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Helping a Singer with Vibrato

Anyone out there have success with methods to help singers "get" vibrato? I'm working with a very musical man right now who has sung in choirs for years, but has never had voice lessons. His voice is seemingly "held" in a straight tone mode.
My understanding of vibrato is that it will happen naturally, if the singer's vocal mechanism is relaxed and the air flow is steady. Therefore, I've been working with him on breathing, support, and relaxation. He's still singing "straight," and his pitch is quite often flat as well.
Any magic fixes out there?
Replies (34): Threaded | Chronological
on May 20, 2010 1:47pm
Hi, Tom.  In a word, no.  No magic fixes.
What you quote is an article of faith among voice teachers (well, among bel canto voice teachers I should say, since jazz singers and jazz coaches use a  very different approach), and one that my son, who can sing a perfectly beautiful straight tone when needed, agrees with after having spent many years working on it.
I, on the other hand, had no natural vibrato even though I studied with very good voice teachers, and since a single exception negates any generalization ... !
There are also some singers whose natural vibrato is a fast flutter, and who can NOT change it by working on breath support because that isn't the problem, it's just their natural vibrato.  So there are exceptions to every "rule."
The only suggestion I have is to help him find a way to produce a vibrato when it's needed, if standard voice teaching doesn't have that result.  That's what I had to do.
on May 20, 2010 6:12pm
Hi, John. Good to hear from you.
  • What is the different approach which jazz folks use?
  • Is that "fast flutter" you describe also called tremolo? That's the name I know which describes a forced or artificially controlled 'hyper' vibrato.
  • How did you learn to produce a vibrato?
All my best,
on May 20, 2010 8:42pm
Hi, Tom.  The jazz approach I'm thinking of is to start an extended note with a straight tone and then bring the vibrato in as an ornament to beautify it, in much the same way that baroque singers and players apparently did.  There's no question but that vibrato was used, but much more by soloists than by ensemble players, whose intonation it would mess up.  And I believe that vibrato was sometimes listed in ornament tables along with trills, as an occasional ornament but not a continuous one.  In fact some writers wrote AGAINST a continuous vibrato, which tells us (a) that some folks used it, and (b) other folks thought it ugly!  I believe that many sensitive jazz vocalists use vibrato in exactly the same way, just as they learn to use a microphone as an important extension of their vocal production.  Sarah Vaughn had a mic position for every vowel at every volume level, and made the whole thing look like planned and graceful choreography. 
There's a vibrato that does not disturb the pitch, which is what I believe my son uses, and which is NOT the vibrato made infamous by opera singers whose jaws wobble as they sing!  If it isn't a pitch vibrato it can only be an intensity vibrato, which I THINK is what Kentaro is describing.
Yes, the fast vibrato I'm thinking of could well be called a tremolo, but I've known singers for whom it WAS their natural vibrato, and not forced or artificially controled in any way.  All one needs to do is listen to some World Music to realize that the bel canto type of vibrato is NOT the only kind, and that it would be considered rather gross in some cultures.  (I.e., listen to some of the Bulgarian women's choirs!)
And I really can't remember HOW I learned vibrato.  I just started imitating one, and was able to bring it in or out pretty much at will.  I think the clearest argument against "all good singing produces a bel canto vibrato" is the simple fact that many children, with perfectly healthy vocal production and nothing forced at all, do not have a natural vibrato.  My wife heard our older daughter trying to make one, when she thought no one was listening.  In fact that's so true that a child with vibrato almost sounds odd and too "grown up"!  But I'm not a voice teacher (because I know enough to understand how much I do NOT know!), so you can safely ignore my opinions.
All the best,
on May 20, 2010 9:55pm
Love the last comment, John!  I wish more choral leaders were humble about what they don't know as voice teachers - or about what they have successfully applied to themselves and then expect to have it work for everyone.  The art of "teaching singing" is a delicate and individually focused one....
on May 20, 2010 5:45pm
Hi, Tom!
I kind of figured out by myself how to do and controle vibrato when I am 17.
This is not a magic fix, but I usually have pretty good results from students whom I teach  vibrato excersise of flute.
I believe that it helps students to experience how air flow of vibrato feels. I then teach  how to maintain the flow of air when vocalizing. Relaxing is the key here, since it is impossible to maintain the air flow of vibrato and vocalizing at the same time, if a singer has very tight muscles around the throat.
A lot of people have misunderstood that air flow of vibrato is steady (like the same amount of air always).  What is required is steady frequency of change of air flow and steady "support." 
For your male student case, I kind get the feeling that he also doesn't know how to do falsetto (not to be mistaken with the sounds made by false vocal folds).  Am I correct?
on May 20, 2010 6:16pm
Hello Kentaro!
Thanks for your reply.
Could you share a little more about your methods of teaching vibrato? I never learned that it's a "steady frequency of change of air flow," nor do I experience myself controlling it as such. I'd love to know more about your approach.
RE falsetto, I'm not sure if he knows how to produce it. But then, I'm not sure I understand what you mean by the term. Do you mean the high part of a traditional yodel, or something else?
All my best,
on May 21, 2010 2:39am


