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.
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.
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.
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.
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.
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.
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.
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.
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.
Written by Margaret Mittelbach.