It’s a planet of sound! Sound starts with just a simple vibration. It’s at the heart of everything from One Direction’s latest song to the conversations you have with friends. But how does sound work? What turns the motions of molecules into the symphonic sounds of orchestras, whistling tea kettles, and barking dogs? Let’s explore the phenomenal world of sound and vibration—and how this natural phenomenon is used for communication and even navigation. Sound good?

Sound is an invisible form of energy. Most animals have the ability to sense it. Along with sight and smell and the skin’s ability to sense temperature, our sense of hearing tells us what’s happening outside our bodies. It can tell us what’s happening far away and out of sight—and it’s a powerful form of communication. Think about a lion roaring or someone crying for help.

Wolves howl when they are separated from other wolves in their pack. The long, low howls can be heard for miles, and they help wolves stay in touch with each other. (Stayer/ Shutterstock)

A rattlesnake shakes the end of its tail—making a telltale rattling sound—to warn other creatures not to come any closer. (Matt Jeppson/ Shutterstock)

Sounds are made by vibrations. Some vibrations are easy to see. For example, if you stretch out and twang a rubber band, you can see it moving back and forth. Other vibrations are less obvious, but you can feel them. Try putting your hand around your throat and humming a tune. Can you feel the vibrations? Those are your vocal cords moving rapidly back and forth. Without vibrations, the world would be silent.

So how do vibrations travel and get to your ears? The vibrations that create sound must travel through a “medium,” such as air or water—or anything made of molecules. To understand sound, it’s important to remember that air isn’t just empty space. Air is actually a fluid—a fluid we live in, just like fish live in water. Although you can’t see air, you can feel it flow past you, and you can use it to blow bubbles or fill up a balloon. Air can move, flow, and fill up spaces because it is made of invisible gas molecules. The molecules in air are loosely packed, floating and bumping around. It is those air molecules that transmit most sounds.

Here’s how: If you hit a spoon against a drinking glass, it will cause the glass to vibrate. As the glass shudders and shakes with vibrations, it pushes on the surrounding air molecules and causes them to move. With each forward motion, air molecules pulse outward, pushing other air molecules and crowding them together. With each backward motion, the molecules get less crowded. The forward and backward vibration of the glass creates a chain reaction of crowded and not-so-crowded molecules that ripples through the air. This traveling vibration is called a sound wave.

A loudspeaker generates a sound wave: The areas where the air molecules are pushed together are called “compressions.” The areas where the air molecules are more spread out are called “rarefactions.” The blue sine curve on top represents the sound wave, with high points being compressions and low points being rarefactions. The distance from one high point, or compression, to another is called the “wavelength.” (Image via Wikimedia Commons)

Hitting the skin of the drum causes it to vibrate and create sound waves. When the sound waves reach your eardrums, they vibrate, too. (Anna Jurkovska/ Shutterstock)

Sound waves can travel through all kinds of mediums—some better than others—and that is why sound can travel through solid objects (like a wall or closed window). In fact, most solid materials are better at directly transmitting sound than air. For example, if you take an object, such as a spoon, hold it right in front of your nose and tap the far end very lightly with your finger, you probably won’t hear anything. BUT if you put one end of the spoon next to your ear and tap the other end, a sound wave will travel straight through the spoon and you will hear it clearly.

Sound waves can’t travel forever. After a while they lose energy and fade away. They can also be weakened and distorted. When a sound wave traveling through the air encounters an obstacle, such as a tree or wall, some of the energy of the sound wave gets absorbed, so the sound comes out fainter and sometimes garbled on the other side.

Can you hear what’s going on in the next room by putting a glass to the wall and pressing your ear against it? Yes, this trick really does work. The sounds in the next room are transmitted through sound waves into the wall, which absorbs most of the vibrations. The glass can help you pick up the vibrations directly from the wall and amplify them straight into your ear. You can’t hear perfectly, but you definitely hear more. (ollyy/ Shutterstock)

Where is it not possible for sound to travel? In outer space. Sound waves require a medium to travel through. Outer space is a vacuum, and therefore it is silent. However, you can hear inside spacecrafts if air is pumped in. (Image via NASA)

Sound waves travel through different mediums at different speeds. At sea level, sound waves travel through the air at about 760 mph—about five miles a second—which means you can hear nearby sounds almost instantaneously. But they move through water 4 times that fast and through steel more than 17 times as fast.

Why do you see lightning before you hear its thunder? Light travels much faster than sound. The sound of a thunder clap is caused by the lightning heating air molecules and pushing them outward, setting up an enormous sound wave. (Mihai Simonia/ Shutterstock)

When an object moves at the speed of sound, it is said to be traveling at Mach 1. When a jet surpasses the speed of sound, it “breaks the sound barrier” and causes a sonic boom—which sounds like a clap of thunder. When it’s humid, jets form a vapor cone as they break the sound barrier. (Katerina_S/ Shutterstock)

Sound waves move through the air with astonishing speed. But what happens when they reach your ears? In fact, your ears are so fine-tuned they can process information 1,000 times faster than your eyes.

