Sound waves are generated by objects which themselves are moving rapidly to and fro: the kind of movement usually called vibration. Pitch is that characteristic of a sound which allows us to differentiate between a sharp or shrill sound from a grave one. It depends on the frequency and is directly proportional to it. For a shrill sound pitch is higher. In most of the circumstances, the pitch of the sound is determined by how rapidly the object producing sound waves is vibrating.
The number of vibrations in one second is the same as number of sound waves generated, and is referred to as the frequency. The unit of frequency is hertz (Hz). The idea of frequency is applied to any change which involves oscillation, vibration or wave motion, so it is used to describe such diverse things as the movement of pendulum, radiowaves and light, as well as sound. The higher the frequency of sound waves, the higher the pitch of the sound which is heard.