Have you ever wondered what sound is? Sound is invisible. You cannot see it. But you can hear it. Because you can hear it, you know it must be a thing. So what kind of thing is it?
When you throw a stone into a pond, it makes a splash. The impact of the splash makes waves in the water. But the sound of the splash makes waves in the air. It may seem strange that a sound makes waves. But it's true! You see, a sound is a wave. But it’s not a wave of water. It’s a wave of air! When the air gets pushed, it makes a wave. The wave travels to your ears. And then you can hear it! Waves move through the water. But they move through the air, too.
Sometimes you can hear a wave twice. Sound waves can bounce off walls and other surfaces. When this happens, you hear an echo. First the sound comes to your ears. Then it bounces off a wall and comes to your ears again. You hear it twice!
Not all waves are the same. Some waves are bigger than others. The size of a wave is its amplitude. Large waves are loud. Very large waves are very loud! Whenever there is a loud sound, it has a large wave. Small waves are quiet. Very small waves are very quiet! Whenever there is a quiet sound, it has a small wave. Some waves vibrate faster than others. The speed of a wave is its frequency. Fast waves have a high-pitched sound. A whistle has a high frequency. Slow waves have a low-pitched sound. An elephant’s voice has a low frequency.
Some waves cannot be heard. This is because they are too high. Or it is because they are too low. Waves that are too high are called ultrasonic. Waves that are too low are called infrasonic. Bats make ultrasonic noises. We cannot hear them. Whales make infrasonic noises. We cannot hear them either.