Vibrato

Vibrato 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 characterized in terms of two factors: the amount of pitch variation ("extent of vibrato") and speed with which the pitch is varied ("rate of vibrato").

It is with increasing frequency that singers write me and ask the question, "What is vibrato and how do I develop it in my voice?" Another question that arises is "Are there certain singers who just don't have that kind of sound in their vocal production? Can vibrato be developed?" I will try to address these questions and offer some thoughtful advice to those who wish to have vibrato defined. I will also give some concrete ideas on how to vocalize in such a way that vibrato is encouraged to develop.

I define vibrato as a "slight variation of pitch resulting from the free oscillation of the vocal cords". This free oscillation of the vocal cords results from
(a) an open pharynx or what many call the "open throat" along with
(b) healthy "closure of the cords" (see article on vocal cord closure) I consider that vibrato is a result of these two opposites working together: open throat and closed cords.
(c) Another major factor to be considered in regard to vibrato is the even sub-glottic breath pressure. This is regulated by the "support system" which involves the abdominal muscles, lower lumbar/upper gludial muscles, intercostal muscles and pectoral muscles. (See articles on breath and breath management).
Environmental Factor: As a child, I always had a vibrato. Whether developed through vocal freedom or imitation I do not know. I had an older sister who was an excellent soprano and used to sing a lot of opera and operetta in the home. I found great joy in imitating her vocal sound while I was still a boy soprano. I could match it quite well and was later asked to join the Texas Boy's Choir because I had a healthy and even vocal sound. Other cultures encourage different types of vibrato. For example, in some Asian cultures, a wide and slow vibrato is very desirable. Many French pop singers use a faster tighter vibrato. Neither of these examples represents healthy vocalism.

Vibrato Problems:
(1) The Vocal Wobble:
We often hear singers that have a wide and slow vibrato (see article on the vocal wobble.) The causes have been described in the article entitled "The Vocal Wobble". A wide vibrato is usually a lack of proper "resistance of the breath pressure" or a lack of "focus in the tone". It can also be a result of a lack of proper adduction of the vocal cords. One or all of these problems create a sound that our culture defines as "age in the voice". I have found that singers in their 20's can have a wobble. I have students in my studio who are in their 70's who have no sign of such a vocal characteristic. In fact, they have what our culture calls a youthful, and more importantly, healthy sound. I have found the primary cause of the vocal wobble to be misuse rather than age. An unhealthy vocal technique used over a short or long period of time can be the cause such a vocal problem. The solution is quite simple: vocalize exercises that require body support along with focus. The "ng" is a healthy sound that can help develop focus in the voice. The sustained "hiss" can help a singer learn what muscles to use in order to "hold back" the breath pressure or "support the tone".

(2) The Overly-Fast Vibrato:
Some singers have an overly fast vibrato that can be as disconcerting as the wobble. Neither the wobble nor the fast-vibrato is the desired vocal sound for healthy singing. A fast-vibrato can be caused by a number of vocal situations.
(a) Pressure at the root of the tongue. This pressure at the root of the tongue can have its origin at inhalation or at the attack or onset of sound.
(b) Lack of vocal cord approximation: Many singers who do not quite understand that the vocal cords must close after inhalation. This lack of proper adduction of the cords can result in a faster vibrato speed. If the vocal cords do not approximate closely enough, the vibrato can become faster depending upon the size and shape of the vocal cords themselves.
(c) Lack of support is another cause of this vocal problem. Most of us have heard singers with definite vibrato problems and we have experienced singers with healthy vibrato. One key factor in attaining a healthy sound is to be sure that the vibrato is vibrating at an even rate. An uneven vibrato can be caused by sudden changes in the sub-glottic breath pressure. This is caused by a lack of even "body resistance" or support in the body. The vocal cords then begin to separate and vibrate unhealthily. The result is an uneven vibrato sometimes accompanied by pitch problems. The fast vibrato is less noticeable if the rate of vibrato is even rather than sporadic.

