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.

The frequencies, magnitudes, and bandwidths of vocal tract resonances are all important in understanding and synthesizing speech. High precision acoustic impedance spectra of the vocal tracts of 10 subjects were measured from 10 Hz to 4.2 kHz by injecting a broadband acoustic signal through the lips. Between 300 Hz and 4 kHz the acoustic resonances R (impedance minima measured through the lips) and anti-resonances R¯ (impedance maxima) associated with the first three voice formants, have bandwidths of ∼50 to 90 Hz for men and ∼70 to 90 Hz for women. These acoustic resonances approximate those of a smooth, dry, rigid cylinder of similar dimensions, except that their bandwidths indicate higher losses in the vocal tract. The lossy, inertive load and airflow caused by opening the glottis further increase the bandwidths observed during phonation. The vocal tract walls are not rigid and measurements show an acousto-mechanical resonance R0 ∼ 20 Hz and anti-resonance R¯0∼200 Hz. These give an estimate of wall inertance consistent with an effective thickness of 1-2 cm and a wall stiffness of 2-4 kN m(-1). The non-rigidity of the tract imposes a lower limit of the frequency of the first acoustic resonance fR1 and the first formant F1.

Experimental determinations of the acoustic properties of the subglottal airway, from the trachea below the larynx to the lungs, may provide useful information for detecting airway pathologies and aid in the understanding of vocal fold auto-oscillation. Here, minimally invasive, high precision impedance measurements are made through the lips (7 men, 3 women) over the range 14-4200 Hz during inspiration, expiration, and with a closed glottis. Closed glottis measurements show the expected resonances and anti-resonances of the supraglottal vocal tract. As the glottis is gradually opened, and the glottal inertance decreases, maxima in the subglottal impedance increasingly affect the measured impedance spectrum, producing additional pairs of maxima and minima. The pairs with the lowest frequency appear first. Measurements during a cycle of respiration show the disappearance and reappearance of these extrema. For a wide glottal opening during inspiration, and for the frequency range 14-4200 Hz, the impedance spectrum semi-quantitatively resembles that of a single, longer duct, open at the remote end, and whose total effective length is 37 ± 4 cm for men and 34 ± 3 cm for women. Fitting to simple models of the subglottal tract yields mean effective acoustic lengths of 19.5 cm for the men and 16.0 cm for the women in this study.

No abstract

No abstract