A harmonic spectrum is a spectrum containing only frequency components whose frequencies are whole number multiples of the fundamental frequency; such frequencies are known as harmonics. "The individual partials are not heard separately but are blended together by the ear into a single tone."

In other words, if ω {\displaystyle \omega } is the fundamental frequency, then a harmonic spectrum has the form

{ … , − 2 ω , − ω , 0 , ω , 2 ω , … } . {\displaystyle \{\dots ,-2\omega ,-\omega ,0,\omega ,2\omega ,\dots \}.}

A standard result of Fourier analysis is that a function has a harmonic spectrum if and only if it is periodic.