Sound is a vibration. When you clap your hands, the motion pushes the air molecules nearby. Those molecules then push the ones next to them, which push the ones next to them, and so on. This chain reaction creates a wave of pressure that travels through the air until it reaches your ear.

This wave isn't like a wave in the ocean, which carries water across the sea. Instead, the air molecules themselves only jiggle back and forth in place. What travels is the energy, the disturbance itself. This is why sound needs something to travel through. We call that 'something' a medium.

Sound can travel through air, water, or even a solid steel beam. But in the emptiness of space, there are no particles to vibrate. This means there's no medium to carry the wave. If a tree falls in a forest and no one is around to hear it, it still makes a sound. But if a star explodes in the vacuum of space, it happens in total silence.

When we picture a wave, we usually think of an up-and-down motion, like a rope being flicked or a ripple on a pond. This is called a transverse wave. In a transverse wave, the particles of the medium move perpendicular, or at a right angle, to the direction the wave's energy is traveling.

Sound waves are different. They are longitudinal waves. Instead of moving up and down, the particles of the medium vibrate back and forth in the same direction as the wave is moving. Imagine pushing one end of a Slinky. A compression pulse travels down its length. The individual coils of the Slinky just move forward and back, but the wave of energy travels from one end to the other.

These areas where the air molecules are bunched together are called compressions, and the areas where they are spread apart are called rarefactions. It’s this alternating pattern of high and low pressure that makes up a sound wave.

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