4.7 Irregularities in vibrato
The most frequent errors of the vibrato are "tremolo", "rocking", and straight voice".
A tremolo or a "lamb sound" in the voice occurs when the vibrato is too frequent (more than 6-7 cycles per second). To remedy this defect you should pay attention to the removal of an unnecessary stress in the throat so that it could function properly.
For "straight" voices, i.e. the ones without vibrato the exercises on vocal shake as well the exercises for velocity can be helpful.
The "rocking" (less than 6-7-oscillations per second) is the result of the coordination loss in the vocal tract, of muscle fatigue due to overpressure or because of a lowered tone of the neuromuscular system that comes with age. In this case, you should resort to the exercises on scales within fifths and ninths, choosing such tempo that the voice is not swinging. These exercises need to be sung for a long time.