I learned to play a lot of instruments since it is very beneficial for composition. And while I was studying wind and brass instruments, I found that I could apply singing vibrato technique to flute most naturally since it is air-reed instrument and the air-pressure required in the low-to-mid range is most similar to the voice among the wind instruments. I also noticed that no flute (or wind) players would produce very fast vibrato or tremolo we hear in some singers. And I noticed that singers who can sing with good vibrato can almost always whistle with vibrato too.

There are exercises that one can do to learn vibrato on flute. I thought that I could use them for singers, and they seemed to work pretty well.

When we sing with "vibrato" (which usually considered as periodical slight changes of pitch), it will naturally accompany with periodical slight changes in volume and this comes from periodical slight changes in air flow.

Flute method basically doesn't consider the pitch element first (well it is flute!). The player will learn how to do periodic changes of air from about 2 cycle/sec, 3/sec 4/sec, 5/sec, 6/sec etc by as if they are whistling (breath only though). Beginners usually do changes as if there is very awkward on/off switch, but when they practice, they should be able to do as if there is a nice smooth slider.

At this point, the flute players start using real instruments, but for singers, it is time to activate vocal folds. I found the nice round closed "Oo" at soft dynamics in mid-range would be a good choice because air pressure is most similar to the previous practices. If the singer is male falsetto would be better.

If the singer is at the level so that he can sing with relaxed throat, this periodic changes of air flow learned in flute method seems to naturally react or change the vocal folds in a way so that they produce vibrato (pitch change).

This usually produces very "classical" or gentle type of vibratos. I found that from this point on, learning different types of vibratos like operatic, jazz, etc would be easy for singers.

This flute method approach works well and, singers seem to learn how to turn vibrato on and off, and how to change the cycle rate. Singers also seem to learn how to use vibrato without the helping of wobbling jaw. I found that this works pretty well for fixing for those who have tremolo or rather uncontrollable vibrato.

...About Falsetto

Falsetto is a certain way of using vocal folds. Normal voice (or modal voice or range) uses vocal folds fully, but the falsetto uses only the tip of vocal folds.

The important thing to remember is that we human beings have two vocal producing organs. One is the vocal folds, and the other is the false vocal folds which are located a bit above the vocal folds. Although it is difficult, it is possible that we could use both organs and create two distinct sounds, for example,

The problem is that sometime falsetto (which is produced by vocal folds) sounds very similar to sounds which are produced by false vocal folds (I called false sounds).

For example, this is falsetto without vibrato

And this is same pitch produced by false vocal folds.

So, singers, especially male singers, sometimes learned to sing false sounds instead of falsetto. This causes problems because correct falsetto can move from and to modal range without break since they are produced by same organs, but false sounds can't. Additionally, if one can do falsetto, he could move very wide range (with modal range) without break like

But, if he uses false voice, there will be a break since he would be switching actual organs that produce sounds.

