Vibrato
Vibrato (Italian, from past participle of "", to vibrate) is a musical effect consisting of a regular, pulsating change of pitch. It is used to add expression to vocal and instrumental music. Vibrato is typically characterised in terms of two factors: the amount of pitch variation ("extent of vibrato") and the speed with which the pitch is varied ("rate of vibrato").
In singing it can occur spontaneously through variations in the larynx. The vibrato of a string instrument and wind instrument is an imitation of that vocal function.
The terms vibrato and tremolo are sometimes incorrectly used interchangeably, although they are properly defined as separate effects with vibrato defined as a periodic variation in the pitch (frequency) of a musical note, and tremolo as a fast repetition of the same note (usually a semiquaver) in order to produce the audible effect of a longer note, especially on instruments which do not have the ability of producing long sustained notes, such as the guitar. In practice, it is difficult for a singer or musical instrument player to achieve a pure vibrato or tremolo (where only the pitch or only the volume is varied), and variations in both pitch and volume will often be achieved at the same time. Electronic manipulation or generation of signals makes it easier to achieve or demonstrate pure tremolo or vibrato.
A Leslie speaker (best known through its historical and popular association with the Hammond organ) creates vibrato as a byproduct of tremolo production. As a Leslie speaker is moved by the rotating mechanism on which it is mounted, it moves closer to or farther away from any given object (such as a listener's ears) not also mounted on the mechanism. Because amplitude varies directly with sound pressure (A = k1P) and sound pressure varies directly with distance (P = k2d), such that amplitude also varies directly with distance (A = k1(k2d) = k1k2d), the amplitude of the sound as perceived by the listener will be greatest when the speaker is at the point in its rotation closest to the listener and least when the speaker is farthest away. Because the speaker is constantly moving either toward or away from the listener, however, the mechanism's rotation is constantly affecting the listener-perceived sound's wavelength by either "stretching" the wave (increasing wavelength) or "squeezing" it (decreasing wavelength) — and because frequency, i.e., pitch, is inversely proportional to wavelength, such that increasing wavelength decreases frequency and vice versa, any listener for whom the speaker's motion changes the sound's perceived amplitude (i.e., any listener whose distance from the speaker is changing) must also perceive a change in frequency. However, the size of this effect is likely to be tiny compared against the tremolo effect since the distance oscillation is very small.
...
Wikipedia