What are overtones or harmonics?
In most cases where a sound is created by natural means (such as a musical instrument or the human voice), a number of higher tones is produced in addition to the fundamental pitch. These higher tones are called overtones, partial tones, or harmonics. The sequence of these tones is called the overtone series, harmonic sequence, or sequence of natural tones. The combination of the fundamental and the overtones is the frequency spectrum of a tone.