(3) The Straight Tone:
So often I have singers who come into my studio with a straight tone (no vibrato). Some of these singers are not aware of vibrato or how it is developed in the voice. Many come into my studio with the express desire to develop vibrato in their sound realizing that their voice is lacking in that particular area. In over 25 years of teaching, I have never had a singer in my studio that could not develop healthy vibrato.
Some straight tone singers have sung in choirs where the director has demanded straight tone. This is potentially a damaging circumstance. Straight tone singing is extremely unhealthy for the voice. Vocal nodules can result from such vocal production because of too much pressure held at the glottis to prevent vibrato from occurring in the tone. Choral blend is developed through vowel and acoustical alignment, not squeezing the voice into straight tone sound. The proper vowel and acoustical alignment can create a beautiful vocal blend. I experienced this personally in Berlin when 11 of my singers sang "Let Your Garden Grow" from Candide as a finale along with a men's choir. The resulting vocal sound of these 11 singers who were trained on the "ng" ring was amazing to say the least. Because these singers were trained with vowel and resonance alignment, the resulting sound was one of beautiful blend of tone along with fullness of vocal sound and blend. Several conductors from German opera houses came to me after the concert to ask about the training of these singers.
Most straight tone singers cannot use the idea of vocal cord closure during the first part of their training because "too much pressure" has been held at the vocal cords for too long a time period. I have found that vibrato comes into the voice when the singer achieves proper balance in the "support muscles" and when the singer keeps the feeling of the "u" vowel in the pharynx. The Italian "u" is a crucial part of a singer's training in order for vibrato to occur. The "u" vowel allows a healthy adduction of the vocal cords without too much pressure at the glottis. It is understandable that this vowel is crucial in the Italian School. I find that the "u" vowel must be produced without the "bunching" of the back of the tongue and with a "high and wide soft palate" in order to be efficient acoustically. The result is beauty and resonance simultaneously. 

(4) Diaphragmatic Vibrato:
A diaphragmatic vibrato is the pulsating of the diaphragm during a sustained tone to "create" a false vibrato. Music theatre singers develop this damaging vocal habit in order to have some sort of vibrato when none is present in the tone. This is a huge mistake. A diaphragmatic vibrato is difficult to repair because the lower abdominal muscles memorize the pulsating sensation so deeply. This situation can be repaired with lots of time and hard work. Solution: Use the idea of the sustained "hiss" and memorize what the body "feels" during this function. Then sing a tone while keeping the same "feel" in the body. This will stabilize the shaking diaphragm.

(5) The role of the vocal trill:
In some cases, a singer can begin to awaken the vibrato function by using a trill. A trill is an educated yodel at the vocal cords that may or may not be easy for a singer to produce. Some connect with this idea and begin to release the "over-squeezing" of the vocal cords, therefore allowing for the development of a vibrato.
Healthy vibrato can be achieved in a rather short period of time. Usually the time factor is dependent upon the singer's mind/body coordination. Some singers have more of a connection to their body than others. I recently had a large-voiced baritone who began to develop some vibrato in his sound after only three lessons. I have had others who have taken much longer because of singing straight tone for so many years. At any rate, any singer can achieve a properly regulated and even vibrato with concentration, proper instruction, and by embracing the process rather than the result. Patience is a most important aspect while training and balancing vocal production.
The use of vibrato is intended to add warmth to a note. In the case of many string instruments the sound emitted is strongly directional, particularly at high frequencies, and the slight variations in pitch typical of vibrato playing can cause large changes in the directional patterns of the radiated sound. This can add a shimmer to the sound; with a well-made instrument it may also help a solo player to be heard more clearly when playing with a large orchestra

Typical rate and extent of vibrato
The rate and extent of the variation in pitch during vibrato is controlled by the performer. The extent of vibrato for solo singers is usually less than a semitone (100 cents) either side of the note, while singers in a choir typically use narrower vibrato with an extent of less than +/- a tenth of a semitone (+/- 10 cents). Wind and bowed instruments generally use vibratos with an extent of less than +/- half a semitone

