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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.