Sound varies from low to high pitch in direct relation to the frequency of the sound wave. Sound waves oscillate, and the number of times per second a sound wave oscillates is known as its frequency. A higher frequency produces a higher pitch, while a lower frequency results in a lower pitch.
Sound waves are analogous to the ripples created on a pond from a tossed stone; the distance between the ripples is representative of the frequency, or the pitch, of the sound wave. More ripples moving past a given area over the course of one second would be commensurate with a higher pitch, while fewer ripples would indicate a lower tone.
Another example of the frequency and pitch relationship is the Doppler effect. This phenomenon occurs when a sound source, or the listener, is in motion and also occurs when both are moving, according to About.com. As a moving sound source moves toward a listener, or the reverse, the leading sound waves are slightly closer together, and when the source or listener passes, the trailing sound waves spread farther apart. This is observed when a train or ambulance passes a particular location, as the sound produces a steadily higher pitch as it approaches, then a progressively lower pitch as it moves away.