During speech and singing, the vibrating vocal folds are acoustically loaded by resonant ducts upstream (the trachea) and downstream (the vocal tract). Some models suggest that the vocal fold vibration (at frequency f) is more stable at frequencies below that of a vocal tract resonance, so that the downstream load is inertive (mass-like). If so, vocal fold vibration might become unstable when f and resonance frequencies "cross over" and the load varies rapidly in phase and magnitude. In one experiment, singers produced a slow diphthong at constant pitch, thus shifting the first tract resonance R1 across fixed f. In another, pitch glides took f across the tract and subglottal resonances. Few instabilities occurred when singers could change lip geometry and thus alter R1. This suggests that avoiding resonance crossings can aid vibrational stability. In experiments in which R1 was constrained using a mouth ring, instabilities occurred at frequencies above R1. When subjects sang into an acoustically infinite pipe, which provided a purely resistive load at the lips, R1 was eliminated. Here, instabilities were reduced and concentrated near the lower limit of the head voice.

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