Sound spectrum

A harmonic series is the sequence of sounds—pure tones, represented by sinusoidal waves—in which the frequency of each sound is an integer multiple of the fundamental, the lowest frequency.

Pitched musical instruments are often based on an acoustic resonator such as a string or a column of air, which is capable of oscillating at multiple modes simultaneously. At the frequencies of each vibrating mode, waves travel in both directions along the string or air column reflecting from the ends, reinforcing each other to form standing waves. Interaction with the surrounding air causes audible sound waves, which travel away from the instrument. Because standing waves only occur at frequencies at which the length of the resonator is equal to an integer number of half-wavelengths, the resonant frequencies of the resonator usually occur at integer multiples, or harmonics, of a lowest frequency, the fundamental frequency, and such multiples form the harmonic series (see harmonic series (mathematics)).

The musical pitch of a note is usually perceived as the lowest partial present (the fundamental frequency), which may be the one created by vibration over the full length of the string or air column, or a higher harmonic chosen by the player. The musical timbre of a steady tone from such an instrument is strongly affected by the relative strength of each harmonic.

A "complex tone" (the sound of a note with a timbre particular to the instrument playing the note) "can be described as a combination of many simple periodic waves (i.e., sine waves) or partials, each with its own frequency of vibration, amplitude, and phase." (See also, Fourier analysis.)

...
Wikipedia