand down a slope made from a thick, heavy, chipboard work surface lined with fabric. According to their theory, the sand falls in a synchronized avalanche, with grains bumping over each other at a regular rate, turning the top of the dune into a loudspeaker and producing a distinct note. But why the grains should synchronize is not known. If the theory is right, then the waveguide measured by Nathalie Vriend might just embellish the sound rather than being the underlying cause. Or maybe the waveguide aids the synchronization of the grains.

  Wind plays an important role in sifting the grains of musical dunes. The mustard-colored sands at Kelso rise incongruously out of the surrounding landscape of desolate scrub and distant granite mountains. The prevailing westerly winds pick up sand from the Mojave River sink at the mouth of Afton Canyon and deposit their cargo at Kelso. Eddies form that drop sand onto the 180-meter-high (600-foot) dunes. Sand is made up mostly of grains—most commonly pieces of quartz, with smaller particles called fines. The unusual flow of winds sifts the sand so that the grains all have a similar diameter on the leeward size of the dune and there are very few fines.

  The burping happens because the grains of sand are rounded and are all very similar in diameter. A grain varnish seems to be an important part of the sound production. French physicist Stéphane Douady found that his laboratory samples of sand could lose their voice. He then discovered that rinsing and drying the sand at high temperature with salt got them speaking again. The process added a varnish of silica-iron oxides to the sand, changing the friction between neighboring grains.

  Diane Hope and I set off from the Kelso campsite at sunrise on day two, so that we could climb the dune when it was cooler and there was no wind. It was the summer solstice, and as we packed away the tents, a spectacular V-shaped sunbeam lit the sky through the tops of the nearby mountains.

  When I checked Nathalie Vriend’s scientific paper about Dumont Dunes in California, I noticed that her measurements of the boom were done over much longer slopes than I had been sliding down the day before. The paper also indicated that a steeper angle, about 30 degrees, was needed. Climbing up the dune, Diane and I scanned the hill for the longest, palest sand free of vegetation. On day one we had already learned that the sand with the gray tinge would not burp; it was easier to walk on, and the sand did not flow easily. Nearly all dunes that sing tend to do so on the leeward face, so we aimed for a ridge that was not at the top of the dune but had a long, steep slope more perpendicular to the prevailing wind than the places we had tried the day before.

  With trepidation, I did a trial slide. It immediately felt different from the previous day’s slope. I could feel the ground vibrating under my bottom. For a fleeting moment the sand broke into song. We had found the dune’s audio sweet spot; all I needed to do now was perfect my sliding abilities. As you slide down, the sand bunches up around you. You need to avoid digging in too deep and coming to a halt, while still getting enough sand moving to create the boom.

  Many writers assign a musical quality to the sound because it has a distinct frequency (88 hertz, from one of our measurements—equivalent to a low note on a cello) colored by a few harmonics. It reminded me of the drone of a taxiing propeller aircraft at an airport. The Marquess Curzon of Kedleston wrote, “First there is a faintly murmurous or wailing or moaning sound, compared sometimes to the strain of an Aeolian harp . . . Then as the vibration increases and the sound swells, we have the comparison sometimes to an organ, sometimes to the deep clangor of a bell . . . Finally, we have the rumble of distant thunder when the soil is in violent oscillation.”55 What this description misses is the whole-body experience that accompanied my triumphant slides. The drone moved my eardrums, the avalanche was vibrating my lower body, and the rest of me was quivering with excitement because I had gotten the dune to sing.

  The Quietest Places in the World

  W

  hile on my expedition to record singing sand dunes, I experienced something quite rare: complete silence. The scorching summer heat kept visitors away; most of the time my recording companion, Diane Hope, and I were on our own. We camped at the foot of Kelso Dunes, in a barren, scrubby valley with dramatic granite hills behind us. Virtually no planes flew overhead, and only very occasionally did a distant car or freight train create noise. The conditions were wonderful for recording. No noise meant there was no need for second takes. Much of the day, however, there was a great deal of wind, which often whistled past my ears. But at twilight and early in the morning the winds calmed down, and the quiet revealed itself. Overnight I heard the silence being interrupted only once, when a pack of nearby coyotes howled like ghostly babies, unnerving me with their near-musical whistling and chattering.

