The messa di voce, or swell tone (from pp to ff and back to pp again, as smoothly as possible), has historically been regarded as one of the most important tests and training exercises of vocal technique. It is considered most challenging in the upper range of the male voice, and not all authorities agree on whether the pp extreme should include the falsetto register. The electroglottograph (EGG) signal, which reveals the pattern of contact between the vocal folds and allows us to derive the closed quotient (CQ) -- the percentage of the glottal cycle in which the glottis is closed to airflow -- is particularly interesting to follow in the messa di voce maneuver.
The robustness of a given voice appears to be
related to the size of the closed quotient that is habitually maintained in
singing, and this quantity is an important piece of information in the profile
of an individual voice. The messa di voce tests both the limits of the
individual CQ and the ability of the singer to vary it. The figures show messe
di voce of two tenor subjects, with waveforms of EGG and audio taken from
the extremes of ff and pp (marked by the cursors in the
spectrograms). The CQs are given just below the EGG line. Both these tenors
reach the high value of 3/4 closed in ff, but there is a marked contrast
in their ability to diminish to the softest sounds. While the one still has a CQ
of 2/3 at the soft end, the other reduces his CQ to around 1/4 in a
quasi-falsetto production.