Vibrato's use in various musical genres
Vibrato is sometimes thought of as an effect added onto the note itself, but in some cases it is so fully a part of the style of the music that it can be very difficult for some performers to play without it. The jazz tenor sax player Coleman Hawkins found he had this difficulty when requested to play a passage both with and without vibrato by the producer of a children's jazz album to demonstrate the difference between the two. Despite his technique, he was unable to play without vibrato. A symphony saxophonist was brought in to play the part.
Many classical musicians, especially singers and string players, have a similar problem. The violinist and teacher Leopold Auer, writing in his book Violin Playing as I Teach It (1920), advised violinists to practise playing completely without vibrato, and to stop playing for a few minutes as soon as they noticed themselves playing with vibrato in order for them to gain complete control over their technique.

In classical music
The use of vibrato in classical music is a matter of some dispute. For much of the 20th century it was used almost continuously in the performance of pieces from all eras from the Baroque onwards, especially by singers and string players. A drastic change in approach cannot be understood wholly without regarding the rise of notionally historically accurate ("period") performance from the 1970s onwards. However, there is no actual proof that singers performed without vibrato in the baroque era. Vocal music of the renaissance is almost never sung with vibrato as a rule, and it seems unlikely it ever was. There are only a few texts from the period on vocal production, but they all condemn the too frequent use of vibrato. However, it should be understood that "vibrato" occurs over a wide range of intensities: slow, fast, wide, and narrow. Most sources in condemning the practice seem to be referring to a wide, slow, perceptible oscillation in pitch, usually associated with intense emotion, whereas the ideal for modern vibrato, and possibly in earlier times as well, was to imitate the natural timbre of the adult singing voice, from which a measure of vibrato (it has since been shown) is rarely absent.

Leopold Mozart’s Versuch einer gründlichen Violinschule (1756), for example, provides an indication of the state of vibrato in string playing at the end of the baroque period. In it, he concedes that “there are performers who tremble consistently on each note as if they had the permanent fever”, condemning the practice, and suggesting instead that vibrato should be used only on sustained notes and at the ends of phrases when used as an ornament. This however, does not give anything more than an indication of Mozart's own personal taste, based on the fact that he was an educated late Rococo/Classical composer. Mozart acknowledges the difference between the heavy, ornamental vibrato that he finds objectionable, and a more continuous application of the technique less obtrusively for purposes of improving tone quality (in which case he does not refer to it as "vibrato" or "tremolo" at all; describing it merely an aspect of correct fingering). In this respect he resembles his contemporary, Francesco Geminiani, who advocated using vibrato "as frequently as possible" on short notes for this purpose. Although there is no aural proof, as audio recordings were not around for more than 150 years, that string players in Europe did not use vibrato, its overuse was almost universally condemned by the leading musical authorities of the day.

Certain typtes of vibrato, then, were seen as an ornament, but this does not mean that it was used sparingly. In wind playing too, it seems that vibrato in music up to the 20th century was seen as an ornament to be used selectively. Martin Agricola writing in his Musica instrumentalis deudch (1529) writes of vibrato in this way. Occasionally, composers up to the baroque period indicated vibrato with a wavy line in the sheet music. Again, this does not suggest that it was not desired for the rest of the piece any more than the infrequent use of the term in 20th century works suggests that it is not used elsewhere.

Vibrato wars
Music by late Romantic composers such as Richard Wagner and Johannes Brahms is now played with a fairly continuous vibrato. However, some musicians specialising in historically informed performances such as the conductor Roger Norrington argue that it is unlikely that Brahms, Wagner, and their contemporaries would have expected it to be played in this way. This view has caused considerable controversy. The view that continuous vibrato was invented by Fritz Kreisle and some of his colleagues is held to be shown by early sound recordings, which allegedly demonstrate that vibrato appeared only in the 20th century. Against this are cited sources which are said to prove that early 19th-century Viennese string players like Franz Clement and Joseph Mayseder were noted for their tasteful use of vibrato. These musicians (and the two Hellmesbergers) are said to represent the school on which Fritz Kreisler based his stylistic approach.