  High up on the dune, early on the second morning, I was waiting for Diane to set up some recording equipment. Since she was some distance away, I had a chance to contemplate real silence. The ear is exquisitely sensitive. When perceiving the quietest murmur, the tiny bones of the middle ear, which transmit sound from the eardrum to the inner ear, vibrate by less than the diameter of a hydrogen atom.1 Even in silence, tiny vibrations of molecules move different parts of the auditory apparatus. These constant movements have nothing to do with sound; they stem from random molecular motion. If the human ear were any more sensitive, it would not hear more sounds from outside; instead, it would just hear the hiss generated by the thermal agitation of the eardrum, the stapes bone of the middle ear, and the hair cells in the cochlea.

  On the dunes, I could hear a high-pitched sound. It was barely audible, but I worried that I might be experiencing tinnitus—that is, ringing in the ears, perhaps evidence of hearing damage caused by my excessively loud saxophone playing. Medics define tinnitus as perceiving sound when there is no external source. For 5–15 percent of the population tinnitus is constant, and for 1–3 percent of people it leads to sleepless nights, impaired performance at tasks, and distress.2

  Theories of tinnitus abound, but most experts agree that it is caused by some sort of neural reorganization triggered by diminished input from outside sounds. Hair cells within the inner ear turn vibrations into electrical signals, which then travel up the auditory nerve into the brain. But this is not a one-way street; electrical pulses flow in both directions, with the brain sending signals back down to change how the inner ear responds. In a silent place, or when hearing is damaged, auditory neurons in the brain stem increase the amplification of the signals from the auditory nerve to compensate for the lack of external sound. As an unwanted side effect, spontaneous activity in the auditory nerve fibers increases, leading to neural noise, which is perceived as a whistle, hiss, or hum.3 Maybe what I was hearing on the dunes was the idling noise of my brain while it searched in vain for sounds. One thing I noticed was that this high-frequency whistle was not always there—maybe a sign that, after a while, my brain habituated to the noise.

  Figure 7.1 The anechoic chamber at the University of Salford.

  In contrast to the variable silence on the dunes, at my university there is an anechoic chamber, a room that provides unchanging, guaranteed silence, uninterrupted by wind, animals, or human noise (Figure 7.1). The anechoic chamber never fails to impress visitors, even though the entrance is utilitarian and uninspiring. Just outside the entrance they see dusty metal walkways, and nearby, builders are often making lots of noise constructing test walls in a neighboring laboratory. These walls will be analyzed for how well they keep sound from passing through them. Guarding the anechoic chamber are heavy, gray, metal doors. In fact, you have to go through three doors to reach the chamber, because it is a room within a room. To make the place silent, several sets of heavy walls insulate the innermost room, stopping outside noise from entering. Like a modern concert hall, the chamber is mounted on springs to prevent unwanted vibration from getting into the inner sanctum.

 
The chamber is the size of a palatial office. First-time visitors are usually circumspect, not least because the wire floor is like a taut trampoline. Once inside, with the doors closed, they notice vast wedges of gray foam covering every surface, including the floor beneath the wire trampoline. When showing visitors around, I like to say nothing at this point because it is fun to watch the realization sweep across their faces as they adjust to this unbelievably quiet space.

  But it is not silent. Your body makes internal noises that the room cannot dampen. Sound recordist Chris Watson described his experience in such a chamber: “There was a hissing in my ears and a low pulsing that I can only guess was the sound of my blood circulating.”4 The internal noises are not the only oddity. The foam wedges on the floor, ceiling, and walls absorb all speech; there are no acoustic reflections. We are used to hearing sound bouncing off surfaces—floor, walls, and ceiling—which is why a bathroom is lively and reverberant, and a bedroom muffled and subdued. In the anechoic chamber, speech sounds very muffled, like when your ears need to pop in an airplane.

