Sound is an enormous part of our everyday existence, yet we may never even think about how sound works. What makes some sounds louder than others? What causes us to hear notes that are pitched low differently from those that are pitched high? Why does a lion's roar sound so different from a bird's whistle? Sound is actually a series of waves created by the vibration of an object. The waves that are created are called mechanical waves because they transfer energy from one particle to the next. Sound waves move in all directions from their source, like the circular ripples that occur after you throw a rock into a pool of water. Sound moves through gases, such as air, but it also moves through liquids and solids. In fact, the only place sound cannot travel is in a vacuum. If an astronaut on the moon plays a guitar, no one would be able to hear it. This is because as sound waves move, they vibrate the matter around them, causing the atomic particles to move and to transfer energy. If there is no matter to vibrate, such as in outer space, there will be no sound. One might think that sound moves fastest through the air, slower through a liquid, and even slower through a solid. In fact, the opposite is generally true: the speed of sound is faster in solid materials and slower in liquids or gases. It is easier for sound waves to go through solids than through liquids because the particles are closer together in solids. The closer the particles are to each other, the less energy it takes for them to pass the sound to each other and the faster sound can travel. Solids move sound vibrations quite efficiently. Similarly, it is harder for sound to pass through gases than through liquids, because the particles in gases are farther apart. With the help of a friend, you could demonstrate this. If you stood in a tunnel that had metal pipes running the length of the tunnel, and your friend stood at the other end, you could show that sound moves faster through a solid than through a gas. If you tapped on the pipes, and at the same time called out to your friend, your friend would hear the tapping before hearing your voice! Just as there are differences in the speed of sound, there are also differences in the waves of sound. Different sounds create different types of waves. Imagine the waves of the ocean—there are valleys between the wave peaks. In sound, the distance between one peak and the next is called a wavelength. If you hear chimes tinkling in the wind, the wavelengths of the sounds they create are short. In other words, the peaks are close together. If you hear bass drums booming, the wavelengths of the sounds they create are long. When wavelengths are short (waves are close together), we hear higher sounds, and when they are long (waves are far apart), we hear lower sounds. Pitch is the quality of sounds that makes them seem "higher" or "lower." Deep sounds, like those of bass drums, have a low pitch. They have fewer wavelengths per second than sounds with a higher pitch because the wavelengths of deep sounds are spread out, or farther apart. Just as the waves can be spread out or pushed close together, they can also vary in size. The greater the distance between the peaks and valleys of sound waves, the louder the sound is. You can remember this by imagining an ocean storm—the waves are enormous and crash loudly. On a calm day, ocean waves are small and make a quiet lapping sound. When you listen to a sound—whether it comes from the squeal of brakes, the wail of an electric guitar, or the rumble of a passing train—think about the unique characteristics of the sound waves that are moving invisibly through matter to bring the sound to your ears. There are two graphs for readers to compare. This graph shows the long wavelengths of the sound of thunder. Three waves fit in the length of the graph. There are two graphs for readers to compare. This graph shows the long wavelengths of the sound of thunder. Three waves fit in the length of the graph. This graph shows the short wavelengths of the sound of a flute. Ten waves fit in the length of the graph. This graph shows the short wavelengths of the sound of a flute. Ten waves fit in the length of the graph. There are two graphs for readers to compare. This graph shows the short wavelengths of the sound of a whisper. There are two graphs for readers to compare. This graph shows the short wavelengths of the sound of a whisper. This graph shows the tall wavelengths of the sound of a jet airplane. The waves are so tall they touch the top and bottom edges of the graph.