Sound

Sound is a kind of energy that travels as a wave. When something moves quickly back and forth, it pushes on the air around it. Those pushes spread out in all directions, like ripples from a stone dropped in a pond. When the ripples reach your ear, your brain hears them as sound.

The back-and-forth movement is called vibration. Pluck a guitar string and you can see it shaking. Tap a drum and you can feel the drumhead bouncing. Even your own voice comes from vibration. Two small flaps in your throat, called vocal cords, flutter when you speak or sing.

Sound needs something to travel through. This something is called a medium. Air is the most common medium, but sound also travels through water, wood, metal, and even the ground. Sound cannot travel through empty space. That is why astronauts on the Moon had to use radios to talk, even when they were standing right next to each other.

How fast does sound travel? In dry air at room temperature, sound moves about 767 miles per hour. That is fast, but light is almost a million times faster. This is why you see lightning before you hear thunder. The flash reaches your eyes right away, while the boom takes several seconds to cross the sky.

Sound travels even faster through water and through solid materials. In water it moves about four times faster than in air. In steel it moves about 15 times faster. Whales use this to their advantage. A blue whale's call can travel for hundreds of miles underwater and be heard by another whale far away.

Sounds can be high or low. A bird's chirp is high. A tuba is low. The difference is called pitch. Pitch depends on how fast something vibrates. Fast vibrations make high sounds. Slow vibrations make low sounds. Sounds can also be loud or soft, which depends on how big the vibrations are. Loudness is measured in units called decibels. A whisper is about 30 decibels. A rock concert can reach 120 decibels, loud enough to damage your ears if you stand too close for too long.

Humans can hear a wide range of pitches, but not all of them. Dogs can hear higher sounds than we can, which is why a dog whistle seems silent to people. Bats and dolphins can hear even higher. They use these sounds to find food in the dark, by listening for the echoes that bounce back.