  According to the Guinness Book of Records, the anechoic chamber at Orfield Laboratories in Minneapolis is the quietest place in the world, with a background noise reading of –9.4 decibels.5 But how quiet is that? If you chatted with someone, your speech would measure around 60 decibels on a sound-level meter. If you stood quietly on your own in a modern concert hall, the meter would drop down to a level of about 15 decibels. The threshold of hearing, the quietest sound a young adult can hear, is about 0 decibels. The test room at Orfield Laboratories, like the chamber at Salford University, is quieter than that.

  An anechoic chamber has an impressive silence because it simultaneously presents two unusual sensations: not only is there no external sound, but the room puts your senses out of kilter. Through their eyes, visitors obviously see a room, but they hear nothing that indicates a room. Add the claustrophobic drama of being enclosed behind three heavy doors, and some begin to feel uneasy and ask to leave. Others are struck with fascination at the oddness of the experience. I know of no other architectural acoustic space that regularly has such a strong effect on people. But it is remarkable how quickly the brain gets used to the silence and the contradictory messages from the senses. The exotic sensory experience is filed in memory, and the extraordinary becomes more normal. The magical impact of the first visit to an anechoic chamber is never really experienced again. Not only are anechoic chambers very rare, but our brains ensure that the experience is mostly ephemeral.

  However, there is more to silence than experiencing the quietest rooms on Earth. Silence can be spiritual; it can even have an aesthetic and artistic quality, as epitomized in John Cage’s famous silent composition 4′33″. When one of my teenage sons learned I was going to see this piece performed, he expressed shock that I would spend money to hear nothing. Cage composed the piece in 1952 after a visit to the anechoic chamber at Harvard University. There, surround by thousands of fiberglass wedges, he had expected to find silence. But it was not entirely quiet, because of the noises within his own body. He also described hearing a high-frequency sound that might very well have been caused by tinnitus.

  The performance of 4′33″ that I heard took place nine months before my trip into the desert. It was carried out with all the usual pomp and ceremony of a normal concert. The house lights were dimmed, and the musician strode on stage and bowed to the applause from the audience. He then sat down at the piano, adjusted the seat to make it just the right height, turned the page on the score, opened the keyboard lid of the piano, closed it again, and started a timer. Nothing else happened, apart from the occasional turning over of the empty sheet music, and the opening and closing of the keyboard lid to signify the end and beginning of the three movements. At the end, the pianist opened the keyboard lid for one last time, stood up to accept the applause of the audience, bowed, and left. Amusingly, the work comes in different orchestrations, and I guess the full orchestral version is very popular with the Musicians’ Union, maximizing the number of people being paid to play no notes.

  The first surprise occurred before the pianist entered the stage. As the doors of the auditorium were closed and the house lights dimmed, I felt a sudden frisson of excitement, even greater than I get before a normal concert. A modern concert hall is one of the quietest places to be found in a city. At the Bridgewater Hall in Manchester, England, tour guides like to recount the story that when the largest peacetime bomb ever detonated in Great Britain exploded in 1996, workers within the auditorium did not hear the bang, because the hall was so well isolated from the outside world. Planted by the Irish Republican Army (IRA) in the city center, the bomb destroyed shops, broke virtually every window within a kilometer (half-mile) radius, and left a 5-meter-wide (16-foot) crater.

  It is worth taking the backstage tour of a modern concert hall to see the precision needed to achieve the noise isolation. The tour guides are usually very proud of the fact that the auditorium is built on springs. Like a souped-up car suspension, the springs stop vibration from entering the concert hall. If ground vibration were to set parts of the auditorium moving, the tiny vibrations of the hall would set air molecules into motion, creating audible noise. Everything connected to the hall that might transmit vibration—electricity cables, pipes, and ventilation ducts—have to be carefully designed with their own little suspension systems. The attention to detail is staggering.

