Once again, imagine that the red dots are air molecules. Now, an ear is nearby. The yellow dot will expand, and then collapse, creating a wave in the air molecules. Press "Play" to see this.

Hearing is all about a wave in the air pushing on the inside of the ear, specifically, on the ear drum. When a series of these waves pushes on the ear drum, a tone is heard. The number of times per second that a wave pushes on the ear drum is called the frequency of the sound. The frequency of a sound is measured in "hertz," abbreviated Hz.

Try this link to hear sounds that have different frequencies...