Sorry, this is getting so long and my writing is unclear… Gee…

on May 21, 2010 5:52pm
Thanks, Kentaro. Most interesting to listen to those sound clips! I'm going to do some more research on falsetto, as well -- thanks for the inspiration there. All my best, Tom
on May 21, 2010 3:04am
I learned to sing with vibrato when I was an advancing flute player.     There's a point in wind instrument playing that you simply can't go on to the more challenging works without having a well moderated and controlled vibrato.   Even disregarding the musical aspects, you'd run out of breath.  
 The catalyst which helped me was a book by James Galway, the brilliant Irish flute player, simply called Flute.    The book contains a small section on vibrato and explained very clearly how to approach it.     My memory is going back over 20 years, but I believe he started with a couple of bars of quarter notes on one note, which are played or sung on one vowel emphasising each note but not breaking between them.    Then he doubled that to eighth notes, then doubled that to 16th notes.       In singing, you start at about M.M = 80 for the quarter notes.   By the time you are singing 16th notes you are getting the feel of what vibrato is.    It's just a start, but the synchronization with a metronome was a real light globe moment for me.   
Hope this helps, Tom.    It would be especially good if you could read what Galway originally wrote in case it's become warped in my mind over the years!
on May 21, 2010 6:30am
1.  Make sure the air is flowing freely--no obstructions (minimize tension in the jaw and neck), no "controlling" it.
Once you're sure of that, try
2.  Having him shake his hand (hand loosely open, fingers pointing up) very quickly back and forth in the air next to his face--in a rapid rotating motion, like jazz hands.  (It's easier to demonstrate than to describe.)  Often the movement of the hand or hands will take the singer's focus away from the desire to hold the larynx in place, and a natural vibrato will come in.
The trick is to free the mechanism as much as possible.
I hope that helps!
on May 21, 2010 5:58pm
Thanks, David. I'll try this hand-shaking technique.
All my best,
on May 21, 2010 7:05am
 Dear Tom,
Over the last thirty years, I have had the opportunity to address the use of vibrato in the voice studio and in choral situations.  Vibrato is one of those expressive tools that is applied based on stylistic or historical context.  Opera choruses singing 19th and 20th century productions use vibrato.  Madrigal groups tend to favor straight-tone singing.  Pop singers add and subtract vibrato.  A great comparison of vibrato usage can be found in listening to the singing styles of the Anglican boy choirs of Britain as opposed to the Vienna Boy Choir.  The former sing with straight-tone production, while the latter uses vibrato.  Finally, vibrato is essentially a western, high art technique.  Vibrato is not natural.  It is learned, though usually unconsciously, by 'osmosis.'  Traditional singing styles in Asia, Africa, and other non-western cultures are generally devoid of vibrato.
Like instrumentalists, trained singers should be able to add and subtract vibrato, even speed up or slow down (within the bounds of good taste) according to the needs and appropriate context of the music.  I have developed a step-by-step process for learning to add vibrato, or to learn to control a vibrato that has degenerated to a wobble.  John Howell is right in pointing out that vibrato is an oscillation in amplitude, not pitch.  If anyone is interested in receiving this process, I will be happy to send it to them.
on May 21, 2010 7:09am
At the other end of the scale, let us not forget the singers in Japan, who from childhood have been encouraged to think that vibrato was the only way to go.
(in traditional Japanese singing it's called 'kobushi', and it's good; the only problem is, that you can't do both -- I tried it and it just doesn't work. )
But I am still on excessivly friendly terms with the Sumo Jinku group who were doing it the other way -- can you guys imagine what it is like to get out of your home station after a loong day's work, meet a small group of middle-aged men dressed in parrot colours -- and be greeted by name!)
This advice passes virtually all Japanese singers of an older generation. In the choirs of the two churches on the same site, we arrange jont services.

But the person we have to avoid at all costs is the extremely talented Japanese lady, whose vibrato has widened so far that to sing beside her is like singing
with someone who is sobbing at an extremely slow pace.

The times, they are a-changing -- and unfortunately it's not a generational thing...

