Spectrums

SOUND

These go to eleven.

Nigel Tufnel, This Is Spinal Tap

Sound is movement. Specifically, sound is the movement of molecules in a medium. And our sense of hearing is actually an extension of feeling—as though our ears were fingers that could reach out and stroke the ripples that run through the air around us. Hearing sound, like perceiving light, is another one of our evolutionarily clever ways of stretching our awareness beyond our boundaries, whether in search of food, concern for danger, or desire for connection.

“I don’t care much about music. What like is sounds.”

—Dizzy Gillespie, jazz musician

If you stand next to a speaker playing loud music, you can often feel the sounds against your skin—rumbling low bass notes and buzzing high-pitched tones. You’re feeling waves of air pressure, molecules moving and bouncing into each other. Our ears are so sensitive to these changes in pressure that we can detect when air molecules move one-tenth their diameter—that’s about one-millionth the size of the smallest dust speck you can see.

The molecules move because of energy. Somewhere, somehow, something releases energy—a string is plucked, a dog barks, a reed flutters, or what have you. This is physical, kinetic energy; it moves, and the movement flows outward in waves. One molecule pushes into another, but the molecules can’t move far until they affect another molecule, like people crowding to get out of a theater after the show.

So as one molecule pushes the next, each moves only a little, but the energy continues. It propagates. It travels. It transmits, like a message being passed from one person to another. Each passing wears down the energy a little, but with enough thrust, enough power, it can travel a long way—perhaps even all the way to your ear.

The Medium Is the Message Of course, if there is no medium, there is no sound. The great Greek thinker Aristotle, student of Plato and teacher of Alexander the Great, first pointed out that we hear sound through the medium of air. Want the ultimate in soundproofing for your office or apartment? Encase yourself within a perfect vacuum of space, for where there is nothing (literally no-thing, no molecules to push around), there is no sound.

The statement in the movie Alien that “in space, no one can hear you scream” is true; those movie scenes in which inhabitants of one spaceship can hear a sound in another nearby spaceship are impossible. Without some medium—some solid, gas, or liquid to transmit the energy waves from one location to another—there is no message.

This is distinctly different from electromagnetic energy. Electromagnetic waves—whether X-rays, radio waves, or even the small sliver of the spectrum called visible light—don’t require molecules and can voyage through space indefinitely, easily traveling the 24 trillion miles (40 trillion km) of nothingness between us and our nearest star, Proxima Centauri. But a sound, no matter how great, peters out at the edge of the atmosphere.

Sound isn’t limited to earth’s atmosphere, of course. We are awash in the radiation of our sun, but we rarely consider the extraordinary noise that this ball of gas is making. You think a crackling fire is loud on a rainy night? The turbulence of gas and plasma churning, burning, exploding, at 16,000,000°C is staggering, sending incomprehensible shockwaves outward in all directions through the sun’s mixture of hydrogen, helium, oxygen, and gaseous metals. The sound energy pounds out toward the surface of the burning ball, and then, as it reaches the boundary of the sun’s atmosphere, the gas eventually stops, and with it the sound. Between our beneficent star and earth is silence.

Of course, if you were actually close enough to hear the tolling of the sun, you would be vaporized. But just as you can “see” sound by watching a rattling window, scientists can see the sounds of the sun from 93 million miles away using precise Doppler measurements taken by the Solar and Heliospheric Observatory (SOHO) satellite. The surface of the sun vibrates with a complex set of resonances. Although the vibrations are too low for the human ear to hear, they can be sped up—compressing forty days worth of sound into a few seconds. The result is like an eerie bell, or a Buddhist bowl gong slowly, endlessly ringing into space.

The Speed of Sound Because sound relies on molecules bouncing into one another, passing along their energy wave, it takes time for it to travel from one place to another. Sound moves fast, but not nearly as quickly as light—in fact, not even as fast as a bullet from a high-powered rifle.

Sound travels through water at different speeds, depending on temperature. So scientists can use hydrophones (underwater microphones) to determine water temperature by measuring the speed of sound in a particular location.

