How Sound Waves Travel

Astronauts can see the Sun shining, and they can watch explosions in movies, but real space is completely silent. Why can light travel through outer space while sound cannot?

The answer is that sound waves are mechanical waves. A mechanical wave is a vibration that has to travel through a material, called a medium, like air, water, or rock. The wave moves because particles bump into their neighbors and pass the shake along. Space is a vacuum, with almost no particles, so there is nothing to carry the sound.

Light is different. Light waves are electromagnetic waves, and they do not need a medium at all. They can cross the empty vacuum of space with no problem, which is why sunlight reaches Earth after traveling through millions of kilometers of emptiness.

Sound does not travel instantly. It moves at a certain speed, and that speed changes depending on what it is moving through. In air at room temperature, sound travels about 343 meters every second. That is fast, but you can still notice the delay if you watch a faraway hammer hit a stake: you see the hit before you hear it.

Sound travels faster in liquids than in gases, and faster still in solids. In water it moves more than four times faster than in air, and in steel it races along more than fourteen times faster. The reason is how tightly the particles are packed. When particles are close together and hold onto each other firmly, they pass the vibration along quickly.

Temperature matters too. Warm air particles jiggle faster, so sound travels a little faster on a hot day than on a cold one.

This section also connects three ideas: the speed of a sound wave, its frequency (how many waves per second, which we hear as pitch), and its wavelength (the length of one wave). Multiply frequency by wavelength and you get the wave's speed.

A wave is a disturbance that moves away from where it started. Waves carry energy from place to place, but they do not carry matter along with them. The particles just wiggle in place while the energy passes through.

Some waves, called mechanical waves, must travel through a material. Sound waves, water waves, and earthquake waves are all mechanical waves. Without air, water, or rock to wiggle, these waves simply cannot move. That is why there is no sound in the vacuum of space.

Light is special because it is not a mechanical wave. It can travel through empty space just fine.

Waves also come in two styles. In a longitudinal wave, like sound, the particles squeeze and spread along the same direction the wave is traveling. In a transverse wave, like a wave on a rope, the particles move up and down while the wave moves forward.

Have you ever heard a church bell or a clocktower go dong, dong across a whole town? That sound started as a vibration and traveled all the way to your ear.

In this unit, students investigate how sounds are made and how they travel. They run experiments to gather evidence that vibrating objects make sound. Touch a ringing bell or a plucked rubber band and you can feel the buzz with your fingers. When the vibration stops, the sound stops too.

Then students explore how the sound gets from the object to a listener. The vibration passes through the air (or through a solid, like a string telephone or a tabletop) as a wave, and your ear receives it. By testing what helps and what blocks the sound, they build a model of how sound travels and how we know it really moves from one place to another.