One of the displays of VoceVista (Figure 4, below) shows the waveforms during a short segment of time: above, the microphone (audio) signal, and below, the electroglottograph (EGG), which allows one to follow the pattern of vocal fold contact in the glottal cycle. The moment of closing of the glottis (increasing contact) is usually abrupt and can be determined fairly precisely in the EGG signal. The opening (theoretically, the moment of the steepest falling slope) can be more problematical to determine, but an experienced observer can make consistent and useful estimates.
Fig. 4. Waveform signals of three glottal cycles, taken from spectrogram (left) of sustained G2-flat (89 Hz): above right, microphone signal; below right, electroglottograph. The horizontal cursor is manually adjusted to the estimated moment of glottal opening. The values of the glottal period (11.21ms) and the closed quotient (0.54) are displayed. The two signals have been adjusted to align their moments of glottal closing.
Because of the distance the sound must travel from the glottis, the microphone signal lags slightly behind, but the moment of closing is usually apparent in the mike signal as well. Using a convenient operation of the software, an experienced operator can align the closing moment in EGG and audio, gaining more information from the combination of the two than from either signal alone.
The closed quotient–the percentage of the glottal cycle in which the glottis is closed, preventing the flow of air–is an important variable in the singing voice, giving indications of both ‘register’ (chest or falsetto vocal-fold vibration pattern — see Figure 5, below) and vocal effort. By adjusting a horizontal cursor in the EGG signal to the estimated point of opening of the glottis, one can get a readout of this quantity.
Fig. 5.Spectrogram of mezzo soprano arpeggio, A3 to A4, with waveform signals shown of C4-sharp (above, middle register) and A3 (below, chest register). Note the difference in the closed quotient (CQ) and form of the EGG signal.