It’s relatively easy to measure the speed of sound: Place two people a mile apart, giving one a sports starter pistol with blanks and the other person binoculars and a stopwatch. When the pistol is fired (watch for smoke), start the stopwatch, then stop it when you hear the sound. Intuition says we probably couldn’t start and stop it quickly enough, but you’ll actually count almost five seconds before hearing the blast.

This exercise explains a common game during thunderstorms: Start counting seconds when you see a flash of lightning and stop when you hear the thunder. Divide the total number of seconds by 5 to find out how many miles away the lightning struck. (It’s not 1 mile per second, as some people mistakenly believe.) Or divide by 3 to find the number of kilometers.

With careful measurements, you’ll find that sound travels through air about 1,125 feet (343 m) in a second. That works out to about 67,500 feet (20,580 m) per minute, or 767 miles per hour (1,234 km/h).

Notice the word about. The speed of sound can change significantly depending on factors such as temperature. On a freezing cold day, it drops to 330 m/sec. On a hot day it can increase to 350 m/sec. The difference is due to the rate the molecules are moving through the gas we call air. As the sun warms the air, the molecules move faster, bounce into each other more often, and allow signals (such as sound wave energy) to transmit faster.

We need to be clear here: The molecules are not traveling very far. When a book falls off a table across the room, the affected molecules don’t travel from the book to your ear. That would be wind, not sound. But like a series of billiard balls ricocheting across a pool table, the energy within the sound finds its way to you.

The makeup of the air also affects the speed of sound. For example, the speed of sound is faster in helium gas, which contains far lighter molecules than air, leading to the time-honored party game of talking with a lungful of helium. The pitch of your voice actually stays the same, but it moves through the gas faster, causing it to sound sped-up, like Donald Duck. If you’re lucky enough to have a supply of heavier-than-air xenon gas handy, you can reverse the effect and sound like a “slow-talking cowboy” through the magic of slowing the speed of sound.

Sound can travel through liquids and solids, too, and at a very different speed than gas. The exact speed changes radically, based partly on the elasticity and density of the material. In freshwater, where the molecules are packed together in a slightly sticky and viscous solution, sound waves travel more than four times faster than in air: 1,482 meters per second (about 3,315 mph). In seawater, the speed of sound increases by a few percent, depending on temperature, depth, and salinity.

In a solid, where molecules are bound even more securely and one can barely move without affecting its neighbor, energy can pass even faster. A soft, relatively elastic material such as lead transmits sound waves at just over 2,000 m/sec, but in steel the speed of sound is about 5,960 m/s (that’s 21,450 km/h or 13,300 mph). That’s why you can hear a train approach by pressing your ear to the tracks; you’ll hear it 17 times faster through metal than through the air. If the tracks were made of an incredibly hard substance, such as beryllium or diamond, you’d hear it almost 40 times faster than through air.

“Wherever we are, what we hear is mostly noise. When we ignore it, it disturbs us. When we listen to it, we find it fascinating.”

—John Cage, composer

Curiously, sound does not tend to pass very well from one medium to another. Sound waves act like light waves in this respect. Just as light reflects and refracts when moving from one medium (like air) into another (say, water), sound waves bend and bounce at these boundaries. So a crack of a hammer against the end of a long steel bar may travel beautifully through the metal, but little of it will transfer into the air on the other side. Similarly, sound waves from the air don’t penetrate well into water, and vice versa. Anglers, take note: Feel free to chat with your buddies; the sound won’t scare the fish!

A noisy object creates sound waves that expand out in all directions, like tiny ripples in a still pond. If the object starts to move, then those ripples keep expanding, but they become elongated, falling farther behind the object than in front of it, similar to the wake behind a boat. Just as waves don’t hit the shore until long after the boat has passed, we don’t hear the rumble of a jet airliner flying far overhead until the plane is almost out of sight.

But a funny thing happens as a jet plane nears the speed of sound: The sound waves compress and bunch up like wrinkled cloth in front of the nose of the aircraft. It’s as though the molecules of air can’t get out of the way quickly enough, and the pilot begins to experience increased aerodynamic drag, like an invisible hand pushing back.