on May 21, 2010 9:16am
>>let us not forget the singers in Japan, who from childhood have been encouraged to
>>think that vibrato was the only way to go.
Yes, for those in 60s and up who grew up with Enka or Minyo... :)
Technically, kobushi includes many different things related to pitch ornamentation and tone quality. So, one usually can't get good overall kobushi, if s/he tried to imitate the aspects which could be recognized as similar to vibrato. 
on May 21, 2010 5:57pm
Thanks so much to all who replied!
I find the notion that vibrato is an unnatural phenomenon that requires training or emulation absolutely fascinating, and am going to do more research on that. For those who learned vibrato as a conscious manipulation of air flow, I'd love to learn more. I'd also be very interested in any particular method that you've devised.
Feel free to contact me directly, but I have a feeling that others would enjoy reading your techniques as well....
All my best,
on May 22, 2010 1:00pm
The Thought Plickens...
I did some googling, and every supposed expert and site I could find says that vibrato will naturally occur if the support is present and the tension is not. Is this another area where 'absolute truth' eludes us? I must confess, my own understanding of vibrato (and singing experience) resonates with what I read. I can "pull" a straight tone and do the jazz thang that John and Robert spoke of, but the only way I can NOT sing with vibrato is if I consciously add tension to my voice.
Interesting! More reading about to commence.
on May 22, 2010 2:23pm
Tom et al.
Just got back from playing a wedding (string quartet; we were the hi-class music; much of the rest was some kind of New Age and I didn't recognize a single song!).  There were two singers on the gig, very different voices, very different vibratos, perhaps appropriate to our discussion.
Singer No. 1:  Female, probably has had voice lessons at some point but not an operatic voice.  Very natural vibrato, pleasing, pleasant to hear, no problems.  And I could NOT tell where it was coming from or how it was produced!  No pitch modulation that I could hear, and I listened very closely.  Just a natural, unoforced color added to the tone.  No feeling of pulsation, either.  AND, while she may have been chanelling Sandi Patti or someone, the words "How Beautiful" kept coming back, sung on a rising 5th with the last syllable held out, and she started that syllable with straight tone and brought in the vibrato very tastefully.  Didn't sound studied, just, well, NICE!
Singer No. 2:  Male, obviously no voice lessons, ever!  Some of the most distorted vowels I've heard in a long time!  "Ah" modified towards "Oh," "Ay" modified towards "Ee."  We're in S.W. Virginia, but that was no SW Virginia accent, it was just distortion probably caused by severe muscular tension.  And the vibrato!  When he lost breath support--which was whenever he sang low or softly or got short on breath--it was a definite pulsation--amplitude modulation writ large.  When his breath support kicked in, which was when he sang loud or sang high, it wasn't as obnoxious, but still clearly amplitude modulation rather than frequency modulation.  Did NOT sound "nice," but is a big voice and therefore probably impressive to many in the audience.  (And he played piano the way he sang, trying to overpower the piano and overplay--Contemporary Christian with insufficient piano lessons.)
My conclusions?  Not really helpful.  I can tell a nice vibrato when I hear it, and it enhances the vocal sound.  But I already knew that!  What I CAN'T figure out is how the different kinds of vibrato are actually produced, and until we can identify those types and talk about them intelligently we don't even know whether we're on the same page!!
All the best,
on May 22, 2010 8:22pm
This is all very interesting. Could it be that the people who don't have vibrato -- or those who consciously manufacture it through some kind of muscular pulsation -- believe that it's manufactured. And those who produce it as a result of support and relaxation believe that it's a natural by-product of the same?
I did attend an interest session at an ACDA convention in the last couple of years, at which the two presenters made the claim that it was an acoustic phenomenon rather than a physiological one. I remember saying to myself, "Wait a minute, when I play recorder the sound has vibrato -- the same frequency as when I sing. So it must be related to breath and not sound waves...."
I repeat -- this is all very interesting.
on May 22, 2010 9:32pm
I know that in the (North American) oboe world, at least, there are two schools of thought about vibrato -- one (Robert Bloom/Ray Still et al.) that it must be natural; the other (if I'm not mistaken, John Mack et al.) that it is intentionally structured, and even measured.  Both schools derive from Marcel Tabuteau, so there appears to be a quasi-Talmudic question of origin.  Seems to me that the prevalent British school is also a deliberate vibrato.
