Timbre, also referred to as tone color or tone quality, is the perceived unique sound produced by a particular musical instrument or other object. It is what allows humans to distinguish among different sound sources even when the pitch (or frequency) and volume (loudness) are the same. It can also be defined as the remaining characteristics of a sound other than pitch, volume and duration.
Timbre is a result of the fact that each note from a musical instrument or other sound source is a complex wave containing more than one frequency. Each such mixture of frequencies is heard not as separate sounds, but as a composite. Small differences in the balance of the frequencies, including the number thereof, their relationship to the fundamental pitch, and their relative loudness, create the unique timbre of a note or instrument.
The timbre of a musical instrument is determined by the type of instrument, its size and shape (for example, whether the tube of a wind instrument is cylindrical or conical) and by the materials from which it is made (such as wood, plastic or metal, and the specific type of each). The timbre of a singing voice or spoken words is determined by the specific characteristics of the vocal tract of the individual and is modified by subconsciously opening or constricting various parts of it, including the tongue, lips, and throat.
The human ear and brain are capable of distinguishing very small differences in timbre. For example, a trained listener can not only tell the difference between identical music played on a violin and a viola, but can also distinguish between two different violins or two different violas.