Sound

Sound

When someone shouts hello, billions of molecules push and shove through the crowded air between you, speeding the sound to your ears. If we could watch this happen, we’d see that sounds are waves of energy that squeeze and stretch the air as they travel.

All sounds travel in waves, and what makes one sound different from another is simply the shape of its waves. Unlike waves on water, which snake up and down as they move forward, sound waves push and pull in the same direction that they travel.

Sound is ultimately just another type of energy, like light or heat, but it is special to us because it carries words and music at high speed. Without sound we wouldn't be able to listen to birds singing in the trees or the latest hit songs on the radio. It has the ability to affect our emotions and stir up our interest in the world around us.

How sound travels

If you bang a drum, its skin vibrates, shaking the air molecules around it. These push on nearby molecules, which shake others, and the sound quickly ripples outward, spreading energy in all directions. When the energy finally reaches our ears, it makes the air inside them vibrate too and we hear sounds.

Loudness

It takes more energy to make louder sounds. The harder you beat a drum, the more its skin shakes up and down. That makes the air molecules push and pull harder, producing a louder sound in your ears.

Louder sounds are carried by taller waves (higher amplitude). Quieter sounds are carried by shorter waves (lower amplitude).

Pitch

The pitch (frequency) of a sound comes from how often it vibrates. A tight drum skin vibrates more than a loose one, making higher pitched (higher frequency) sounds. These vibrate more often than lower frequency sounds.

Higher pitched sounds vibrate quicker (higher frequency). Lower pitched sounds vibrate slower (lower frequency).

Speed of sound

We see lightning flash several seconds before we hear thunder claps, because light travels much faster than sound. At ground level, at an air temperature of about 68°F (20°C), sound travels at 1,125 feet per second (768 mph or 1,235 kph). However, it doesn't move at the same speed in every material. Because sound moves by shaking energy through atoms or molecules, its speed depends both on the inner nature of a material - how close together its atoms are - and the temperature.

Speed of sound in different materials

You can walk faster through air than through water, so you might expect sound to do the same. But sound waves travel fastest in solids (since the atoms are closest together), slower in liquids, and slower still in gases. Sound travels over 17 times faster in steel than in air.

Supersonic motion

By the time you hear a jet plane screaming overhead, it's already shot past. Flying faster than the speed of sound, it leaves its own noise far behind. Supersonic planes make so much noise because they ram and squeeze the air in front of them, trailing huge shock waves behind.

The Doppler effect

When a police car hurtles toward you, its siren sounds high-pitched. This is because the car is driving forward into the sound waves sent out by the siren, so the waves get closer together and arrive at your ears more frequently, giving them a higher pitch. After the siren passes, you hear a sudden drop in pitch. This is because the car is moving away in between the sound waves, so they grow further apart, and therefore sound lower pitched. This is called the Doppler effect after the physicist who discovered it.

Shifting sirens: The pitch of a siren is helpful in working out if a car is moving toward or away from you. In the moment the car passes you, you will hear its siren exactly as the driver of the car hears it.

Tuning fork

Bang a tuning fork and it makes a simple, regular, up-and-down sound wave pattern called a sine wave. Each fork produces only one note (frequency) and you need different forks to make other notes.

Flute

Flutes make sounds when you blow into them, vibrating and making waves inside the pipe. The sound waves are similar to the sine wave from a tuning fork, but slightly more complex.