The alleged growth of vibrato in 20th century orchestral playing has been traced by Norrington by studying early audio recordings but his opponents contend that his interpretations are not supported by the actual samples. Norrington claims that vibrato in the earliest recordings is used only selectively, as an expressive device; the Berlin Philharmonic Orchestra were not recorded using vibrato comparable to modern vibrato until 1935, and the Vienna Philharmonic Orchestra not until 1940. French orchestras seem to have played with continuous vibrato somewhat earlier, from the 1920s.

Defenders of vibrato claim that the sonic limitations of 78-rpm recordings, particularly with respect to overtones and high frequency information, make an uncontroversial assessment of earlier playing techniques difficult (although, it must be said, early recordings of operatic singers manage to show clearly the extent to which a vibrato is present [or not] in their voices). In addition, the defenders of vibrato point out a distinction needs to be made between the kind of vibrato used by a solo player, and the sectional vibrato of an entire string ensemble, which cannot be heard as a uniform quantity as such. Rather, it manifests itself in terms of the warmth and amplitude of the sound produced, as opposed to a perceptible wavering of pitch. The fact that as early as the 1880s composers such as Richard Strauss (in his tone poems "Don Juan" and "Death and Transfiguration") as well as Camille Saint-Saëns (Symphony No. 3 "Organ") asked string players to perform certain passages "without expression" or "without nuance" strongly suggests the general use of vibrato within the orchestra as a matter of course.
Despite this, the use of vibrato in late Romantic music is still common, though challenged by Roger Norrington and others of the historically informed performance movement. Performances of composers from Beethoven to Arnold Schoenberg with limited vibrato are now not uncommon. Norrington caused controversy during the 2008 Proms season by conducting Edward Elgar's Enigma Variations, and the Last Night of the Proms, in non-vibrato style, which he calls pure tone. Some take the view that even though it may not be what the composer envisioned, vibrato adds an emotional depth which improves the sound of the music. Others feel that the leaner sound of vibratoless playing is preferable. In 20th century classical music, written at a time when the use of vibrato was widespread, there is sometimes a specific instruction not to use it (in some of the string quartets of Béla Bartók for example). Furthermore, some modern classical composers, especially minimalist composers, are against the use of vibrato at all times.

In opera
All human voices possess the capacity to produce a vibrato. This vibrato can be varied in width (and rapidity) through training. In opera, as opposed to pop, vibrato begins at the start of the note and continues to the end of the note with slight variations in width during the note.
Traditionally, however, the deliberate cultivation of a particularly wide, pervasive vibrato by opera singers from the Latin countries has been denounced by English-speaking music critics and pedagogues as a technical fault and a stylistic blot (see Scott, cited below, Volume 1, pp. 123–127). They have expected vocalists to emit a pure, steady stream of clear sound — irrespective of whether they were singing in church, on the concert platform, or on the operatic stage.

During the 19th century, for instance, New York and London based critics, including Henry Chorley, Herman Klein, and George Bernard Shaw, castigated a succession of visiting Mediterranean tenors for resorting to an excessive, constantly pulsating vibrato during their performances. Shaw called the worst offenders "goat bleaters" in his book Music in London 1890-1894 (Constable, London, 1932). Among those censured for this failing were such celebrated figures as Enrico Tamberlik, Julián Gayarre, Roberto Stagno, Italo Campanini and Ernesto Nicolini—not to mention Fernando Valero and Fernando De Lucia, whose tremulous tones are preserved on the 78-rpm discs that they made at the beginning of the 20th century.

The popularity of an exaggerated vibrato among many (but by no means all) Mediterranean tenors and singing teachers of this era has been traced back by musicologists to the influential example set by the early-19th century virtuoso vocalist Giovanni Battista Rubini (1794–1854). Rubini had employed it with great success as an affecting device in the new Romantic operas of Gaetano Donizetti and Vincenzo Bellini. A host of young Italian tenors—including the renowned Giovanni Mario (1810–1883) — copied Rubini's trend-setting innovation in order to heighten the emotional impact of the music that they were singing, and to facilitate the delivery of fioritura "by, as it were, running up and down the vibrato" (to quote Scott; see p. 126).

