This completes a vocal fold cycle. In this model respiration is the driving force that sets the vocal folds into motion and the interchange between pressure and flow at three critical sites keeps the vocal folds vibrating. The leading edge is set into motion by the air from the lungs. Then negative as the folds recoil to midline. First, you have positive pressure but as the air flow increases the pressure drops.
Pressure is negative as the folds close, but rises as the flow is cut off by the closing glottis.
Pressure and flow are reciprocal. The amount of pressure needed to blow the folds apart varies with the vocal fold tissue, the tissue damping, mucosal wave, flexibility, and mass. The supraglottic pressure assists in sustaining the oscillation via the compression or rarefaction of the air molecules. When the sound represented by the first spectrum is transmitted through a vocal tract that resonates at the frequencies indicated in the second spectrum the resulting sound will be a product of the two. The harmonics of the glottal sound wave which are at or near the spectral peaks of the transfer function of the vocal tract are resonated, and those distant from the resonant frequencies of the tract lose energy and are thus greatly attenuated.