When fighter jets first encountered this during World War Two, some erroneously believed that supersonic flight—flying beyond the speed of sound—was impossible, like traveling faster than the speed of light. However, the soldiers need only to have looked at their artillery. Bullets easily pass the speed of sound, flying through the air at speeds as fast as 5,000 feet/second (1,500 m/sec).

The exploding charge that propels ammunition creates a sound, but the bang actually stems from something unexpected: The pressure waves that build up in front of a supersonic object can no longer get away from their source. The result of this compression is an intense shock wave that travels through the air, often referred to as a sonic boom. Even a gun with a muzzle silencer cannot stop the sound of the bullet as it pierces the air, generating an intensely sharp peak in air pressure, like a thin but powerful wall of sound.

A bullwhip snapped properly creates a tiny sonic boom as the end of the whip (the “cracker”) quickly flips around faster than the speed of sound.

Modern fighter jets create similar, though obviously much larger, sonic booms as they move at transonic speeds. Though you may hear a sudden rumble that hits and then passes, the shock wave actually continues, following behind the plane like a sharp-edged sonic shadow that extends until the air pressure can sufficiently dissipate.

When dealing with very fast-moving objects, it’s often helpful to discuss their speed as multiples of Mach, named in honor of the physicist and philosopher Ernst Mach (1838–1916), who studied (among many other things) sound and ballistics. One half Mach is half the speed of sound, Mach 2 is twice the speed of sound, and so on. For many years, the fastest airplane was the Lockheed SR-71 Blackbird, which broke the speed record at Mach 3 in 1964. Forty years later, NASA’s X-43A scramjet busted the doors off the record, with speeds up to Mach 9.6—nearly 7,000 mph.4

▲ Lockheed SR-71 Blackbird

▲ Heinrich Hertz

For those readers curious about the strange inter-capitalization of kHz (kilohertz): It’s the policy in the scientific community to capitalize letters in abbreviations when those terms derive from a person’s name—in this case, the nineteenth-century German physicist Heinrich Hertz, who greatly contributed to the nascent exploration of electromagnetism.

In order to escape Earth’s orbit, a rocket needs to go even faster—about Mach 23 (17,000 mph), or twice that to propel itself to the moon. Obviously, the sound generated by these engines creates a sonic boom that is very, very loud.

▲ Speed of sound in different materials (m/sec)

How Loud Is Loud? Why are some sounds louder than others? Like ocean waves, sound waves are each created with a crest and a trough, and their sizes—the difference between their peaks and the surrounding air pressure (or sea level, using that analogy)—is called the wave’s amplitude. For example, musicians know you take a small sound signal and make it huge with an amp, or amplifier—a device that increases the amplitude of the wave.

The bigger the difference in air pressure, the bigger the wave, the louder the sound.

We’re not talking about huge differences in pressure here. Pressure is often measured in pascals (Pa), and we live in a bubble of air pressurized at 101,325 Pa. Let’s say a sound wave is moving toward us. The air pressure momentarily increases and decreases by a tiny amount (remember, there is always a crest and a trough, technically called compression and rarefaction). If the pressure changes by 2 Pa (just 0.002 percent), we hear it—not as a whisper, as you might expect, but as the deafening sound of a jackhammer breaking through stone. A quiet conversation alters air pressure by as little as 0.0005 Pa (less than 5 ten-millionths of 1 percent).

Perhaps you can begin to see how sensitive our ears are. Due to a complex system of interrelated bones and membranes in our middle and inner ear, we can pick up a change of less than a billionth of the atmospheric pressure, where air molecules are moved less than the diameter of an atom.

“You can tell a good putt by the noise it makes.”

—Bobby Locke, South African golfer

Instead of talking in pascals, most people refer to loudness using a different value: the decibel (dB), measuring one-tenth of a “bel” (named in honor of the telecommunications pioneer Alexander Graham Bell). The decibel is an almost entirely humancentric measurement: Zero decibels marks not some universal constant but the lower limit of human hearing—the faintest sound we can detect. Below that, you can’t tell the difference between sound and air molecules just randomly bumping up against the eardrum.