Ears are phenomenal sound wave catchers. Their curvy shape funnels sound waves into your ear canal. The sound waves roll into you eardrum—which really is a bit like the skin of a drum. Your eardrum vibrates—and this vibration is sent along a series of connected bones (the hammer, anvil, and stirrup), which are the smallest bones in the human body. The last tiny bone (the stirrup) passes the vibration on to a membrane called the oval window. And here’s where things get really interesting. The oval window is the window to the inner ear. When the oval window vibrates, it causes fluid inside the spiral-shaped inner ear (cochlea) to vibrate, too.

Inside the cochlea, there are 15,000 to 20,000 microscopic sound receptors, called hair cells. The hair cells are “tuned” to different frequencies and the ones that are stimulated send an electrical signal—a form of code—to the brain. The brain interprets the signals and that is how we “hear” sound.

Sound waves are captured by the outer ear and cause the eardrum (tympanic membrane) to vibrate. These vibrations are transmitted via three connected bones: the hammer (malleus), anvil (incus), and stirrup (stapes). The vibrating stirrup tap-tap-taps on the oval window and causes fluid inside the cochlea to pass the vibrations on to the basilar membrane and organ of Corti, which is covered in thousands of microscopic sound receptors (hair cells). From there the vibrations are turned into electrical signals and passed on to the brain for interpretation. Note: The cochlea is shown stretched out here like a straight tube. In reality, it is curled up like a coil. (Alila Medical Media/ Shutterstock)

Many animals—horses, rabbits, kangaroos—have muscles that allow them to swivel their ears toward a sound without turning their heads. This is handy when they want to listen for danger. Here’s an interesting fact: Human ears aren’t really built for swiveling, but the muscles are there and some people can wiggle their ears just a little. Are you one of them? (Filip Fuxa/ Shutterstock)

Very big ears can capture more sounds. This gerenuk is an African gazelle that needs both big eyes and big ears to be on guard against predators, such as lions, cheetahs, and leopards. Want to hear like a herbivore? Try cupping your hands behind your ears and flaring your hands out like the gerenuk’s ears. All of a sudden, everything will seem louder. (Sergey Uryadnikov/ Shutterstock)

Inside the inner ear, the microscopic tips of hair cells look (kind of) like bunches of hair—thus the name. Over time, hair cells can get damaged or destroyed by loud noises and other traumas—and they won’t grow back. That means permanently losing some—or even all—hearing. But scientists have discovered certain animals (such as chickens) CAN regrow hair cells. This view through a scanning electron microscope shows the hair cells of a chicken blown up thousands of times. Scientists are studying these animals’ ears to see if there is a way to help people re-grow hair cells too. (Inside the inner ear, the microscopic tips of hair cells look (kind of) like bunches of hair—thus the name. Over time, hair cells can get damaged or destroyed by loud noises and other traumas—and they won’t grow back. That means permanently losing some—or even all—hearing. But scientists have discovered certain animals (such as chickens) CAN regrow hair cells. This view through a scanning electron microscope shows the hair cells of a chicken blown up thousands of times. Scientists are studying these animals’ ears to see if there is a way to help people re-grow hair cells too. (Peter Gillespie via Cell Image Library)

So what makes all these vibrations sound so different? There are an almost infinite number of sounds in the world, but there are just three main components of sound: pitch, loudness, and timbre.

Pitch is all the different notes of the “song” of life as heard by humans and animals. The deep roar of a lion has a low pitch and the squeak of a mouse has a high pitch. If you were to play every note on a piano, each one would have a different pitch. Pitch is created by sound waves having shorter or longer wavelengths. The longer the wavelength is the lower the pitch. The shorter the wavelength is the higher the pitch. Pitch is measured by the number of wavelengths that travel through the air per second—or the frequency. The frequency of sound waves is measured in Hertz (abbreviated as Hz). If you play Middle C on a piano, it creates a sound wave with a frequency of about 262 Hz. The highest note on a piano creates a sound wave with a frequency of about 4,186 Hz.

Humans can hear in the range of 20 Hz to 20,000 Hz. Anything below about 20 Hz is called infrasonic. Anything above 20,000 Hz is considered ultrasonic.

