Frequency modulation: This refers to the periodic change (which can be controlled) in a characteristic of a sound. There are two signals involved in modulation; the carrier (or original signal) and the signal which produces the change in the original (therefore modulating signal) In music frequency modulation is termed vibrato and amplitude modulation is described as tremolo. Where the modulating signal approaches and goes below the fundamental tone of the original signal you can hear each variation produced as a kind of 'beating'. However where the modulating frequency is above this limit the ear perceives a complex spectrum