The Science of Sound

A World Full of Sound

Close your eyes for a moment and just listen. You might hear birds singing, a car passing by, the hum of a fridge, voices talking, or music playing. Sound is all around us, all the time. We use it to talk to our friends, enjoy our favourite songs, hear danger coming, and understand the world even when our eyes are shut.

But what exactly is sound? You cannot see it, hold it or catch it in a jar. And yet it travels across a room in an instant and right into your ears. In this book, we will uncover the secrets of sound: how it is made, how it journeys through the air, how your ears turn it into the sounds you hear, and what makes some sounds loud, soft, high or low.

Sound Starts with a Shake

Here is the big secret behind every single sound in the universe: sound is made by vibrations. To vibrate means to shake or wobble very quickly, back and forth.

Whenever something vibrates, it makes a sound. When you pluck a guitar string, you can actually see it blur as it vibrates, and you hear the note it makes. When you bang a drum, the skin of the drum shakes up and down. When you speak, special parts in your throat called vocal cords vibrate to make your voice.

You can feel this for yourself. Gently rest your fingers on the front of your throat and hum a tune. Can you feel the tickly buzzing? Those are the vibrations that become the sound of your humming. No vibration means no sound — it really is that simple.

How Sound Travels

So a guitar string vibrates over there, but you hear it over here. How does the sound make the journey across the room to reach your ears?

The answer is that sound travels in waves. When something vibrates, it pushes and squeezes the tiny bits of air around it. Those bits of air bump into the next bits, which bump into the next, passing the wobble along like a wave moving through a crowd. This invisible ripple spreads out in all directions until it reaches your ears. We call it a sound wave.

This means sound needs something to travel through. Usually that something is air, but sound can also travel through water and even through solid materials. Sound actually travels faster and further through water and solids than through air. That is why, in an old story, people would press an ear to the ground or to a railway track to hear something coming from far away.

There is one place where sound cannot travel at all: outer space. Space is almost completely empty, with no air to carry the vibrations. So no matter how big an explosion might be out among the stars, in space it would be completely silent.

How We Hear

Your ears are wonderful sound-catching machines. The part of your ear you can see is shaped like a funnel, perfect for gathering sound waves and guiding them inside your head.

Deep inside your ear is a thin, tightly stretched skin called the eardrum. When sound waves hit the eardrum, they make it vibrate, just like a tiny drum. Behind the eardrum, the vibrations pass through some of the smallest bones in your whole body and into a special spiral-shaped part filled with fluid. There, the vibrations are turned into messages that zoom along to your brain. Your brain reads these messages and tells you what you are hearing — a friend's voice, a barking dog, or your favourite song.

All of this happens in the blink of an eye, every single time you hear something. Because you have two ears, one on each side of your head, your brain can also work out which direction a sound is coming from, which helps you turn toward whoever is calling your name.

Loud and Soft, High and Low

Not all sounds are the same. Some are loud, some are soft, some are high and squeaky, and some are low and rumbling. Sound has two main qualities, and both come from the vibrations that make it.

The first quality is loudness, which is how loud or soft a sound is. Loudness depends on how big the vibrations are. A gentle tap on a drum makes small vibrations and a quiet sound. A hard whack makes big vibrations and a loud sound. Very loud sounds, like a jet engine or fireworks, can even hurt your ears, which is why it is wise to protect them.

The second quality is pitch, which is how high or low a sound is. Pitch depends on how fast something vibrates. Fast vibrations make high sounds, like a whistle or a tiny bird's chirp. Slow vibrations make low sounds, like a big drum or a rumble of thunder. On a piano, the thin short strings vibrate quickly and play high notes, while the thick long strings vibrate slowly and play low notes.

The Magic of Music

When people put sounds together in a careful, pleasing way, they make music. Every musical instrument is really just a clever machine for making vibrations and turning them into sounds we enjoy.

Different instruments make their vibrations in different ways. String instruments like the guitar and violin make sound when their strings vibrate. Drums and other percussion make sound when a surface is hit and shakes. Wind instruments like the flute and trumpet make sound when air vibrates inside a tube. By choosing high notes and low notes, loud parts and soft parts, fast rhythms and slow ones, musicians turn plain vibrations into something beautiful.

Even your own voice is a kind of instrument. When you sing, you control the vibrations in your throat to make different notes, high and low. Every person's voice sounds a little different, which is how you can recognise a friend just by hearing them speak.

What We Learned

Sound is made by things vibrating, or shaking quickly back and forth. It travels as invisible waves through the air, and also through water and solids, but never through the empty silence of space. Our ears catch these sound waves and turn them into messages for the brain to understand. How loud a sound is depends on how big the vibrations are, and how high or low it is depends on how fast they shake. From a whisper to a roar of thunder to your favourite song, all of it is the wonderful science of sound.

To explore another kind of wave that races to your eyes, read The Story of Light. Or discover the energy behind every vibration in Energy All Around Us.