Sound is the result of the disturbance of air by some kind of movement. These disturbances of the air are called sound waves. Examples of the types of movement that cause this disturbance of air are the vibrations of a tuning fork, a guitar string, or a rubber band.
Let's take the guitar string as an example. The disturbance in the air is the movement of air molecules as a result of the movement of the string back and forth. When a guitar string is plucked, the string quickly moves back and forth. As it goes in one direction, the string pushes the air molecules closest to it. These air molecules then get closer to the air molecules surrounding it. This is called compression.
Here's a little note about air molecules. They prefer to be equidistant from each other. If an air molecule gets too close, the surrounding air molecules move away, attempting to reestablish the status quo.
So, when the air molecules closest to the string are compressed against the surrounding molecules, a chain reaction is set up, in which the surrounding air molecules move away from the first ones, and are then compressed against other air molecules, which then move away, and so on, and so on.
But that's not the end of it. As stated above, the guitar string moves back and forth. So, after the guitar string moves in the first direction, causing compression, it then moves in the opposite direction. As it does so, it pulls away from the surrounding air molecules. When this happens, those air molecules are now farther away from the air molecules on the other side of the string. This is called rarefaction. Since air molecules prefer to be equidistant from each other, they will move towards the molecules that are too far away. This, in turn, pulls them away from the surrounding air molecules, which then move to restablish the correct distance.
Then, the guitar string moves back again, causing compression, and the whole thing starts over again.
The following animation is an illustration of compression and rarefaction.
The arrow represents the movement of the guitar string, and the individual circles represent air molecules. The chain reaction through the surrounding air caused by compression and rarefaction is the sound wave.
As the guitar string vibrates at a certain number of times within a second, the surrounding air molecules within a certain distance from the string will move back and forth at that same number of times within a second.
The next section discusses the parts of a sound wave and how to calculate them.