In performance, the singerΓÇÖs worst fear is of his voice cracking on a high note. Tenors are particularly prone to this disaster, which is caused by uncontrolled muscular tension and tiredness. In these conditions, the sound can become forced - the balance of pressures is lost, and with it the true resonance of the voice. The vocal folds simply stop vibrating altogether, or are forced to increase their speed of vibration causing a higher note to suddenly come out - a sort of uncontrolled yodel. This is, of course, horribly noticeable to the audience, and can unnerve the singer for the rest of the performance.