Best regards,
Jerome Hoberman
Music Director/Conductor, The Hong Kong Bach Choir & Orchestra
Principal Conductor, Baguio Cathedral International Music Festival (Philippines)
on July 25, 2010 1:16pm
Someone has said, "Straight singing is like a DVD on 'pause'!"  I agree that it is static. Much college choral singing is static, in MY OPINION.
Another extreme is the over emphasis on the beginnings of notes and the mindless* disintegration that follows. Vibrato is a natural recurrent in proper/efficient phonation. Briefly, I would suggest simple flexibility vocalises, such as: two sixteenth notes ascending to a quarter on ah. **(doh re mi).  Then, on ah- sol doh, doh sol ,sol doh, doh sol, Then twice as fast sing s d s d s d s d s___. The latter like a wide trill. Also, have him quasi trill a minor third with no perceivable movement or adjustment of the throat area. It is unnecessary. These might loosen the setness that causes the straightness.
Have you noticed that the straight vocal production has little flexibility, and for expression relies largely on note values and intervallic changes- mesa di voce,
  being quite unavailable.
Any tinkering, trying to induce vibrato  for its own value is a waste of time, in MY OPINION. The above will give you a useful tool, flexibility!
For your best,
*By mindless, I simply mean inattentive. **Staccato (separate) then legato, but leggere.
on July 25, 2010 6:02pm
Hi, Clarence.  You're picking up on an old thread, but it remains an interesting discussion.
I know that when you say, "Vibrato is a natural recurrent in proper/efficient phonation," you believe it to be true, and I accept that it may indeed BE true.  But perhaps only of a certain kind of "proper/efficient phonation," which happens to coincide with the Western European idea of good bel canto vocal production.  But that is patently not the only kind of vocal production possible, and certainly not the only kind used in various styles in various parts of the world.  And of course most childrens' voices--which I hope we can agree are examples of healthy vocal production although not TRAINED vocal production--do not have a natural vibrato.
What we can probably agree on is that there are various kinds of vibrato, that some voices have it naturally and other voices (mine, for instance) do not, and that it can vary from a kind of coloration that can enhance a voice to an exaggerated wobble that should be banned from performances!  And that some singers can control it, and even turn it on and off, while others cannot and are at the mercy of their own "natural" vibrato.
But simply assuming that a non-vibrato tone means tension simply can't be supported, at least not as ageneralization.  Every voice is different, and so every vibratois different.
All the best,
on July 26, 2010 7:53am
Hello John, Tom, too.
Your emphatic response to the little definition I offer is overwhelmingly negligent of the solution offered, that of flexibility. The flexibility of which I speak and possess allows one to do many things that the straight singer cannot. I have a dry- voiced bass in my little church choir who has increased his range by more than a perfect fourth by use of my suggested routine (plus other instruction) to Tom. It may even wok for, well, whoever. Oh, and count your blessings if you have a well-reading steady bass in your choir, Tom!
I presume that Tom is working in the English language as appreciated by most of this country, so far, and that his subject is in that language; therefore we are dealing in the Western trdition.
John, a child's vocal apparatus is too small to shake, don't you think?
Well, all this rumenation is fun! Hopefully, someone will profit by it!  Check YouTube,        Malotte   Palmer
C. Edward
on July 26, 2010 9:50am
Clarence: I may have misread you, and if so I apologize. I do think that flexibility is one of the most important things a singer can have, and it applies to much more than vibrato.
Interesting that you would say, "a child's vocal apparatus is too small to shake, don't you think," since it implies that size alone creates vibato, and that vibrato involves shaking. In my own experience, which admittedly is limited but which includes our own children, children simply don't have a "natural" vibrato unless and until they listen to and start imitating adult singers. Some children DO have vibrato, of course, but perhaps they simply learned at an earlier age?
And of course there is no one and only "Western tradition," as operatically-inclined voice teachers tend to think. The "Western traditions" of folk music, country and traditional music, ethnic musics of many different kinds including Eastern European and Balkan traditions, musical theater and jazz of course, and even Sacred Harp singing would all seem to be equally valuable. Bel canto is simply one particular tradition, highly respected and eagerly sought-after, of course, but still only a single tradition. And it would sound as ludicrous in Noh plays as it does in attempting to sing in jazz style.