The logarithmic nature of the decibel system means that for each additional 10 decibels, ten times more power is required, but it doubles the perceived loudness of a sound. That is, a normal conversation (about 40 dB) is about twice as loud as a quiet library (about 30 dB), but that 30 decibels reflect 1,000 times more power than near silence. A large truck driving by can throw 94 decibels—carrying almost ten million (10⁷) times the power of a whisper.

In a famous 1976 concert, The Who was measured at 126 dB, 100 feet (30 m) from the stage. More recently, the band KISS hit 136 dB—the equivalent of standing next to a jet airplane taking off—during a 2009 Canadian concert, just before being forced to “turn it down” by local law enforcement. That’s 17,000 times louder and 10 trillion times more powerful than a heartbeat. One can only hope earplugs were liberally distributed before these shows, as permanent hearing damage can be caused by sound above 120 decibels.

Of course, a rock concert is nothing compared with the new international “sport” of dB drag racing—where competitors build cars that contain virtually nothing but an engine and audio equipment. The goal is to create the loudest car, if only for a few seconds. The vehicles have two-inch-thick windows and doors bolted and clamped closed so as not to rattle off their hinges. Participants stand outside and throw a switch to create a pulse of noise so powerful it can literally melt the metal in the speakers. The world-record car, at about 180 dB, is 60 times louder and reflects a million times more power than a typical concert.

In fact, that car nearly ties the loudest sound on record, which most historians identify as the 1883 volcanic eruption at Krakatau, Indonesia. The cataclysm, in which most of the island was destroyed and ash was propelled 50 miles (80 km) high, had an intensity of just over 180 dB and was audible 3,000 miles (5,000 km) away in Mauritius. The shock wave traveled even farther, reverberating literally around the world over the next five days.

But could a sound be even louder than that? It depends on how you define sound. There is no maximum strength of a shock wave. Blow up a few hundred pounds of TNT, and you’ll create about 200 dB of pressure—a wave of such power that it would likely kill any human nearby. Nuclear explosions reach over 275 dB. However, if you limit the definition of sound to a pressure wave with a crest and a trough, a signal that conveys a message through the air beyond a distorted, deadly boom, then you’re limited to a wave no bigger than atmospheric pressure itself. That is, the rarefaction (the low point on the wave) cannot drop below zero Pa, the pressure in a vacuum. And a drop from normal atmospheric pressure of 101,325 Pa to zero Pa results in a maximum possible sound volume of 194 dB.

▲ As volume increases, sound waves eventually get clipped.

Sound power is measured in watts per square meter or W/m2. The ratio of power required to generate the faintest sound we can hear, up to the level of “Ow, that hurts!” is 1:100,000,000,000,000 (a hundred million million). This is yet another example of how incredibly dynamic and sensitive human hearing is.

The farther from a sound source, the less loud you hear it. Specifically, the sound intensity drops by about 6 dB each time you double the distance.

[* Employers in the United States must provide hearing protectors to all workers exposed to continuous noise levels of 85 dB or above.]

What’s the Frequency, Kenneth? Our ears are clearly sensitive to a sound wave’s amplitude, but they’re just as sensitive to the distance from the crest of one wave to the crest of the next—the wavelength. This measurement, along with the speed the sound is traveling, determines the amount of time it takes for one full wave to crash into our eardrums, like waiting for one ocean wave to finish before the next comes ashore.

That math-loving Greek Pythagoras, about 2,500 years ago, first observed that a string held taut and plucked vibrates at a particular rate. The vibration of the string then causes a sound. But tighten the string, use a thinner filament, or shorten it, and the pitch rises to a higher note. A string half as long produces a sound exactly one octave higher. A string twice as long is an octave lower.

A string is a helpful tool to understand waves because we can literally see it quiver. A loose, flapping wire makes no sound we can hear, but tighten it until it’s vibrating more than 20 times per second, and you perceive a moan; keep tightening, and the sound rises to a groan, then a growl, each slightly higher in pitch. The tone is based entirely on the frequency of the waves—that is, the number of vibrations each second. Lower frequencies—longer waves and fewer cycles per second—sound lower to us. Higher frequencies (shorter waves, more coming at us each second) sound high-pitched.

