Explanation of the Concepts

If 'Amplitude Modulation' was the process of continuously varying the amplitude of a signal, then one may correctly assume that 'Frequency Modulation' is the process of continuously varying the frequency of a signal. Modulating the frequency of an audio signal by a sub-audio signal can create effects like vibrato and various siren-like sounds. Modulating the frequency of an audio signal by another audio signal is an easy way to create all manner of new and harmonically rich sounds. One may even modulate the frequency of a signal that is in turn modulating an audio signal for even more control and interest.