Oscilloscopes are machines that measure and create images of sound waves, such as the two shown here. The blue sound wave is higher pitched but softer. The yellow sound wave is lower pitched but louder. The loudness, or amplitude, is measured by the height of the wave. The frequency (pitch) is determined by the distance between the “hills” (the wavelength). (Khen Guan Toh/ Shutterstock)

Now let’s take loudness—this his might seem pretty obvious: Yelling is loud and whispering is quiet. But loudness is actually a measure of the change in air pressure created by a sound wave. In fact, extremely loud noises can create so much pressure that they damage your hearing. Different levels of loudness are created by sound waves that have different amplitudes, or heights. The bigger the sound wave is the louder the noise. The smaller the sound wave is, the quieter the noise. Loudness is measured in decibels (abbreviated as dB). Normal conversation is about 60 dB. A crying baby is about 110 dB. Anything louder than 120 decibels can cause your ears to hurt. It’s said that the loudest sound ever heard was the volcano Krakatoa exploding in 1883—it was heard 3,000 miles round the world and may have been as loud as 180 dB.

What about timbre? Why does a violin playing a piece of music sound totally different than a flute playing that exact same piece of music? Why does everyone in the world have a different-sounding voice—almost like a fingerprint? The answer is that most sounds are not made of one sound wave but many. These sound waves combine to form the distinctive sound of your voice—and the unique signature of different musical instruments.

This is a spectrogram of the sound of a violin playing a low G note. It shows the pitch in Hertz on the x axis and the loudness in decibels on the y axis. The first green peak shows the frequency of the fundamental tone at 196 Hz, while the subsequent green peaks are overtones—which together create the violin’s distinctive timbre. (Image via Wikimedia Commons)

Drawing a bow across a violin’s strings causes them to vibrate. But the vibrations of the strings aren’t strong enough to create very loud or musical sounds. The violin is built so the vibrations from the strings travel through the bridge (shown just above the bow) to the body of the violin. The body of the violin is a sound box and it vibrates, too, creating a rich, round sound and amplifying it. (Artem Furman/ Shutterstock)

Electric guitars don’t sound like much when they’re unplugged. Instead of sound boxes, electric guitars have “pickups” which transform the strings’ vibrations into electrical signals that can be played and amplified through a speaker. (tobkatrina/ Shutterstock)

Everybody has heard an echo. Maybe you’ve been in a tunnel or a cave and shouted, “HELLO, OUT THERE!” and heard your voice faintly repeated back to you—maybe even multiple times: HELLO, HELLO, HELLO… [Note: Text is shrinking… can that be done online?] What causes a sound to echo? Just like a mirror reflects light, an echo is a reflection of sound. When a sound wave rolls through the air and hits a flat surface (such as a wall), some of the energy from the sound wave will be absorbed or transmitted and some of the energy will be reflected so that it bounces back through the air. Under certain conditions, the sound reflection is so good that you can hear an echo.

Today, people display art in special places—like museums. Ancient people put their art in special places, too. Scientists have discovered that some cave paintings were done in locations where there are strong echoes. They even found that in places where there are ancient paintings of horses that the echoes sound like hoof beats! These hand stencils in Patagonia, Argentina, are about 10,000 years old. What do you think it would sound like if you clapped your hands nearby? (sunsinger/ Shutterstock)

Echoes can be fun (and sometimes spooky) to hear, but they are also surprisingly useful. For example, many species of bats can use echoes to find food on the wing—a skill called echolocation. Bats make rapid-fire high-pitched calls or clicks and listen for the echoes of the sounds off objects. The bats’ ears and brains are so fine-tuned that they can hear the echoes coming back from tiny objects like mosquitoes. They can use the echoes to create a “sound picture” that helps them avoid obstacles while flying and swooping in on their miniscule prey. Bats can hear higher pitches than humans—and the calls used for echolocation are too high for people to hear. Such sounds are called “ultrasound.”

Echolocation in bats is a form of natural sonar. Sonar stands for “SOund NAvigation and Ranging.” Submarines and fishing boats use sonar to locate objects underwater. Sonar equipment emits ultrasounds and measures how long it takes for the ultrasounds’ echoes to bounce back. With that information, the sonar equipment can calculate how far away underwater objects are and even what shape they are.

Dolphins use echolocation underwater to find food and navigate. Dolphins make rapid-fire clicking noises, called “click trains” at different pitches. The higher the pitch, or frequency, the more detail they can get back about the object—and the smaller the object they can locate. Dolphins can hear frequencies as high as 200,000 Hz. (Tory Kallman/ Shutterstock)

Using an ultrasound machine, millions of high-frequency ultrasonic sound waves are transmitted into the human body—and the reflections, or echoes, they send back are used to see what’s going on inside. Ultrasound is commonly used to look at how fetuses are doing inside their mother’s wombs. This is a picture of an ultrasound done when the fetus is 17 weeks old. (Darren Brode/ Shutterstock)

Humans first guessed that elephants used infrasound when a scientist named Katy Payne visited elephants in a zoo and felt the air around the elephants “throbbing” with vibrations. Payne named the elephants’ infrasonic calls “silent thunder.” (Jonathan Pledger/ Shutterstock)

Written by Margaret Mittelbach.

 

[wp-simple-survey-31]