We see examples of vibrations all around us. A bird flapping its wings 2 or 3 times per second creates no sound we can hear. But a bumblebee wing, flapping about 200 times per second, creates a low hum. A mosquito wing, moving 600 times per second, is an annoying whine. Again, our hearing comes to our rescue, enabling us to “reach out” and find the insect invader.

Scientists replace the phrase “waves or cycles per second” with the simple word hertz (Hz). You could say a clock ticks off time at 1 hertz (once per second), though the tick or tock it makes is actually a sound wave vibrating far faster. In fact, the lowest pitch humans can hear is a wave vibrating around 15 Hz. A tone at 30 Hz sounds about the same, but an octave higher; the same can be said for 60, about the throb of a hummingbird flying by. Double that is 120, about the typical frequency of a man’s voice. Another doubling brings us to the range of a woman’s voice, though the human voice can actually span from about 80 to 1,100 Hz.

When the sound waves are compressed into the thousands-per-second range, we start measuring in kilohertz (kHz). Children can easily hear sound up to 20 kHz (20,000 wave cycles per second), though that ability tends to wear out for one reason or another until, at middle age, we tend not to be able to hear anything higher than 15 or 16 kHz. Marketers have taken advantage of the difference. A Welsh security company created the Mosquito sound repellent that emits screeches in the 17 kHz range, designed to repel teenagers from loitering in front of shops. Of course, the tables were soon turned when the high-frequency sound was converted into a cell phone ringtone—one that kids can hear but adults (such as teachers and parents) cannot.

Note that this range, from 20 waves per second to 20,000, is an extraordinary span, reaching over ten octaves (where each octave represents a doubling of frequency). Compare that with our eyes, sensitive to only a single octave of the electromagnetic spectrum between about 400 and 780 terahertz.

Plus, imagine the speed at which our ears are processing information. First, the sound waves are captured by our pinna (those are the fleshy things sticking out from the side of your head), which act as sophisticated sound-processing gear, cleverly amplifying and filtering sounds before focusing them into the ear canal. The waves then vibrate a thin but rigid piece of skin, technically called the tympanic membrane but commonly called the eardrum.

“There’s one thing I hate! All the noise, noise, noise, noise!”

—Dr. Seuss, How the Grinch Stole Christmas

While the analogy of a drum seems apt at first, with the sound waves beating against it, the truth is that the compression and rarefaction (the positive and negative changes in pressure) actually push and pull at the eardrum. This physical movement is then transferred to an astonishingly complex mechanism in which the three tiniest bones in your body (you remember from school: hammer, anvil, stirrup) act like a hydraulic lever, amplifying the faint sound signals 22 times while pressing against the fluid-filled snail-shell-shaped cochlea. Finally, the waves pass through the cochlear fluid to stimulate more than 20,000 minuscule hairs, like underwater currents wagging long strands of seaweed attached to the ocean floor. A sound’s wavelength translates directly into how far along the cochlear spiral the wave breaks, exciting the hairs. High-frequency sounds release their energy by moving hairs early on; lower-frequency waves stimulate hairs farther along.

Finally, the hairs convert their movement into electrical signals and send them on to the brain. And it all happens in an instant.

“The world is never quiet, even its silence eternally resounds with the same notes, in vibrations which escape our ears.”

—Albert Camus, The Rebel

Echo, Echo When we explored light and its wavelengths, we were dealing with extremely small fluctuations in electromagnetic radiation—sizes on the order of millionths and billionths of a meter. Sound waves are far longer. Even the highest pitch we can hear—the one with the most vibrations per second and therefore the smallest waves—represents a wave about 1.7 cm long, from crest to crest. That’s about 20,000 times longer than the longest visible electromagnetic wave we can see, red light!

You can easily calculate a wavelength by dividing the speed of sound by the frequency of the sound. So the musical note middle C, with a vibrational frequency of about 262 Hz, corresponds to a wave about 4.3 feet (1.3 m) long. The lowest pitch humans can hear is an astounding 75 feet (22 m) long.

