Sharon Isbin’s : Classical Guitar Vibrato Technique
Here’s a video I came across in which Sharon Isbin talks about how she uses and applies vibrato.
Vibrato is given to a note by applying and relaxing pressure to a string in a lateral direction. Lateral direction along the line of the string. The pressure towards the bridge with the hand motion coming towards the body will diminish the tension of the string thus resulting in a slight drop in pitch. When you apply pressure in the opposite direction, towards the head of the guitar away from the body, the string tension is increased thus increasing the pitch a little. When you do this motion back and forth fairly rapidly you achieve vibrato. The speed of your motion will determine how fast the vibrato is.
Sharon adds more pressure by applying more fingers to the note, which results in a bigger or wider vibrato, that is, more movement in pitch.
Vibrato helps to shape notes and adds to the expression of a phrase or passage. It also helps to round the pitch of the note by allowing the ear to hear an area of pitch instead of just a single pitch (or frequency). Vibrato works best in higher positions because the string tension is less as you play towards the middle of the string.
skip to 4’31” for vibrato technique