All the best,
on July 26, 2010 1:16pm
I am only referring to the voice as the typical college choral director attempts to refine/use it, not all the miscellaneous traditions which you so knowledgably list.
The size of the larynx does not dictate wobble or no wobble. I don't think Tom wishes a wobble/shake for his student; he just wants a "live" sound. Again, I want to say                                       that the straight sound, adored by so many, is actually a tense production, lacking flexibility and expressive capabiity. But care is warranted when speaking obout
intentional vibrating. Again, if the sphinxian riddle of "phonation" is solved, vibrance will happen. We must not tinker!!
I may forward to you my chapter on phonation, and the one titled, "THe Wizardry of Aahs."
Hang in there,
C. Edward----
on July 26, 2010 10:20pm
I'm enjoying the entire discussion, gentlemen! Thanks for all the great ideas and willingness to share disparate opinions. With respect and appreciation, ~Tom
on September 29, 2010 8:14pm
I too can remember acquiring my vibrato, and I remember the exercise that prompted it.  My teacher at the time had me bend forward slightly at the waist and let my tongue hang loose as I sang, which relaxed my tongue and throat enough to release my tone.  To remind myself of that feeling, because I am also a string player, I played vibrato as you would on a cello on my collarbone.  
I can think of a few other simple things which may help.   Having him sing an example while gently moving his head around can force him to sing without tension in his throat, as the muscles cannot hold their position if the head is moving.  If he is often sounding flat, check that he can hear the problem, then have him discover his soft palate and learn to lift it in front of a mirror.  It may be that it is vowel shape holding back the proper pitch.
Above all, encourage constant breath.  I sometimes tell my students to waste their breath, because they often find that if they use more, there's more available to them, and it really helps their tone.
Hope something sticks!
AJ Lund
on April 12, 2013 7:29am
I'm joining this discussion rather late and, probably, from the wrong perspective!
Trying to ACHIEVE a vocal vibrato is something totally foreign to my experience. Except in children and those sort of voices that never seem really to mature - a few well-known contemporaries shall remain nameless - most voices have an intrinsic 'instability' that gives a living and vibrant quality to the music.
My real aim is to find out why, these days, there is a general, widespread and altogether ugly tendency to 'wobble'. I use this term because it reflects the a gross mis-management of vibrato. It seems that hardly a solo singer is immune from this ghastly practice. And these practices involve not merely changes in intensity but also in speed and pitch to the extent that it is often difficult to determine what pitch is actually intended.
There is no doubt that teaching which, having no underpinning of vocal anatomy and physiology, concentrates on cultivating unnatural breathing practices is often responsible: the vocal folds being subject to breath pressure uncommensurate with their intrinsic strength. But, apart from that, there is a general tolerance among the public for musical performances which are totally marred by a distortion of sound and beauty - and composers' intentions.
I have a suspicion that the fashion among pop and jazz singers to cultivate this wobble - and, because of the ubiquity of the pop culture - the unavoidable contamination of children's singing - has infiltrated the singing of classical and operatic repertoires and to make it more or less acceptable
Curiously enough, the greatest singers of today, do NOT wobble. This must say something about the public's perceptions of good singing but still the degeneration continues.
PLEASE CHOIRMATERS, BE CAUTIOUS IN YOUR INTRODUCTION OF THIS INSIDIOUS TENDENCY. Take a note from the practice of those conductors who even deplore teh vibroto of stringed instruments in period music.
on April 13, 2013 5:21am
I once attended a workshop with Linda Spevacek, "The Choral Director as Voice Teacher.". She offered to help singers with no vibrato to acquire one by demonstrating her own way of helping someone in this pursuit. A volunteer raised his hand and was called forward. Linda made her hand into a fist shape and rapidly pulsed it against the singer's abdomen while he sang a tone, which was of course a straight tone. The fist stayed in contact with the belly at all times, but rapidly pressed in and out with some force. It almost looked like an assault because of the force required. A vibrato appeared in the singer's tone, which he could then reproduce at will without her anymore. He was amazed and grateful. The audience was likewise amazed, as evidenced by the "oohs" and "ahhs" heard throughout the room.