Prior to the advent of the charismatic Rubini, every well-schooled opera singer had avoided using a conspicuous and continuous vibrato because, according to Scott, it varied the pitch of the note being sung to an unacceptable degree and it was considered to be an artificial contrivance arising from inadequate breath control. British and North American press commentators and singing teachers continued to subscribe to this view long after Rubini had come and gone.

Accordingly, when Enrico Caruso (1873–1921) — the most emulated Mediterranean tenor of the 20th century — made his acclaimed New York Metropolitan Opera debut in November 1903, one of the specific vocal attributes for which he was praised by music reviewers was the absence of a disruptive vibrato from his singing. The scholarly critic William James Henderson wrote in The Sun newspaper, for example, that Caruso "has a pure tenor voice and [it] is without the typical Italian bleat". Caruso's gramophone recordings support Henderson's assessment. (Other prominent Mediterranean tenors of the late 19th century-early 20th century who, like Caruso, did not "bleat" were Angelo Masini, Francesco Tamagno, Francesco Marconi, Francisco Viñas, Emilio De Marchi, Giuseppe Borgatti and Giovanni Zenatello, while the phenomenon was rare among French, German, Russian and Anglo-Saxon tenors of the same period—see Scott.)

The intentional use of a pronounced vibrato by Mediterranean tenors is a practice that has died out over the course of the past 100 years, owing in no small measure to Caruso's example. The last really important practitioners of this style and method of singing were Alessandro Bonci (in the 1900-1925 period) and Giacomo Lauri-Volpi (in the 1920-1950 period). Both of them featured bel canto works, dating from Rubini's day, in their operatic repertoires, and both of them can be heard on recordings which faithfully capture the distinct shimmer inherent in their timbre.

Italian or Spanish-trained operatic sopranos, mezzo-sopranos, and baritones exhibiting a pronounced vibrato did not escape censure, either, by British and North American arbiters of good singing. Indeed, Adelina Patti and Luisa Tetrazzini were the only Italian sopranos to enjoy star status in London and New York in the late-Victorian and Edwardian eras, while such well-known compatriots and coevals of theirs as Gemma Bellincioni and Eugenia Burzio (among several others) failed to please Anglo-Saxon ears because, unlike Patti and Tetrazzini, they possessed unsteady, vibrato-laden voices—see Scott for evaluations of their respective techniques. To give an additional female example from a later date, whenever the vivacious mezzo-soprano of the 1920s and '30s, Conchita Supervia, performed in London, she was admonished in print for her exceedingly vibrant and fluttery tone, which was unkindly likened by her detractors to the chatter of a machine-gun or the rattle of dice in a cup.

In 1883, Giuseppe Kaschmann (né Josip Kašman) — a principal baritone at La Scala, Milan—was criticised for his strong vibrato when he sang at the Met, and the theatre's management did not re-engage him for the following season, even though other aspects of his singing were admired. (Kaschmann never performed in Great Britain but he remained a popular artist in the Latin countries for several decades; in 1903, he made a few recordings which exhibit only too well his perpetual flutter.) Similarly, another one of Italy's leading baritones, Riccardo Stracciari, was unable to turn his pre-World War I London and New York operatic engagements into unambiguous triumphs due to an intrusive quiver in his tone. He subsequently moderated his vibrato, as the discs that he made for Columbia Records in 1917-1925 show, and this enabled him to pursue a significant career not only in his homeland but also at the Chicago opera.

There is another kind of vibrato-linked fault that can afflict the voices of operatic artists, especially ageing ones—namely the slow, often irregular wobble produced when the singer's vibrato has loosened from the effects of forcing, over-parting, or the sheer wear and tear on the body caused by the stresses of a long stage career.

Techniques for producing vibrato
Not all instruments can produce vibrato, as some have fixed pitches which cannot be varied by sufficiently small degrees. Most percussion instruments are examples of this, for instance the xylophone.