Of course, traveling at 343 m/sec, these long waves, with their alternating increase and decrease of air pressure, still roll over us in a matter of milliseconds.

“There is geometry in the humming of the strings, there is music in the spacing of the spheres.”

—Pythagoras

The length of a sound wave affects how we hear it in another way, too. When a wave of any kind hits an obstacle, the result depends on the wavelength. A wave shorter than the obstacle tends to reflect off it. We can see that easily with light waves, far smaller than even the tiniest object we can see with our eyes. Shine a flashlight on something and it causes a shadow where the light does not reach.

The same thing happens with sound waves: You can shout close to a wall and hear its reflection. Submariners take advantage of this effect, navigating the murky depths with sonar—flashes of sound that echo back the location of large objects in the water.

But due to their size, sound waves don’t reflect off small things—anything smaller than the size of the wave itself. Instead, they diffract—they bend around. That’s why you can hear music from a stereo down the hall, though it tends to sound muted with too much bass. The higher-frequency short-wavelength sounds get blocked at doorways and don’t diffract as well. Lower-frequency sounds—those with longer wavelengths—tend to bend around corners quite well, allowing them to travel far and wide.

This is also why you can close your eyes and turn your head left to right and you are able to locate where in the room someone is speaking: Your own head literally makes a sound shadow, and the higher frequencies of the voice go in one ear more than the other. Similarly, stereo enthusiasts know that it doesn’t matter where in a room you put a low-frequency subwoofer; the huge wavelengths diffract so much that you’re unlikely to be able to tell if the source is in front of or behind you.

There’s another reason sound quality changes through space: Matter (the air, your chair, whatever) absorbs sound energy, converting it into a hardly detectable amount of heat. A large room or a deep canyon may echo every sound, but the quality of the tone tends to be somewhat muted because higher-frequency smaller-wavelength sounds are absorbed more quickly than those low-frequency thumpers. This explains why a thunderclap sounds like a sharp crack when it’s near you but only a low rumble a mile or two away.

Beyond Our Sound The fact that as we age we naturally lose the ability to hear higher frequencies may make you wonder if there are other sounds “out there” that you’re not hearing. The answer is absolutely, though they’re not necessarily sounds you want to hear.

Sound waves at frequencies higher than 20 kHz are called ultrasonic (that’s different from supersonic, which means faster than sound). Dogs can hear ultrasonic vibrations up to 45,000 cycles per second, and cats probably hear a bit beyond that. The reason is likely evolutionary: If you’re hunting a small animal like a mouse, you want to hear it—and the call of a young mouse in distress can easily hit 40 kHz.

Hunting by sound is also the key to a practice known as echolocation. If you can make a sound wave narrow enough to reflect off whatever it is that you want to eat, you can find it—sense it, almost feel it—by listening to the echoes around you. The classic example is bats, who can bark at well over 100 kHz. The sound is very loud and very short (usually only a few milliseconds in duration), but the wavelength is just right, bouncing off anything as small as 2 or 3 millimeters—quite helpful when looking for insects or avoiding the branches of trees. When it comes to larger animals or objects, bats can even “see” small features, letting them know what something looks like, what kind of animal they’re approaching, and so on. Even more astonishing is the fact that bats often fly in packs of hundreds or even thousands but can still navigate by recognizing their own voices.

Humans have found a number of clever ways to make use of ultrasound. Dentists use it to clean teeth, doctors focus it to break up kidney stones noninvasively (a practice known by its tongue-twister name lithotripsy), therapists use it to apply “deep heat” muscle treatments, and diagnosticians and engineers use ultrasonography to visualize the interior of the human body as well as test the structural integrity of plastics, wood, or metal beams. However, these practices all use sound waves with frequencies far beyond those audible to animals, with waves reaching from 50,000 cycles per second up to 18 million hertz (18 MHz).