  In recent decades, classical concert halls have been built to be quieter and quieter, giving conductors and musicians access to the widest possible dynamic range to exploit and create drama. In a good modern hall, the collective noise from audience members breathing and shuffling in their seats is actually louder than any background sounds from outside noises or ventilation systems.6

  What the audience hears during a performance of 4′33″ depends on the isolation of the auditorium and the quietness of the audience. The hall I was in did not have the best sound insulation, and I could occasionally hear buses on the busy road outside. The audience was small, about fifty people, whom I could hear fidgeting and coughing. With these distractions, as the piece progressed I found my mind wandering. But were these really distractions, or the actual music? Although there was a musician on the stage, what Cage’s piece does is shift the focus from the performers to the audience. And that change from being a passive member of the audience to being part of the performance was at the heart of my second surprise. When the piece was over, I felt a strong sense of a communal achievement with everyone else in the audience and the performer. As the audience clapped and a few shouted “More!” and “Encore!” I had an overwhelming sense of a shared experience. We had all just done something that was completely pointless—or was it?

  Moments of silence are commonly used in the arts—famously so in theater by playwrights Harold Pinter and Samuel Beckett. For Pinter, silence forces the audience to contemplate what the character is thinking. For Beckett, silences might symbolize the meaningless and eternity of existence.7 Short silences are also used regularly in music. A jazz group in full flow may stop abruptly for a moment, before resuming exactly together a couple of beats later and carrying on as though the pause had never happened. The silence adds dramatic tension by subverting expectation in a way that the brain finds pleasurable.

  Imagine a musician walking up to a piano and repeating a snippet of a favorite melody over and over again. The predictability would soon become tedious. Similarly, one would derive little pleasure from the more random approach of letting a cat run about on the keys playing notes haphazardly. Successful music is neither completely repetitive nor entirely random. It lies somewhere between, having some regular rhythmic and melodic structure, but with changes to maintain the listener’s interest.

  One task the brain does when listening to music is try to break down the rhythmic structure, the beat or groove. The seemingly simple task of finding and tapping along to a beat involves several brain regions and is not fully understood. The basal
ganglia buried deep inside the cerebrum seem to play a role, as do the prefrontal cortex at the front of the brain and other areas used for processing sound.8 The basal ganglia play a vital role in initiating and regulating motor commands; when they are damaged in Parkinson’s disease, patients have difficulty starting movements.

  As the brain decodes the information bombarding it during a tune, it is constantly attempting to predict when the next strong beat will occur. It draws on past experiences of similar music, and recent notes from the piece, to work out where the rhythm is going. Correctly anticipating the next strong beat is satisfying, but there is also a delight in hearing skillful musicians violate that regular tempo, subverting the listener’s expectation. One way of flouting expectation is to add unexpected silences, even very brief ones. The brain seems to find pleasure in adjusting itself to remain synchronized with the musical beat.9

  A sudden pause in music also transfers the responsibility for the beat to the audience, because for a moment they have to carry the tempo until the musicians resume playing. Like John Cage’s work, the pause takes the focus of the music making away from the stage. The second piece in the concert that featured 4′33″ was a more conventional piano sonata by Charles Ives that required no audience participation. As the pianist raced his fingers up and down the keyboard, he seemed to be trying to make up for the lack of notes in Cage’s work. The piece left me entirely cold, and I kept wishing I could hear silence again.

  Sound mixers generally avoid complete silence in film soundtracks, with one famous exception. In 2001: A Space Odyssey, Stanley Kubrick boldly used lots of quiet. If a film director attempted this nowadays, it would be the film equivalent of 4′33″, and all you would hear would be endless crunching and slurping of junk food and soda by fellow cinemagoers. Often when the audience thinks there is silence, there are actually quite a few audio tracks of “nothing” playing. Charles Deenen, head of audio for Electronic Arts, described to me how he became obsessed with silent rooms when developing a video game soundtrack. Turning up the volume on recordings that he had made in empty rooms revealed “amazing creepy tones” and “amazing squeaking things happening.”10 Charles also described how he might take a sound, like a camel moan, and digitally manipulate it, shifting it down many octaves and listening for distinctive tones or ringing that might appear and create the right creepiness. Game players or a film audience might not be consciously aware of these background sounds, but they are an important part of setting the emotional feel of a scene.