Great singing is a matter of certain measurable attributes. In my previous post I covered a couple of these aspects which are the fundamental pitch and the harmonic richness that great voices produce. These are two absolutely essential and quantifiable qualities that all great singing voices have the ability to wield.
In addition the these two qualities there are some other elements every great singer has. One of these is a proper vibrato action. The vibrato is an "on and off" nerve impulse from the brain to the muscles of the larynx. The vibrato also has a pitch fluctuation as well as a pulse. The pulse is about 6 cycles per second ideally. Of course someone can have a bit faster pulse or a bit slower pulse. However, if a singer has too fast a pulse it goes into a tremolo action which is not desirable. It is anti-musical as it forgoes the proper pitch fluctuation and also gives the feeling of agitation. If the singer's vibrato is too slow then they have a wobble which is also anti-musical.
The proper pitch fluctuation of a vibrato changes based on intensity and pitch range. The lower a singer is in their range the less pitch change there is generally speaking. The higher they go the wider the pitch change. This is also the case in reference to loudness. The louder a singer sings the more pitch fluctuation there should be in their vibrato - and the quieter the sing the less pitch change they should have. This can be easily seen in an analysis of great singers on a spectograph.
Additionally, the pitch fluctuation has a particular accent to it. The top of the pitch fluctuation, or the "peak", is generally 5 decibels louder than the bottom of the pitch change or the "trough". Also, there is a difference in the peak and the trough relative to the fundamental pitch. The peak will be about 1/3 above the fundamental pitch while the trough will be about 2/3 below. So here we have the peak being only 1/3 above the pitch, but 5 decibels louder than the trough which is 2/3 below the pitch. Consequently, our ears hear this action as being on the fundamental pitch. If this proper fluctuation of the vibrato is off then the voice of the singer is affected negatively. Some examples of this are as follows:
1) Tremolo vibrato - too little pitch fluctuation and too fast a pulse.
2) Wobble - too wide of a pitch fluctuation and too slow a pulse.
3) Inverted vibrato - the fluctuation in too big on the peak and to shallow in the tough. In this set up the singer will sound "sharp" or "driven". It is an unsettling vibrato.
4) The vibrato is evenly above and below the pitch which sounds more like a trill.
All of these improper vibrato actions affect the singer's voice negatively. So this is another measurable attribute of a great singing voice.
Another important fact to remember about the vibrato is that it is semi-reflexive. What this means is that the vibrato, just like blinking, will come in naturally by itself or it can be controlled by taking it out altogether or speeding it up, trilling, slowing it down etc. If, however, the vibrato becomes a reflex action then it is completely out of the singers control. This happens in the case of the tremolo vibrato. In this case the speed of the vibrato cannot be changed as it is a reflex.
Consequently, since the fioratura (passages that require rapid scale movement) are done on the vibrato action, if the vibrato becomes a reflex - as in a tremolo vibrato - it will be problematic for the singer for a couple of reasons. First, if the fioratura is at a different speed than the tremolo there will be unsteadiness and inaccuracy. Secondly, sometimes a singer needs to straighten out their vibrato action altogether when passages move so rapidly that they are faster than the vibrato action, giving no time for the singer to vibrato at all; And also there is a musical reason for taking out the vibrato at times in an aria. Singing with no vibrato causes a sense of heightened tension. This tension is then relieved once the vibrato action is added. Great singers use this affect to their advantage when singing. Additionally, there will be times when a singer will want to speed up their vibrato for emotional affect; e.g., at the end of a high note or long held out note. This also has an emotional effect on the listener.
Thus, a singer's control over their vibrato action is crucial for expression, vocal freedom and movement. Paramount to gaining this control is understanding what the proper action is and what controls it and/or affects it. For example, the proper breathing coordination is crucial to gaining control over the vibrato. You can see my past articles on breathing in my blog. If the breathing is set up incorrectly it will not be possible for the vibrato to be correct. The vibrato is also affected by laryngeal position, which is affected by the tongue and so on. In singing all of these components work together so it is vitally important that singers learn systematically and accurately what these components are and how they properly function together. This will result in the most vocal freedom, resonance, beauty and skill that a singer can attain.