Keyboard instruments
Some types of organ however, can produce the effect by altering the pressure of the air passing through the pipes, or by various mechanical devices (see the Hammond or Wurlitzer Organs for example). The clavichord, though technically a fixed-pitch keyboard instrument, is capable of producing a type of vibrato known as Bebung by varying the pressure on the key as the note sounds. Some digital keyboards can produce an electronic vibrato effect, either by pressure on the keys, or by using a joystick or other MIDI controller

String instruments
The method of producing vibrato on other instruments varies. On string instruments, for example, the finger used to stop the string can be wobbled on the fingerboard, or actually moved up and down the string for a wider vibrato.
Many contemporary string players vary the pitch from below, only up to the nominal note and not above it, although great violin pedagogues of the past such as Carl Flesch and Joseph Joachim explicitly referred to vibrato as a movement towards the bridge, meaning upwards in pitch, and the cellist Diran Alexanian, in his 1922 treatise Traité théorique et pratique du Violoncelle, shows how one should practice vibrato as starting from the note and then moving upwards in a rhythmic motion. In a 1996 acoustic study by the Acoustical Society of America, along with Wellesley College and the Massachusetts Institute of Technology, found that the perceived pitch of a note with vibrato "is that of its mean", or the middle of the fluctuating pitch.

The guqin, a Chinese bridgeless zither, has documents describing over 25 different types of vibrato that can be executed. Most peculiar is the vibrato ting yin (literally "still vibrato"); ancient manuals state that the finger on the left hand that is pressing the string should only move or rock ever so slightly so as to alter the pitch minutely, and some manuals say that the finger should not move at all but let the pulse of the finger do the vibrato.

Wide vibrato, as wide as a whole-tone, is commonly used among electric guitar players and adds a vocal-like expressiveness to the sound. This effect can be achieved both by the movement of fingers on the fretboard and by the use of a tremolo arm, a lever that adjusts the tension of the strings.

Wind instruments
Players of wind instruments generally create vibrato by modulating their air flow into the instrument. This may be accomplished either through stomach vibrato, the pulsing of the diaphragm slightly up and down, or throat vibrato, a variation of vocal chord tension to manipulate air pressure as singers do. Players of other instruments may employ less common techniques. Saxophonists tend to create vibrato by repeatedly moving their jaw up and down slightly. Clarinet players rarely play with vibrato, but if they do, the saxophone method is common because of the similarity of the saxophone and clarinet mouthpieces and reeds.

Brasses
Brass instrument players may produce vibrato by gently shaking the horn which varies the pressure of the mouthpiece against the lip. Alternatively, the embouchure can be rapidly altered, essentially repeatedly "bending" the note. On a trombone, a player may provide a slightly more pronounced vibrato by gently moving the slide back and forth, centering on one note to give a lyrical effect.

Auto-vibrato
Some instruments can only be played with constant, mechanical vibrato (or none at all), notably electric organists using a Leslie speaker. Vibrato on the theremin, which is a continuously variable-pitch instrument with no "stops", can range from delicate to extravagant, and often serves to mask the small pitch adjustments that instrument requires

David Bowie has a fast one. Mary J. Blige has a slow, sultry, one. Maynard James Keenan doesn’t have a trace of one. Singing with vibrato is a matter of taste. Having a choice is a matter of control. For most singers, the subtle, rhythmical movement of vibrato feels more like fate. When you want vibrato, it hides on you; don’t think about it, and it shimmers on the end of a note. Vibrato brings vitality to a voice. Sound without variation is boring. Compare a refrigerator to a fly buzzing around. The steady hum of the compressor quickly becomes background noise while the bug gets harder to ignore. With the exception of rappers and singers like Beck who don’t sustain notes, those without vibrato tend to rely on overdrive to create excitement. This often leads to blow outs. The more vocal colors available on your pallet, like vibrato, breathy, nasal and gritty, the easier it will be to paint an interesting portrait of a song without killing yourself.