At the other end of the sound frequency spectrum, below 20 waves per second, sits the mysterious world of infrasound. Whereas dolphins, porpoises, and orca whales echolocate in the ultrasonic range, when it comes to communication, most marine mammals tend toward this lower end of the sound spectrum. As we’ve seen, lower-frequency sounds can travel farther, and those below 1,000 Hz happen to travel much farther in the saltwater sea. So the humpback and blue whale can sing out extremely loudly (over 150 dB) in the range of 10 to 30 Hz—low, powerful songs that can travel hundreds of miles.

Humpback whales sing songs so loudly that they can be heard more than 100 mi (160 km) away. However, it is sperm whales that emit the loudest sound of any animal, using an air-blowing structure in their heads, curiously named “monkey lips.” These clicks can be as loud as 230 dB!

On land, elephants, hippopotamuses, and alligators also use these infrasonic tones (sounds with a frequency too low for humans to hear) to communicate with their brethren, allowing for widespread coordination of herds, or for males to find mates. A female elephant, for example, signals its availability by creating distinctive rumbling noises that can broadcast over several kilometers. Zoologists report they can feel these calls thrumming through the air even when they cannot hear them. It’s unclear how the animals themselves detect these noises, though as pressure waves travel better through rock than air, it’s possible that they feel the sound through their feet.

Many species have developed specialized organs to produce or detect sounds. Arthropods such as spiders and cockroaches have special hairs on their legs that can sense sound. The antennae of mosquitoes and many other insects can sense minute variations in air pressure. Honeybees appear to communicate through the buzzing vibrations of their wings; ants, crickets, and even some snakes and spiders stridulate (rub body parts together) to create chirps, clicks, or hisses. Some of these can be extremely loud: The African cicada’s sound has been measured at over 105 dB!

Curiously, even though we humans cannot hear infrasonic tones, we can detect them, and the effects can be dramatic. In 2003, a team of researchers in London set up an “infrasonic cannon” in the back of a concert hall, adding very soft (only about 7 dB) and very low (17 Hz) sounds intermittently to several pieces of music played before a large audience. When asked, 22 percent of the listeners reported feelings of intense discomfort, fear, or—on the flip side—a sense of the supernatural or numinous, using phrases such as “an odd feeling in my stomach,” “feeling very anxious,” and a “strange blend of tranquility and unease.”

The fact that infrasound makes humans uncomfortable has been repeatedly documented. Employees have refused to work in certain factory rooms in which they felt inexplicably ill, until it was discovered that vibrating cooling fans were pumping more than just air into the environment. Some scientists now believe that many reports of haunted houses actually stem from underlying and hard-to-trace infrasonic waves. For example, a sound wave at just the right frequency—about 18 Hz—can actually cause the human eye to vibrate, and this vibration may cause mysterious gray apparitions in the peripheral vision. Could ghosts be what Shakespeare’s Macbeth called “full of sound and fury, signifying nothing”?

The lowest sounds ever found—far below the infrasonic thunder of animals, avalanches, or earthquakes, measured at even less than 1 hertz—are those of distant cosmic events. Many galaxies contain a huge quantity of “free-floating” gas—the residue of untold numbers of stars that have grown and exploded over billions of years. Astronomers, looking at a black hole in the Perseus cluster of galaxies, about 250 million light-years away, recently noticed a pattern in the clouds. More dense in some areas, less dense in others, the pattern soon revealed itself to be a sound wave, emanating from the black hole.

The sound is a single note, drawn out not over meters but over billions of meters. To be precise, the researchers have determined that it’s a B-flat 57 octaves below middle C, a million billion times lower than the lowest sound we can hear. If you can imagine, where a 20 Hz sound wave would take 1/20 of a second to pass by, a single wave of the Perseus black hole’s drone would take 10 million years. Truly, the irony is palpable; Plato noted that “the empty vessel makes the loudest sound.”

Of course, as we’ve seen, sound is ultimately absorbed and converted into heat. And scientists estimate that these tones, these rich, vast roars, provide as much energy throughout a galaxy as billions of suns. It’s as though the music of the spheres, heating this interstellar gas, helps create just the right conditions for new stars and galaxies to be born.

