Vibrato is a slight fluctuation of pitch on a sustained tone. It is a kind of ornament used by both singers and instrumentalists to add expression. The variation in pitch is typically 4 to 6 Hz.
Vibrato may be natural but can also be taught. The best singers have full control over their vibrato and use it to accent certain words or phrases for dramatic or emotional effect. Vibrato rate can vary with such factors as vocal intensity, age, and condition of the singer, muscular interference and tension, and imitation of vocal sounds heard throughout life. Excessive vibrato produces what is often described euphemistically as a wobble warble, which is usually the result of bad singing technique.
Recent research indicates that the cricothyroid muscles and the thyroarytenoid muscles are the primary producers of vibrato, and that the movement of the thyroid and cricoid cartilages is the main source of vibrato.
String players produce vibrato by wiggling the left hand back and forth (the right hand holds the bow); wind players and singers use breath control.
The confusion between vibrato and tremolo is exacerbated by the misnaming of the "tremolo bar" (whammy bar) on some electric guitars, by which the tension of all strings can be altered simultaneously to bend entire chords up or down in pitch. This device, strictly speaking, should be called a vibrato bar.