The mechanics of vibrato are simple and reflexive, which is what makes it so elusive. Picture the fret hand of a guitarist sustaining a note. The finger movement alters the length of the string creating a slight waver in pitch. Things are just a little more complex with the voice. Like a stringed instrument, the tension of the vocal folds is varied rhythmically, creating movement in pitch. Along with this tension change, though, is a variation in the thickness of the vocal fold. The combined movements of pitch, volume and tone are what set vibrato apart from tremolo (change in volume only) and wobble (change in pitch only).

Tension squashes vibrato. Not just the obvious neck bulging stuff, but subtle everyday stiffness can neutralize it as well. Like the freedom required to wiggle your finger when sustaining a note on guitar, vocal vibrato requires muscle independence. Backing off the air pressure is the first step to releasing your voice. Let the ability to produce vibrato be your guide. Lay down flat on your back and place your hand on your belly button. Breath so that your hand rises and falls. Now sing a comfortable note and look for the presence of vibrato. If the pitch is stiff notice what your abs are doing. Are they contracting to drive the note? Check the behavior on various pitches. If you push too much from your stomach, the muscles surrounding the larynx will brace and vibrato will be lost. Reduce the volume and try again. The goal is to reduce the air pressure to the point where flexibility is found. Don’t be alarmed if this only happens at very low volumes. With practice, you’ll be able to increase the volume without loading the neck with pressure. Strike the proper balance during a song and vibrato will blossom. That’s why it tends to come in at the ends of notes; once we feel safely on pitch, we ease off the pressure a bit. 

Another check for vibrato-eating throat tension is to rotate your head in a small circle when singing. Pretend you are tracing the outline of a quarter with your nose. Does the rotation stop when you begin to sing? Is it stiffer on high notes? Again, reduce the volume until you find the correct air pressure. Neck tension is not a requirement of singing loud or high. We often see singers so locked up in the neck that they literally have to shake their heads or jaws in order to create vibrato. In the same way, a guitar player who needs to shake the guitar to move a note must be applying a death grip on that fretboard. There’s nothing wrong with using force to make a strong statement. Too often, though, the statement it makes is that we are overcompensating to mask weakness. Be brave and do the dirty work in private. Use vibrato as your guide and discover the power within. 

How To Develop An Outstanding Singing Vibrato
You see, when vibrato is flowing effortlessly in your voice it means that something good is happening. Vibrato is a sign of good vocal technique. Plus it sounds fantastic. he chances are that once you master your singing vibrato, you will be able to connect with all areas of your voice, low and high. And your voice will most likely sound very balanced as well.
So where does vibrato come from, and how can you develop an effortless one? Let's explore this further. Vibrato happens when your voice shifts quickly between two pitches. So if you were to sing a middle C, and then a C# and then a C again, followed by a C# followed by a C, you would be singing with vibrato. The key to a great vibrato is the speed in which you change between the two pitches. If you can oscillate between the two pitches around six times per second, you will have a very natural sounding vibrato. 

Let's Do An Exercise To Get Your Singing Vibrato Happening!
Here is a simple exercise that will allow you to see what singing vibrato feels like. Please understand that this is not a finished sound. It's an exaggerated version that will later be honed into something more usable. 

Here's how to do the exercise.
1. Place your hands at the bottom of your chest and feel where your ribs come together in the middle. Now move your hands slightly below this point. (it's the soft area a few centimeters above your belly button)
2. Now sing a note on one pitch in your easy range. Any note will do.
3. As you're singing this note, push in gently with your hands. The key is to push in, then pull out, and push in, then pull out and so on. Try doing this at a rate of around 3 to 4 cycles a second. 

Listen to how your voice wavers in a singing vibrato like fashion. Now it probably sounds a little exaggerated at this point, but this exercise is just to get things going. There is a home study course that includes many highly effective exercises that will help you develop an amazing vibrato. You will also learn how to increase your vocal range, improve your tone quality, and gain greater control over your voice.


Source :
http://en.wikipedia.org/wiki/Vibrato
http://www.voiceteacher.com/vibrato.html
http://www.voicelesson.com/html/lessons/free_lessons_07.htm
http://www.become-a-singing-master.com/singing-vibrato.html