The Sound Book: The Science of the Sonic Wonders of the World (27 page)

I immediately confirmed that the hum was the same sound I had heard in my garden, which meant that it was carrying at least 4 kilometers (2½ miles) across the city. Ironically, it was difficult to get a good recording because it was so windy. Gusting air was creating turbulence across my microphone; the same physics causing the building to sing was ruining my recording. I put a foam windshield over the top of the microphone to alleviate this problem, but in such high winds it was almost useless.

The hum came and went as the wind gusted. It was an eerie, long note from a bass musical instrument, a distinct tonal sound at 240 hertz (roughly B below middle C). Because it was distinctive, the note was easy to pick out among the traffic noise, and this is probably why residents found it so disturbing. Our hearing finds it hard to ignore such tones—sounds one can sing along with—because they might contain useful information. After all, the vowels in speech (
a
,
e
,
i
,
o
,
u
) are often pronounced in a singsong way with distinct frequencies. Knowing that tones stand out to us also explains the simple short-term solution used by the television sound recordists for
Coronation Street
. By the addition of very quiet broadband noise to the sound track—the rumble of a distant busy road would have been a good choice—the hum was hidden by a noise less likely to grab a listener's attention.

But just as the burping on the sand dunes is only the starting point of the sound, the noise created as wind whistles past the edges of the glass in Beetham Tower is only the initial impulse. Both the sand and the wind noise require amplification. For the dunes, what causes the amplification is still debated. One theory focuses on a layer of dry, loose sand, roughly 1½ meters (5 feet) deep, that sits on top of harder-packed material lower down.

Nathalie Vriend explained to me that the layer hypothesis came from her doctoral supervisor Melany Hunt, of the California Institute of Technology. To test Hunt's theory, Nathalie has carried out field measurements on various dunes in the southwestern US. To reveal the underlying structure, she turned to geophysics, using ground-penetrating radar and seismic surveys. She also described using a probe 1 centimeter (about half an inch) in diameter to take samples from the dune. Getting the probe to go through the top layer of loose sand was straightforward, but about 1.5 meters (5 feet) down it hit a layer as hard as concrete: “We had our biggest, most muscular guy hitting that probe with the hammer and he couldn't get it in any further.”
⁵⁴
A sample from the top of this very hard layer showed moist sand grains welded together with calcium carbonate, forming a largely impenetrable barrier to sound.

The top layer of loose sand acts as a waveguide for sound, similar to the way an optical fiber channels light. The avalanching sand produces a range of frequencies. The waveguide then picks out and amplifies a particular note. Similarly, the wind whistling past the louver of Beetham Tower creates sounds at many frequencies. The resonance between the glass panes then selectively amplifies particular notes, creating the audible hum.

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.

W
ind 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.

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.