When violinists play, their left hands always seem to shake. It can't be that violinists are always nervous, because their right hands don't shake (most of the time, anyway). The explanation is that they're using vibrato.
Vibrato, from the Italian word for "shaken," or "vibrated," is both a technique and an effect. The technique, used by violinists, violists, cellists, and double bass players, is to rock the fingers of the left hand rapidly back and forth on the strings as notes are played. The effect is a small, regular, up-and-down fluctuation in pitch for each vibrated note. Vibrato is usually characterized by two factors: speed and width. The rocking motion can be fast or slow, wide or narrow. A wide vibrato causes the pitch to change more with each oscillation than it would with a narrow vibrato, because the wider rocking motion makes the fingertip cover more distance on the string.
Why do string players use vibrato? Because vibrato adds resonance to the sound of a stringed instrument. A vibrated note usually sounds richer or warmer than a nonvibrated note, and a passage played with vibrato has a completely different effect than one played without. It's not just a matter of flipping a switch, however, of turning the mechanism on or off. The accomplished string player has many types of vibrato at his or her command, covering an enormous range of fine gradations and possible combinations of speed and width. Vibrato is thus one of the string player's best tools for varying the sound, for changing tone quality and intensity from phrase to phrase or even from note to note within a phrase.
Singers also use vibrato, although use isn't exactly the right word. Vibrato happens, is more like it. For the string player, vibrato is a tool, something applied and varied by choice. But for the singer, vibrato is simply a by-product of proper breathing and tone production. Singers don't purposely produce vibrato, in other words, and, except for those who specifically try to sing without vibrato for expressive or stylistic reasons, they usually don't vary it on purpose. In the words of Mozart, "The human voice already vibrates of itself, but in such a degree that it is beautiful, that it is the nature of the voice." Studies of great singers have shown that the average speed of a beautiful vibrato is between six and seven oscillations per second, although in musical passages that increase in volume and excitement the vibrato generally speeds up and widens.
When a singer's vibrato is too fast to be considered pleasing, it's called a tremolo, Italian for "trembling." (Tremolo has a different meaning for instrumentalists.) When it's too slow, too wide, or uneven in speed and/or width, it's usually called a "wobble." Ideally in both singing and string playing effective vibrato deceives the ear: we don't notice the actual oscillations in pitch, we just hear a beautiful, warm tone. Unpleasant vibratos make the vibrato noticeable, and it becomes a distraction. For singers, the culprit in tremolos or wobbles is usually vocal tension of some kind, caused by nervousness, fatigue, or improper technique, especially improper breathing technique (or inadequate "breath support," as it's called). Advancing age can play a part, too, although older singers who sing "right" in a relaxed, properly supported manner have far fewer problems than those who sing "tight."
Although woodwind and brass players can produce vibrato in a variety of ways, they tend to use the effect very sparingly. (Flutists are an exception. A certain amount of vibrato is an integral component of most flutists' sounds.) In general, jazz and pop wind players make much greater use of vibrato than their classical colleagues.
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