The acoustic impedance spectrum of the vocal tract, measured during phonation (Hanna et al., 2016. JASA, 139, 2924–2936), shows qualitative and semi-quantitative similarity to that of a simple cylindrical duct of length l. But how does l compare with geometrical length(s) measured from a magnetic resonance imaging (MRI) scan. One male subject (age 34, height 184 cm) (1) performed a neutral /з/ vowel during impedance measurement; (2) was MRI scanned for the same gesture with his lips around a plastic tube with the same dimensions as the impedance head; and (3) was scanned during closed mouth nasal breathing. For the neutral vowel, the effective acoustic length is a weakly increasing function of frequency: the first four acoustic tract resonances give lengths rising from 155 to 195 mm; i.e. the higher resonances occur at slightly lower frequencies than the expected 1:3:5:7 ratios for cylindrical geometry. One factor is that the tract cross-section is on average greater near the lips than near the glottis. I...