Think about a guitar string. When you pluck it, it wiggles back and forth very quickly. That wiggle is a vibration. Sound is just a vibration that travels from a source to our ears. Because it needs to travel through something, sound is known as a mechanical wave.

When an object vibrates, it disturbs the particles of the medium around it. Imagine dropping a pebble into a still pond. The pebble creates ripples that spread outward. The water itself doesn't travel across the pond; instead, the energy of the splash is passed from one water molecule to the next. Sound works in a similar way, but with a key difference in how the particles move.

Waves on a pond move up and down, while the wave itself travels forward. Sound waves are different. They are longitudinal waves.

Let's go back to that guitar string. As it vibrates, it pushes forward into the air molecules nearby, squishing them together. This area of high pressure is called a compression. Then, as the string moves back, it leaves a space with fewer air molecules, creating an area of low pressure called a rarefaction.

This process repeats rapidly, creating a chain reaction of compressions and rarefactions that travel away from the string. The air molecules just jiggle back and forth in the same direction the sound wave is moving. That's what makes it a longitudinal wave.

It's this traveling pulse of pressure changes that your eardrum picks up, allowing your brain to interpret it as sound.

Because sound relies on particles bumping into each other, it absolutely needs something to travel through. This “something” is called a medium. A medium can be a gas (like air), a liquid (like water), or a solid (like a wall).

Without a medium, there are no particles to vibrate. This is why the classic sci-fi movie scenes with loud, fiery explosions in space are technically inaccurate. Space is mostly a vacuum, meaning it's empty of matter. With nothing to carry the vibrations, sound cannot travel.

You can hear sounds through solids and liquids, too. If you put your ear against a closed door, you can often hear conversations from the other side more clearly. And whales communicate over vast distances underwater. In fact, sound often travels better through liquids and solids than it does through air because the particles are packed more tightly together, allowing vibrations to pass between them more efficiently.

So, sound is a vibration that needs a substance to journey through. It's a physical push and pull on the particles of a medium, creating a wave of pressure that our ears can detect.