Timbre, like pitch and loudness, is a psychoacoustical property of sound. In other words, it depends not only the physical or objective characteristics of a sound, but also upon how that sound is perceived and experienced by the listener. Timbre is the tone color, or quality of sound, that distinguishes one instrument or voice from another. Unlike amplitude, which is measured in decibels (dB), and frequency, which is measured in hertz (Hz), timbre is difficult to quantify. Timbre is loosely defined as the tone, color, or texture that enables the brain to distinguish one type of instrument sound from another. The term generally encompasses all the qualities of a sound besides loudness and pitch, and is characterized by such descriptions as "smooth," "rough," "hollow," "peaceful," "shrill," "warm," and so on. In simple terms, timbre is the sonic difference between a violin and a trumpet playing the same note at the same amplitude level.
Much of a sound's unique timbre is a result of its particular transient qualities. Transients are the attack and decay, or beginning and ending characteristics, of a sound. For example, the quiver of a violin bow as it strikes the strings or the brief squawk of a saxophone as the air begins to vibrate the reed are transients. Different instruments have unique transients that affect the way we hear a series of notes being played together. In addition, almost all vibrations are compound: for example, a sounding violin string may be vibrating not merely as a whole but also at one and the same time in halves, thirds, quarters, and other fractions, and these fractions are producing notes according to their varying lengths, such as are not easily identifiable by the ear but are nevertheless present as factors in the ensemble of the tone. It is the absence or presence, or greater or lesser comparative strength, of the various sounds thus produced, including harmonics, overtones, and upper partials, which condition the timbre.
A sound's timbre stems from more than just the acoustic properties of the particular instrument being played (including the human voice) or the object being hit or vibrated. It also depends on the acoustics of the environment in which a sound is produced.
The term timbral refers to the overall frequency balance of a system. In a perfect world, all systems would have complete tonal neutrality. With current technology, this ideal is approached but not met. Listening to many equally "good" speakers will reveal that some sound warmer than others, some sound brighter etc. In a surround sound system it is important that all speakers have a close timbral match for the highest degree of sonic realism.