A simple sound wave can be described by its frequency and by its amplitude. The frequency of a sound wave is the rate at which the pressure changes occur. It is measured in Hertz (Hz), where 1 Hz is equal to 1 cycle-persecond. The range of frequencies audible to the human ear extends from a low of about 20 Hz to a high of about 20,000 Hz. In practice, a sound source such as a voice usually produces many frequencies simultaneously. In any such complex sound, the lowest frequency is called the fundamental and is responsible for the pitch of the sound. The higher frequencies are called harmonics and are responsible for the timbre or tone of the sound. Harmonics allow us to distinguish one source from another, such as a piano from a guitar, even when they are playing the same fundamental note.
(from Audio Systems Guide for Houses of Worship, Shure Educational Publication)
