in acoustics, tone sounding above the fundamental tone when a string or air column vibrates as a whole, producing the fundamental, or first harmonic. If it vibrates in sections, it produces overtones, or harmonics. The listener normally hears the fundamental pitch clearly; with concentration, overtones may be heard.

Harmonics are a series of overtones resulting when the frequencies are exact multiples of the fundamental frequency. The frequencies of the upper harmonics form simple ratios with the frequency of the first harmonic (e.g., 2:1, 3:1, 4:1). In the case of ideal stretched strings and air columns, higher harmonics result when the full length of the vibrating medium is divided into more and more equal parts.

Some musical (music) instruments (musical instrument)—among them those whose sounds result from the vibration of metal, wood, or stone bars (e.g., marimbas or xylophones); of cylinders (e.g., orchestral chimes); of plates (e.g., cymbals); or of membranes (e.g., drums)—produce nonharmonic overtones—that is, the frequencies of the overtones are not multiples of the fundamental frequency.

Musical timbre, or tone colour, is affected by the particular overtones favoured by a given instrument. The “woody” sound of the clarinet comes from its emphasis on low-frequency odd harmonics, whereas the more nasal sound of the oboe comes from the presence of all harmonics and a greater emphasis on the higher frequencies.