95 Sound-High

Sound Written by Robert N. Knight

What Is Sound? Can you describe what sound is? You might say that sound is something we hear with our ears. How does sound move from one place to another? How does sound get to your ears? How do your ears hear sound? This book will help you understand the answers to these and other questions about sound.

A thick blanket of air surrounds Earth. This air is made of tiny, invisible particles. Sound travels through the particles in air to your ears. But how does this happen? First of all, it's important to know that sound is a kind of energy. Like all energy, sound has the ability to move things and make things happen. Sound moves through air by making the tiny particles in air vibrate, or move back and forth. Think About It A jar full of air and a jar full of sand have something in common. They are both filled with particles. The jar full of air has many invisible air particles that you can't see.

Suppose you bang on a drum or hit a baseball with a bat. These actions cause the drum or the bat to vibrate. A vibrating drum or bat then makes the particles of air around it vibrate. The vibrations of one particle cause the particle next to it to vibrate. The vibrations spread out to other particles, causing the sound energy to move away from the object making the sound. Wowser! Sound moves a lot faster than most things on Earth. It moves through air near the ground at a speed of over 1,200 kilometers per hour (750 mph). It moves more slowly high above Earth, where there are fewer air particles.

How Do Waves Travel? Sound energy travels through air as sound waves. They are like waves you see in the ocean. But sound waves are different from water waves. Water waves only travel across the surface of water, while sound waves travel out from a vibrating object in every direction. Every sound you hear comes to you as a sound wave.

A sound wave has two parts. In one part, the particles of air are squeezed together, or compressed. In the other part, the air particles are spread out. Look at the sound waves below. Each compressed part of each wave is followed by a part where the particles are spread out. All sound waves have a regular pattern like the waves in the drawings. Different sounds make different wave patterns. But sound waves all have parts where particles of air are squeezed together and parts where they are spread out. Let's find out how waves made by soft and loud sounds are different.