Many thanks to http://www.music.vt.edu/musicdictionary/ for the Definition.Vibrato - The pulsating or vibrating element of some sounds that is produced by a full, resonant quality of tone. Vibrato is a very slight fluctuation of the pitch of a note; it was known as early as the 16th century, but until the 19th century it was used mainly as ornamentation. Since the 19th century, vibrato has been used almost constantly because of its enhancement of tone.
Vibrato is a left hand technique (assuming that you are playing right handed) that allows the fretless bassist to modulate the pitch of a note similar to the way a vocalist may use vibrato. This technique, when done well, permits you to impart a pleasing, and musical, characteristic into your music. Violin style vibrato enables control of micro-tonality, musical nuance, beauty, and emotive character. Characteristics that are, in my opinion, fundamental to the fretless bass as well as voice.
When done correctly vibrato also makes it easier to maintain intonation or pitch control. In essence vibrato is modulating the pitch above and below a perfect note in an oscillating manner, thus the ear hears the average, or center of your vibrato as the pitch you are playing even though the reality is that you are not maintaining a constant pitch. If your intonation is not quite perfect the human ear tends to select the correct pitch that is part of the vibrato (providing that you are at least close to correct pitch).
Good vibrato technique will make your playing more musical.
On string instruments there are basically two types of vibrato, violin type vibrato, and guitar vibrato. Violin vibrato is executed with a rocking or rolling motion of the intonating finger, where this rocking is alternately up-string, and down-string (toward sharp, and back toward flat, etc.). Guitar type vibrato is usually a sideways motion where the player pulls or pushes the string across the fret to change the pitch. Guitar vibrato changes the pitch by increasing the tension of the string when the player pushes or pulls it, and returning to normal tension when released. Guitar vibrato does not sound the same as violin vibrato because the pitch goes from perfect, to sharp, and back to perfect never modulating an equal amount on both sides of perfect (sharp and flat) as does violin vibrato.
A third type of vibrato is produced by a mechanical device such as a whammy bar or Bigsby vibrato tailpiece. Guitarists can also simulate violin vibrato by pushing on the back of the neck to induce tension change.
On the fretless bass vibrato technique comes directly over from the upright bass which is a large member of the violin family, thus the vibrato we usually use is violin type and not guitar type.
The basic mechanics of vibrato is to make your finger pad roll back and forth over the note. If you will press your right thumb against the fleshy pad of one your left fingers you should observe that by moving the thumb laterally (sideways) the pad of the finger is compliant and moves while the bone inside that finger stays motionless. Conversely, when you press a note against the fingerboard and consider the pad of your finger stuck there, moving the bone in that same finger flat and sharp causes the fleshy pad to roll between the finger bone and the fingerboard creating vibrato.
To start getting your vibrato down play a C note on your G string (5th position, using the 4th finger). Just holding down the note will not get you any vibrato, you need to add your personal touch to it. Roll the pad of your 4th finger toward sharp so the pitch increases a few cents. Now roll the pad of your 4th finger flat in opposite direction. Repeat this over and over until it becomes a continuous smooth back and forth rolling motion, plucking the string as needed.
Some players use their wrist to get this motion, others rock the entire forearm. Use that which works best for you and does not involve unnecessary motion.
The amount of pitch variation and frequency (rapidity) in your vibrato is something you decide to suit the music. Vibrato can be subtle or extreme. I tend to prefer a light vibrato, I use it often and to good effect to make the music more beautiful and maintain good intonation.
Sliding Vibrato:
If you desire a more dramatic effect, sliding vibrato where the intonating finger pad is NOT planted in a position rather slides up and down, will provide a greater excursion in pitch and a more extreme vibrato. Try sliding a note up and down fingerboard approximately an interval of one quarter-tone flat and sharp. Greater effect comes with using a wider sliding vibrato interval. Technically this is not vibrato but should be considered a form of Trill played using Glissando.
Vibrato with double and triple stops:
When playing double and triple stops proper intonation is slightly more difficult because you have to maintain two or three notes on pitch instead of just one. I find that vibrato helps tremendously when playing chordal passages but there is a new requirement, you have to add vibrato for each intonating finger AND keep them in proper position. Good pitch when playing chords is one the greatest challenges for the fretless bassist. Multi-finger vibrato is a powerful tool in the bag of tricks to get your chords sounding good and maintaining intonation.
Vibrato lesson:
Develop a nice violin style vibrato, work on varying the vibrato from slow/subtle/gentle to fast/nervous/irritating. There is a whole world of expression available.
Slowly play a scale where each note is executed using a gentle vibrato. Repeat this ascending and descending until you have command of the technique and can use it as desired.
Work on gaining control of the expression you are attempting to communicate, your mood, or the mood of the music, may be expressed through your fingers.
Learn when to use it, and when to turn it off. There are many vibrato expressions, try to get the entire range within your bag of tricks. Vibrato is most effective on long notes and ballads. Here's another Italian word for you - Legato. Vibrato and Legato are the best of friends. When playing fast tempo passages (Presto) or a fast walking bass, vibrato is usually not needed nor advantageous.
Once you have the basics this technique should come naturally, it is not difficult and yields a great benefit.
I do occasionally use guitar type vibrato, usually on a very low note where a side pull provides a distinctive sonic quality.