In the last post, I dealt mainly with the nature of vibrato (that it is inherent in the voice) as opposed to the nurture theory (that a vibrato is made, not inherent to the voice). In this post I would like to address unbalanced vibrato. The two common qualitative issues relative to vibrato are bleats and wobbles, mistakenly referred to as fast and slow vibratos respectively.
It is important to know that a vibrato is defined not only by its rate, the number of oscillations per second, but also by its extent, the pitch range it covers. The vibrato rate does not change from one moment to the next or from one day to the next unless there are issues to the efficiency of nerve function or dysfunction, which can be influenced by the effects of old age. But not necessarily! Indeed vibrato rate (speed) is hardly the issue at all.
The problems we tend to identify as a slow or fast vibrato have to do with the pitch range that the vibrato covers or the vibrato extent. Scientists have gauged an acceptable vibrato (one that does not distract the listener’s attention) to have a pitch range of about a semitone or less. Vibratos have been measured far in excess of a semitone (100 Cents: In acoustical measurements of vibrato extent, one Cent is defined as 1% of a semitone) among professional singers. What is particularly interesting is that a singer’s vibrato extent may vary greatly between one note in their range and another. Extents may vary prominently between different registers as well. All of it depends on the individual singer and this is the question that occupied my mind until a couple of years ago when I began to consider different modes of singing one pitch, relative to the dynamic activity in the antagonistic relationship between the Cricothiroyd muscles that stretch the vocal folds and the Vocalis that resists the stretch and causes tautness in the vocal folds (Scientists have shown that tautness of the folds make for more efficient pitch making than simply fold lengthening).
In my opinion, good singing begins with efficient pitch-making. One pitch can be sung in different ways on a continuum of antagonistic activity between the two main muscle groups. Given that pitch is basically based on the timing of one vibration cycle, there are many variations on how a pitch can be sung. The length of the cycle depends on the vertical depth of the folds on the one hand (since the vibration pattern or mucosal wave occurs on the vertical axis of the folds) and the firmness or looseness of the closure between the folds. A given cycle length (pitch, in simple language) can be produced either with folds that are vertically very thin and firmly pressed, vertically very deep and loosely closed or somewhere in between. The correct in between is what we seek. I believe that wobbles and bleats occur when the production is at the extremes. Thick and loose production tend to cause flatness. A singer with a good ear will always seek to compensate and so the mechanism will try to inch its way to balance between cycles. This means that one moment the tone may be flat and the next in tune. This variation registers in our ears as a slow vibrato when in fact the vibrato rate (how many vibrato cycles per second) has not changed at all. The average pitch range of the vibrato relative to the frequency of the desired pitch is low. Likewise, a pitch that is produced thin and pressed would hang on the higher side of the desired pitch and continually try to adjust to the desired pitch. The average range of the vibrato would be higher but there would always be a move toward the center of the pitch. The vibrato extent in both cases is actually wide, but the pitch range, sharp or flat, makes us perceive the vibrato as fast or slow respectively.
A thick or thin production can both be pressed. A thick production that lacks good breath support would begin to squeeze and the pitch would lower audibly. Such a singer over a long period of time, especially with older age when the breath mechanism can become lethargic, could end up with what sounds like a slow vibrato even if vibrato rate does not change (vibrato rate can change with older age). Likewise a singer who sings thin and lacks adequate breath coordination could end up with an instability that sounds like a bleat, goat-trill or the Italian term caprino. Such instabilities can occur at any point in life, but are more common in older singers, whose breathing function may become weaker due to lower physical activity and atrophy. Singers who on the other hand have a balanced phonation (neither thin nor thick) tend to not exhibit a wobble even in advanced age. A balance phonation is generally accompanied by excellent breath coordination, which maintains good muscular fitness as the singer ages.
One thing is certain, wobbles and bleats develop over time from phonation habits, which may not even have to do with principles of singing but rather the singer’s normal speaking habits. Poor speaking habits can undo good singing practice such that the singer never develops a since of security and consistency.