In the beginning there was silence, and then, suddenly, there was light. When the astronomer and science-fiction writer Fred Hoyle coined the term “big bang” in 1949, he didn’t intend to describe the sound of creation, and in fact it’s a misnomer—with no medium, the explosive expansion of the new universe would have been a light show without a sound track. But it wasn’t long before vibration began to pulse through the blinding sea of photons, and then later the primordial soup of early atoms. The effects of these earliest waves can be detected even today, 13.7 billion years later, as astronomers map the heavens. We can see galaxies—each filled with billions of stars—clumping together every 500 million light-years or so in alternating crests and troughs, compression and rarefaction.

Complex Sounds Today, on our small planet, we are saturated with an astonishingly rich sound field—one in which the spectrums of frequency and amplitude are woven together alongside rhythm, cadence, harmony, and many other audible ingredients. Even more amazing is that we can make sense of the melee. You may be in conversation with someone at a dinner party, overhearing another discussion at the table, tapping your toe to the music in the background, and suddenly catch the cry of a baby in the next room.

One reason we can distinguish among these various signals is that sound is rarely pure. If a flute and a piano each played a slightly flat A by emitting a perfect 436 Hz frequency, we’d never tell the difference between them. But musical instruments, and most objects that produce a sound, create overtones—combinations of additional frequencies, usually at even multiples above the fundamental, lowest note, called harmonics. So play a flute—essentially a metal tube with some holes in it—at 436 hertz and you’ll invariably find a strong second tone at 872 hertz added to the mix, along with a dash of 1,308 Hz and 1,744 Hz. You’ll also hear a faint jumble of many other frequencies between, creating the characteristic breathy sound. A piano combines the same ingredients in very different amounts to concoct a completely different flavor. The exact blend of frequencies helps define what we call the instrument’s tone, or timbre.

Nevertheless, the processing required to decipher the billions of overlapping waves we hear during a few moments at that dinner party seems beyond possibility. Yet it’s just another day at the office for our ears.

And, it should be noted, for our skin—for even the deaf can sense and appreciate sound waves. Many deaf people enjoy dancing to the rhythms of loud, rumbling bass-laced music that they can feel. At music concerts, some deaf audience members hold balloons between their fingertips that act like external eardrums, resonating and amplifying the sound, allowing a richer appreciation.

While at first glance this may seem like a completely different activity from hearing, remember that hearing is feeling. In fact, brain researchers have recently discovered that deaf people experience these physical vibrations in the same part of the brain where hearing people process sound from the ears. It’s clear that humans are designed to detect and understand sound, one way or another.

There is no doubt that sound is a crucial part of our human experience, and perhaps even beyond. Virtually every faith tradition focuses on the creative and healing power of sound. In the Sufi Muslim tradition, music and chant are the secret of bringing one closer to Sirr, the center of inner consciousness where contact with the Divine is possible. As the influential twelfth-century Islamic philosopher Abu Hamid al-Ghazzali said, “There is no way to the extracting of [the heart’s] hidden things save by the flint and steel of listening to music and singing, and there is no entrance to the heart save by the antechamber of the ears.”

And the idea is certainly not without merit, as the phenomenon of resonance is easily shown. Every object, every material, has a natural frequency at which it vibrates. For example, tink the side of a wineglass to hear its special pitch. If you play a matching tone, your sound’s waves add to the object’s—make it loud enough and you can cause the material to shake to the point of breaking. The stories of singers shattering glass goblets are true!

You can see this trick of physics in any kind of wave. Pushing a child in a swing at intervals that match the swing’s resonant frequency makes it go higher, even with very little effort. Push at a faster or slower rate, and neither you nor your child will enjoy it. A piano or cello string does the same thing all by itself: Play a similar note on another instrument—either the same pitch or one that shares the same harmonic overtone—and the string sings out in reply.

So who is to say if the sound of song or prayer could not excite that which is beyond our seeing?

As our ears capture signals from the waves, we become aware of our interconnectedness with the matter around us. A footstep, a word, our favorite song, a brush of silk—we discover significance in the sounds that travel to us and through us, just as we create sound to convey meaning and relationship. These cycles of energy are powerful influencers, from the dark rushings of the mother’s womb to the explosive death of stars.