In acoustics, the harmonics of a sound all have a higher frequency than the fundamental frequency. The fundamental frequency is the lowest resonant frequency that a sound generator produces.
Harmonics are whole-number factors of the fundamental frequency. For example, the second harmonic has a frequency that is double that of the fundamental, while the third harmonic is triple the frequency.
Any object that vibrates at a regular frequency, such as a guitar string or the air in a brass instrument, generates a harmonic frequency structure.
Sound quality, or timbre, depends on the frequency and distribution of the harmonics, as well as the different media that give rise to these vibrations.