What is sound?
People use sound all the time. We rely on sounds to communicate. Unexpected noises may warn us of danger. The sounds we hear tell us a lot about our surroundings. Many animals also use sound. This is especially true of animals that live in water. Since only the very top of the ocean receives light from the sun, animals have difficulty using their eyes to find their way around in deeper waters. Once animals dive, they rely heavily on their hearing to sense their environment.
If you had to draw a picture of a sound, what would you draw? Maybe something like this?
Or perhaps something like this?
In fact, both of these pictures are right! Sound is a wave, similar to the ripples on a pond or the ocean waves you might see crashing on a beach. Instead of being a wave on the ocean surface, sound is a wave that travels through air or water.
Because waves move, you can think of a sound wave moving through water. Imagine a tiny section of the water as a particle. This water particle receives a tiny push, then a pull, as sound travels through it. This causes the water particle to vibrate backward and forward around the spot where it was before the sound wave came through. This spot is called its equilibrium position. The movement of the water particle is called vibration.
The water that sound travels through is called a medium. A medium can be anything - a liquid (such as water), a solid (such as the seafloor), or a gas (such as air). Did you know that sound cannot exist if it doesn't have something to travel through? For example, sound cannot travel through outer space because it is a vacuum that contains nothing to carry sound.
A sound wave is called a compressional or longitudinal wave. This is a picture of a longitudinal wave. The particles in a longitudinal wave move parallel to the direction in which the wave is traveling.
As you can see, there are places where the particles are squashed together (compressed) and places where the particles have been pulled apart (expanded). The places where the particles are compressed are regions of high pressure. The places where the particles are pulled apart are regions of low pressure. What is pressure? Have you ever gotten a cut and had someone say to you, "Put pressure on that cut to help stop the bleeding?" They were asking you to put your hand over your cut and squeeze or compress it. A sound wave alternately compresses and expands whatever medium it is traveling through.
Water waves on the ocean surface are called transverse waves because the particles move up-and-down as the wave moves left-and-right. The particles in a transverse wave move perpendicular to the direction in which the wave is traveling. You can see this below in the picture of a pond. If a rock is thrown into the pond, it creates a wave. The water moves up-and-down while the wave moves away from where the rock landed. That is called a transverse wave.
If you have ever "done the wave" at a football game, you have created a transverse wave.
Here's the same movement, with a more circular path, as water particles move in response to a passing surface wave.