on April 13, 2013 11:16am
Hello friends (hi dad),
Vibrato...  What an interesting word...
This has been a great thread to follow, and has made me think about whether we (voice teachers/singers) are doing a good job of making the phenomenon understood. The characteristic sound of a western classical music solo singer is an aesthetic choice, and vibrato is a result of the technique required to achieve that aesthetic choice. Amplitude alone does not make an artist, but it is the #1 job requirement for unamplified solo singers. The increase in both hall and orchestra size since the 17th century has required singers to make technical accommodations, especially in the 20th century. There are two factors at play that cause vibrato to be perceived as "louder" than straight tone production. First, there is a basic psycho-acoustic phenomenon that causes us to perceive oscillating sounds as louder than non-oscillating sounds. I don't know why this is, but I saw a convincing presentation on it given by faculty from the Shenandoah Conservatory (by David Meyer, I believe) at the Voice Foundation Conference last year. That is worthy of further study. That being said, when a singer oscillates their pitch, it increases the average width of each harmonic, increasing the chance of catching a vocal tract formant. You can see this looking at spectrograms of elite tenors singing in their higher range. Sometimes the upper harmonics will have far greater acoustic energy on one side of the harmonic (higher or lower). Sung without vibrato, the added acoustic energy from "catching" the formant would have been lost. 
Look at this spectrogram of Pavarotti singing Ab4, Bb4, Ab4, followed by a little flourish to G4. (For those of you who have not read a spectrogram, it is like music. Time is left to right, frequency (pitch) increases from bottom to top. Each wavy line is a harmonic of his voice.) The fourth harmonic of the Ab4 (approx. the first quarter of the spectrogram from left to right) is red (high energy). The fourth harmonic of the Bb4 (the green cursor crosshair in placed in the middle of the Bb4, on the fourth harmonic) is only highly energized at the bottom of the oscillation. The harmonic is mostly yellow, with little dots of red at the bottom. (This image is from Donald Miller's software VoceVista based on an example from "Resonance in Singing." Used completely without permission, but I don't think he'll mind! Check out his VoceVista website to see more like this.) 
So vibrato appears to be a function of pitch oscillation, with secondary implications for changes in amplitude. We may perceive something different (spinning, releasing, flowing, variation in airflow, &c.), but the result is the same. There have been studies done that suggest that we perceive vibrato of around 4.5-7 oscillations per second, with a pitch deviation of no more than a quarter step above and below the fundamental pitch, as "pleasing" and characteristic of a "well-produced" western art music solo sound. That is around the oscillation rate of an alpha brain wave, so there may be something there...?
McCoy does suggest that airflow varies slightly when producing an oscillating tone. However, I do not believe there is much to support the contention that intentionally varying the rate of airflow is recommended to achieve a healthy sung vibrato. Interestingly, this is how many wind players create vibrato, so confusion is understandable. It may be a quick fix, but true (free) vibrato comes from the sort of production that allows the larynx to quiver by itself. Airflow will adjust itself once the conditions in the larynx and pharynx are ideal. Besides, do we really expect our students to consciously manage a variation in airflow pressure that changes 7 times per second? At the very least that runs counter to the common notion that breath support (breath energy flow, &c.) is a continuous, rather than rapidly changing, process. 
I have had the best luck bringing about healthy vibrato in students by first correcting tightness in respiration, then constriction in the pharynx. This "allows" the folds to function well without over/under adducting. Then, I follow a similar approach to Mr. Palmer above (and supported by McKinney). Using portamento (if they cannot, go back to addressing tightness in airflow and the pharynx), sing do re do re &c. on quarter notes, quarter note triplets, eighth notes, eighth note triplets, sixteenth notes, &c., until the sound is a trill. If the student can do this without losing their portamento or overly favoring the upper or lower pitch, everything is in place to achieve a healthy vibrato. Then demonstrate it and ask them to repeat it back to you. 
To the second theme in this thread, the question of value judgments attached to certain kinds of vibrato or straight tone singing, of course it is a matter of taste. Excellent operatic vibrato requires a highly "athleticized" coordination of the body. Vibrato is a natural response to that coordination, but no singing is really "natural." Neither is throwing a fastball well, regardless of how efficient and fluid a good pitcher can make it look. To imply that only uncoordinated singing (or sounds made by still developing voices) is natural is just as silly. We either are using the full potential of our instruments or we are not. Along that continuum, we are able to produce an amazing variety of aesthetically pleasing sounds. The conflicts, I find, come into the discussion only when a singer wants to train in a certain way (necessitating or precluding vibrato), and is pulled by a teacher in the opposite direction. There is such a thing as a well-resonated straight tone that is quite appropriate for certain music styles. It wouldn't cut over an orchestra by itself, but that's fine. I take some professional issue with the suggestion that a singer should be able to "do it all." At a certain point you don't ask your elite sprinter to run a marathon, and vice versa. :-)
All, of course, imho
Ian Howell
Teaching Fellow in Voice and Vocal Pedagogy
The New England Conservatory of Music
Applauded by an audience of 1
on April 13, 2013 7:58pm
[edit] ...pitch deviation of no more than a half-step above and below :-). Sorry, was writing too fast!
on April 13, 2013 11:18am
I am new to Choralnet, and love these threads!  Here is a little trick I have used to check myself or any of my students to see how "wide" the vibrato is: standing at the piano, press only the damper pedal, and sing a tone.  Let's say you sing a C, then all the c strings will respond to that and vibrate...if you sing a wide wobbly C then lots of others strings near the C up and down the keyboard will sympathetically vibrate also...ugg...mud!  Works well with the lid open.
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